Merge pull request #29450 from YHNdnzj/run-mandatory

[thirdparty/systemd.git] / man / systemd.exec.xml

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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->

<refentry id="systemd.exec" xmlns:xi="http://www.w3.org/2001/XInclude">
  <refentryinfo>
    <title>systemd.exec</title>
    <productname>systemd</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>systemd.exec</refentrytitle>
    <manvolnum>5</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>systemd.exec</refname>
    <refpurpose>Execution environment configuration</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <para><filename><replaceable>service</replaceable>.service</filename>,
    <filename><replaceable>socket</replaceable>.socket</filename>,
    <filename><replaceable>mount</replaceable>.mount</filename>,
    <filename><replaceable>swap</replaceable>.swap</filename></para>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para>Unit configuration files for services, sockets, mount points, and swap devices share a subset of
    configuration options which define the execution environment of spawned processes.</para>

    <para>This man page lists the configuration options shared by these four unit types. See
    <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry> for the common
    options of all unit configuration files, and
    <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
    <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
    <citerefentry><refentrytitle>systemd.swap</refentrytitle><manvolnum>5</manvolnum></citerefentry>, and
    <citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry> for more
    information on the specific unit configuration files. The execution specific configuration options are configured
    in the [Service], [Socket], [Mount], or [Swap] sections, depending on the unit type.</para>

    <para>In addition, options which control resources through Linux Control Groups (cgroups) are listed in
    <citerefentry><refentrytitle>systemd.resource-control</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
    Those options complement options listed here.</para>
  </refsect1>

  <refsect1>
    <title>Implicit Dependencies</title>

    <para>A few execution parameters result in additional, automatic dependencies to be added:</para>

    <itemizedlist>
      <listitem><para>Units with <varname>WorkingDirectory=</varname>, <varname>RootDirectory=</varname>,
      <varname>RootImage=</varname>, <varname>RuntimeDirectory=</varname>, <varname>StateDirectory=</varname>,
      <varname>CacheDirectory=</varname>, <varname>LogsDirectory=</varname> or
      <varname>ConfigurationDirectory=</varname> set automatically gain dependencies of type
      <varname>Requires=</varname> and <varname>After=</varname> on all mount units required to access the specified
      paths. This is equivalent to having them listed explicitly in
      <varname>RequiresMountsFor=</varname>.</para></listitem>

      <listitem><para>Similarly, units with <varname>PrivateTmp=</varname> enabled automatically get mount
      unit dependencies for all mounts required to access <filename>/tmp/</filename> and
      <filename>/var/tmp/</filename>. They will also gain an automatic <varname>After=</varname> dependency
      on
      <citerefentry><refentrytitle>systemd-tmpfiles-setup.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
      </para></listitem>

      <listitem><para>Units whose standard output or error output is connected to <option>journal</option> or
      <option>kmsg</option> (or their combinations with console output, see below) automatically acquire
      dependencies of type <varname>After=</varname> on
      <filename>systemd-journald.socket</filename>.</para></listitem>

      <listitem><para>Units using <varname>LogNamespace=</varname> will automatically gain ordering and
      requirement dependencies on the two socket units associated with
      <filename>systemd-journald@.service</filename> instances.</para></listitem>
    </itemizedlist>
  </refsect1>

  <!-- We don't have any default dependency here. -->

  <refsect1>
    <title>Paths</title>

    <para>The following settings may be used to change a service's view of the filesystem. Please note that the paths
    must be absolute and must not contain a <literal>..</literal> path component.</para>

    <variablelist class='unit-directives'>

      <varlistentry>
        <term><varname>ExecSearchPath=</varname></term>

        <listitem><para>Takes a colon separated list of absolute paths relative to which the executable
        used by the <varname>Exec*=</varname> (e.g. <varname>ExecStart=</varname>,
        <varname>ExecStop=</varname>, etc.) properties can be found. <varname>ExecSearchPath=</varname>
        overrides <varname>$PATH</varname> if <varname>$PATH</varname> is not supplied by the user through
        <varname>Environment=</varname>, <varname>EnvironmentFile=</varname> or
        <varname>PassEnvironment=</varname>. Assigning an empty string removes previous assignments
        and setting <varname>ExecSearchPath=</varname> to a value multiple times will append
        to the previous setting.
        </para>

        <xi:include href="version-info.xml" xpointer="v250"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>WorkingDirectory=</varname></term>

        <listitem><para>Takes a directory path relative to the service's root directory specified by
        <varname>RootDirectory=</varname>, or the special value <literal>~</literal>. Sets the working directory for
        executed processes. If set to <literal>~</literal>, the home directory of the user specified in
        <varname>User=</varname> is used. If not set, defaults to the root directory when systemd is running as a
        system instance and the respective user's home directory if run as user. If the setting is prefixed with the
        <literal>-</literal> character, a missing working directory is not considered fatal. If
        <varname>RootDirectory=</varname>/<varname>RootImage=</varname> is not set, then
        <varname>WorkingDirectory=</varname> is relative to the root of the system running the service manager. Note
        that setting this parameter might result in additional dependencies to be added to the unit (see
        above).</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>RootDirectory=</varname></term>

        <listitem><para>Takes a directory path relative to the host's root directory (i.e. the root of the system
        running the service manager). Sets the root directory for executed processes, with the <citerefentry
        project='man-pages'><refentrytitle>chroot</refentrytitle><manvolnum>2</manvolnum></citerefentry> system
        call. If this is used, it must be ensured that the process binary and all its auxiliary files are available in
        the <function>chroot()</function> jail. Note that setting this parameter might result in additional
        dependencies to be added to the unit (see above).</para>

        <para>The <varname>MountAPIVFS=</varname> and <varname>PrivateUsers=</varname> settings are particularly useful
        in conjunction with <varname>RootDirectory=</varname>. For details, see below.</para>

        <para>If <varname>RootDirectory=</varname>/<varname>RootImage=</varname> are used together with
        <varname>NotifyAccess=</varname> the notification socket is automatically mounted from the host into
        the root environment, to ensure the notification interface can work correctly.</para>

        <para>Note that services using <varname>RootDirectory=</varname>/<varname>RootImage=</varname> will
        not be able to log via the syslog or journal protocols to the host logging infrastructure, unless the
        relevant sockets are mounted from the host, specifically:</para>

        <para>The host's
        <citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry>
        file will be made available for the service (read-only) as
        <filename>/run/host/os-release</filename>.
        It will be updated automatically on soft reboot (see:
        <citerefentry><refentrytitle>systemd-soft-reboot.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>),
        in case the service is configured to survive it.</para>

        <example>
          <title>Mounting logging sockets into root environment</title>

          <programlisting>BindReadOnlyPaths=/dev/log /run/systemd/journal/socket /run/systemd/journal/stdout</programlisting>
        </example>

        <xi:include href="system-or-user-ns.xml" xpointer="singular"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>RootImage=</varname></term>

        <listitem><para>Takes a path to a block device node or regular file as argument. This call is similar
        to <varname>RootDirectory=</varname> however mounts a file system hierarchy from a block device node
        or loopback file instead of a directory. The device node or file system image file needs to contain a
        file system without a partition table, or a file system within an MBR/MS-DOS or GPT partition table
        with only a single Linux-compatible partition, or a set of file systems within a GPT partition table
        that follows the <ulink url="https://uapi-group.org/specifications/specs/discoverable_partitions_specification">Discoverable Partitions
        Specification</ulink>.</para>

        <para>When <varname>DevicePolicy=</varname> is set to <literal>closed</literal> or
        <literal>strict</literal>, or set to <literal>auto</literal> and <varname>DeviceAllow=</varname> is
        set, then this setting adds <filename>/dev/loop-control</filename> with <constant>rw</constant> mode,
        <literal>block-loop</literal> and <literal>block-blkext</literal> with <constant>rwm</constant> mode
        to <varname>DeviceAllow=</varname>. See
        <citerefentry><refentrytitle>systemd.resource-control</refentrytitle><manvolnum>5</manvolnum></citerefentry>
        for the details about <varname>DevicePolicy=</varname> or <varname>DeviceAllow=</varname>. Also, see
        <varname>PrivateDevices=</varname> below, as it may change the setting of
        <varname>DevicePolicy=</varname>.</para>

        <para>Units making use of <varname>RootImage=</varname> automatically gain an
        <varname>After=</varname> dependency on <filename>systemd-udevd.service</filename>.</para>

        <para>The host's
        <citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry>
        file will be made available for the service (read-only) as
        <filename>/run/host/os-release</filename>.
        It will be updated automatically on soft reboot (see:
        <citerefentry><refentrytitle>systemd-soft-reboot.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>),
        in case the service is configured to survive it.</para>

        <xi:include href="system-only.xml" xpointer="singular"/>

        <xi:include href="version-info.xml" xpointer="v233"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>RootImageOptions=</varname></term>

        <listitem><para>Takes a comma-separated list of mount options that will be used on disk images specified by
        <varname>RootImage=</varname>. Optionally a partition name can be prefixed, followed by colon, in
        case the image has multiple partitions, otherwise partition name <literal>root</literal> is implied.
        Options for multiple partitions can be specified in a single line with space separators. Assigning an empty
        string removes previous assignments. Duplicated options are ignored. For a list of valid mount options, please
        refer to
        <citerefentry project='man-pages'><refentrytitle>mount</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
        </para>

        <para>Valid partition names follow the <ulink
        url="https://uapi-group.org/specifications/specs/discoverable_partitions_specification">Discoverable Partitions Specification</ulink>:
        <constant>root</constant>, <constant>usr</constant>, <constant>home</constant>, <constant>srv</constant>,
        <constant>esp</constant>, <constant>xbootldr</constant>, <constant>tmp</constant>,
        <constant>var</constant>.</para>

        <xi:include href="system-only.xml" xpointer="singular"/>

        <xi:include href="version-info.xml" xpointer="v247"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>RootEphemeral=</varname></term>

        <listitem><para>Takes a boolean argument. If enabled, executed processes will run in an ephemeral
        copy of the root directory or root image. The ephemeral copy is placed in
        <filename>/var/lib/systemd/ephemeral-trees/</filename> while the service is active and is cleaned up
        when the service is stopped or restarted. If <varname>RootDirectory=</varname> is used and the root
        directory is a subvolume, the ephemeral copy will be created by making a snapshot of the subvolume.
        </para>

        <para>To make sure making ephemeral copies can be made efficiently, the root directory or root image
        should be located on the same filesystem as <filename>/var/lib/systemd/ephemeral-trees/</filename>.
        When using <varname>RootEphemeral=</varname> with root directories, btrfs should be used as the
        filesystem and the root directory should ideally be a subvolume which <command>systemd</command> can
        snapshot to make the ephemeral copy. For root images, a filesystem with support for reflinks should
        be used to ensure an efficient ephemeral copy.</para>

        <xi:include href="system-only.xml" xpointer="singular"/>

        <xi:include href="version-info.xml" xpointer="v254"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>RootHash=</varname></term>

        <listitem><para>Takes a data integrity (dm-verity) root hash specified in hexadecimal, or the path to a file
        containing a root hash in ASCII hexadecimal format. This option enables data integrity checks using dm-verity,
        if the used image contains the appropriate integrity data (see above) or if <varname>RootVerity=</varname> is used.
        The specified hash must match the root hash of integrity data, and is usually at least 256 bits (and hence 64
        formatted hexadecimal characters) long (in case of SHA256 for example). If this option is not specified, but
        the image file carries the <literal>user.verity.roothash</literal> extended file attribute (see <citerefentry
        project='man-pages'><refentrytitle>xattr</refentrytitle><manvolnum>7</manvolnum></citerefentry>), then the root
        hash is read from it, also as formatted hexadecimal characters. If the extended file attribute is not found (or
        is not supported by the underlying file system), but a file with the <filename>.roothash</filename> suffix is
        found next to the image file, bearing otherwise the same name (except if the image has the
        <filename>.raw</filename> suffix, in which case the root hash file must not have it in its name), the root hash
        is read from it and automatically used, also as formatted hexadecimal characters.</para>

        <para>If the disk image contains a separate <filename>/usr/</filename> partition it may also be
        Verity protected, in which case the root hash may configured via an extended attribute
        <literal>user.verity.usrhash</literal> or a <filename>.usrhash</filename> file adjacent to the disk
        image. There's currently no option to configure the root hash for the <filename>/usr/</filename> file
        system via the unit file directly.</para>

        <xi:include href="system-only.xml" xpointer="singular"/>

        <xi:include href="version-info.xml" xpointer="v246"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>RootHashSignature=</varname></term>

        <listitem><para>Takes a PKCS7 signature of the <varname>RootHash=</varname> option as a path to a
        DER-encoded signature file, or as an ASCII base64 string encoding of a DER-encoded signature prefixed
        by <literal>base64:</literal>. The dm-verity volume will only be opened if the signature of the root
        hash is valid and signed by a public key present in the kernel keyring. If this option is not
        specified, but a file with the <filename>.roothash.p7s</filename> suffix is found next to the image
        file, bearing otherwise the same name (except if the image has the <filename>.raw</filename> suffix,
        in which case the signature file must not have it in its name), the signature is read from it and
        automatically used.</para>

        <para>If the disk image contains a separate <filename>/usr/</filename> partition it may also be
        Verity protected, in which case the signature for the root hash may configured via a
        <filename>.usrhash.p7s</filename> file adjacent to the disk image. There's currently no option to
        configure the root hash signature for the <filename>/usr/</filename> via the unit file
        directly.</para>

        <xi:include href="system-only.xml" xpointer="singular"/>

        <xi:include href="version-info.xml" xpointer="v246"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>RootVerity=</varname></term>

        <listitem><para>Takes the path to a data integrity (dm-verity) file. This option enables data integrity checks
        using dm-verity, if <varname>RootImage=</varname> is used and a root-hash is passed and if the used image itself
        does not contain the integrity data. The integrity data must be matched by the root hash. If this option is not
        specified, but a file with the <filename>.verity</filename> suffix is found next to the image file, bearing otherwise
        the same name (except if the image has the <filename>.raw</filename> suffix, in which case the verity data file must
        not have it in its name), the verity data is read from it and automatically used.</para>

        <para>This option is supported only for disk images that contain a single file system, without an
        enveloping partition table. Images that contain a GPT partition table should instead include both
        root file system and matching Verity data in the same image, implementing the <ulink
        url="https://uapi-group.org/specifications/specs/discoverable_partitions_specification">Discoverable Partitions Specification</ulink>.</para>

        <xi:include href="system-only.xml" xpointer="singular"/>

        <xi:include href="version-info.xml" xpointer="v246"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>RootImagePolicy=</varname></term>
        <term><varname>MountImagePolicy=</varname></term>
        <term><varname>ExtensionImagePolicy=</varname></term>

        <listitem><para>Takes an image policy string as per
        <citerefentry><refentrytitle>systemd.image-policy</refentrytitle><manvolnum>7</manvolnum></citerefentry>
        to use when mounting the disk images (DDI) specified in <varname>RootImage=</varname>,
        <varname>MountImage=</varname>, <varname>ExtensionImage=</varname>, respectively. If not specified
        the following policy string is the default for <varname>RootImagePolicy=</varname> and <varname>MountImagePolicy</varname>:</para>

        <programlisting>root=verity+signed+encrypted+unprotected+absent: \
        usr=verity+signed+encrypted+unprotected+absent: \
        home=encrypted+unprotected+absent: \
        srv=encrypted+unprotected+absent: \
        tmp=encrypted+unprotected+absent: \
        var=encrypted+unprotected+absent</programlisting>

        <para>The default policy for <varname>ExtensionImagePolicy=</varname> is:</para>

        <programlisting>root=verity+signed+encrypted+unprotected+absent: \
        usr=verity+signed+encrypted+unprotected+absent</programlisting>

        <xi:include href="version-info.xml" xpointer="v254"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>MountAPIVFS=</varname></term>

        <listitem><para>Takes a boolean argument. If on, a private mount namespace for the unit's processes is created
        and the API file systems <filename>/proc/</filename>, <filename>/sys/</filename>, <filename>/dev/</filename> and
        <filename>/run/</filename> (as an empty <literal>tmpfs</literal>) are mounted inside of it, unless they are
        already mounted. Note that this option has no effect unless used in conjunction with
        <varname>RootDirectory=</varname>/<varname>RootImage=</varname> as these four mounts are
        generally mounted in the host anyway, and unless the root directory is changed, the private mount namespace
        will be a 1:1 copy of the host's, and include these four mounts. Note that the <filename>/dev/</filename> file
        system of the host is bind mounted if this option is used without <varname>PrivateDevices=</varname>. To run
        the service with a private, minimal version of <filename>/dev/</filename>, combine this option with
        <varname>PrivateDevices=</varname>.</para>

        <para>In order to allow propagating mounts at runtime in a safe manner, <filename>/run/systemd/propagate/</filename>
        on the host will be used to set up new mounts, and <filename>/run/host/incoming/</filename> in the private namespace
        will be used as an intermediate step to store them before being moved to the final mount point.</para>

        <xi:include href="version-info.xml" xpointer="v233"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>ProtectProc=</varname></term>

        <listitem><para>Takes one of <literal>noaccess</literal>, <literal>invisible</literal>,
        <literal>ptraceable</literal> or <literal>default</literal> (which it defaults to). When set, this
        controls the <literal>hidepid=</literal> mount option of the <literal>procfs</literal> instance for
        the unit that controls which directories with process metainformation
        (<filename>/proc/<replaceable>PID</replaceable></filename>) are visible and accessible: when set to
        <literal>noaccess</literal> the ability to access most of other users' process metadata in
        <filename>/proc/</filename> is taken away for processes of the service. When set to
        <literal>invisible</literal> processes owned by other users are hidden from
        <filename>/proc/</filename>. If <literal>ptraceable</literal> all processes that cannot be
        <function>ptrace()</function>'ed by a process are hidden to it. If <literal>default</literal> no
        restrictions on <filename>/proc/</filename> access or visibility are made. For further details see
        <ulink url="https://docs.kernel.org/filesystems/proc.html#mount-options">The /proc
        Filesystem</ulink>. It is generally recommended to run most system services with this option set to
        <literal>invisible</literal>. This option is implemented via file system namespacing, and thus cannot
        be used with services that shall be able to install mount points in the host file system
        hierarchy. Note that the root user is unaffected by this option, so to be effective it has to be used
        together with <varname>User=</varname> or <varname>DynamicUser=yes</varname>, and also without the
        <literal>CAP_SYS_PTRACE</literal> capability, which also allows a process to bypass this feature. It
        cannot be used for services that need to access metainformation about other users' processes. This
        option implies <varname>MountAPIVFS=</varname>.</para>

        <para>If the kernel doesn't support per-mount point <option>hidepid=</option> mount options this
        setting remains without effect, and the unit's processes will be able to access and see other process
        as if the option was not used.</para>

        <xi:include href="system-only.xml" xpointer="singular"/>

        <xi:include href="version-info.xml" xpointer="v247"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>ProcSubset=</varname></term>

        <listitem><para>Takes one of <literal>all</literal> (the default) and <literal>pid</literal>. If
        <literal>pid</literal>, all files and directories not directly associated with process management and
        introspection are made invisible in the <filename>/proc/</filename> file system configured for the
        unit's processes. This controls the <literal>subset=</literal> mount option of the
        <literal>procfs</literal> instance for the unit. For further details see <ulink
        url="https://docs.kernel.org/filesystems/proc.html#mount-options">The /proc
        Filesystem</ulink>. Note that Linux exposes various kernel APIs via <filename>/proc/</filename>,
        which are made unavailable with this setting. Since these APIs are used frequently this option is
        useful only in a few, specific cases, and is not suitable for most non-trivial programs.</para>

        <para>Much like <varname>ProtectProc=</varname> above, this is implemented via file system mount
        namespacing, and hence the same restrictions apply: it is only available to system services, it
        disables mount propagation to the host mount table, and it implies
        <varname>MountAPIVFS=</varname>. Also, like <varname>ProtectProc=</varname> this setting is gracefully
        disabled if the used kernel does not support the <literal>subset=</literal> mount option of
        <literal>procfs</literal>.</para>

        <xi:include href="version-info.xml" xpointer="v247"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>BindPaths=</varname></term>
        <term><varname>BindReadOnlyPaths=</varname></term>

        <listitem><para>Configures unit-specific bind mounts. A bind mount makes a particular file or directory
        available at an additional place in the unit's view of the file system. Any bind mounts created with this
        option are specific to the unit, and are not visible in the host's mount table. This option expects a
        whitespace separated list of bind mount definitions. Each definition consists of a colon-separated triple of
        source path, destination path and option string, where the latter two are optional. If only a source path is
        specified the source and destination is taken to be the same. The option string may be either
        <literal>rbind</literal> or <literal>norbind</literal> for configuring a recursive or non-recursive bind
        mount. If the destination path is omitted, the option string must be omitted too.
        Each bind mount definition may be prefixed with <literal>-</literal>, in which case it will be ignored
        when its source path does not exist.</para>

        <para><varname>BindPaths=</varname> creates regular writable bind mounts (unless the source file system mount
        is already marked read-only), while <varname>BindReadOnlyPaths=</varname> creates read-only bind mounts. These
        settings may be used more than once, each usage appends to the unit's list of bind mounts. If the empty string
        is assigned to either of these two options the entire list of bind mounts defined prior to this is reset. Note
        that in this case both read-only and regular bind mounts are reset, regardless which of the two settings is
        used.</para>

        <para>This option is particularly useful when <varname>RootDirectory=</varname>/<varname>RootImage=</varname>
        is used. In this case the source path refers to a path on the host file system, while the destination path
        refers to a path below the root directory of the unit.</para>

        <para>Note that the destination directory must exist or systemd must be able to create it. Thus, it
        is not possible to use those options for mount points nested underneath paths specified in
        <varname>InaccessiblePaths=</varname>, or under <filename>/home/</filename> and other protected
        directories if <varname>ProtectHome=yes</varname> is
        specified. <varname>TemporaryFileSystem=</varname> with <literal>:ro</literal> or
        <varname>ProtectHome=tmpfs</varname> should be used instead.</para>

        <xi:include href="version-info.xml" xpointer="v233"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>MountImages=</varname></term>

        <listitem><para>This setting is similar to <varname>RootImage=</varname> in that it mounts a file
        system hierarchy from a block device node or loopback file, but the destination directory can be
        specified as well as mount options. This option expects a whitespace separated list of mount
        definitions. Each definition consists of a colon-separated tuple of source path and destination
        definitions, optionally followed by another colon and a list of mount options.</para>

        <para>Mount options may be defined as a single comma-separated list of options, in which case they
        will be implicitly applied to the root partition on the image, or a series of colon-separated tuples
        of partition name and mount options. Valid partition names and mount options are the same as for
        <varname>RootImageOptions=</varname> setting described above.</para>

        <para>Each mount definition may be prefixed with <literal>-</literal>, in which case it will be
        ignored when its source path does not exist. The source argument is a path to a block device node or
        regular file. If source or destination contain a <literal>:</literal>, it needs to be escaped as
        <literal>\:</literal>. The device node or file system image file needs to follow the same rules as
        specified for <varname>RootImage=</varname>. Any mounts created with this option are specific to the
        unit, and are not visible in the host's mount table.</para>

        <para>These settings may be used more than once, each usage appends to the unit's list of mount
        paths. If the empty string is assigned, the entire list of mount paths defined prior to this is
        reset.</para>

        <para>Note that the destination directory must exist or systemd must be able to create it. Thus, it
        is not possible to use those options for mount points nested underneath paths specified in
        <varname>InaccessiblePaths=</varname>, or under <filename>/home/</filename> and other protected
        directories if <varname>ProtectHome=yes</varname> is specified.</para>

        <para>When <varname>DevicePolicy=</varname> is set to <literal>closed</literal> or
        <literal>strict</literal>, or set to <literal>auto</literal> and <varname>DeviceAllow=</varname> is
        set, then this setting adds <filename>/dev/loop-control</filename> with <constant>rw</constant> mode,
        <literal>block-loop</literal> and <literal>block-blkext</literal> with <constant>rwm</constant> mode
        to <varname>DeviceAllow=</varname>. See
        <citerefentry><refentrytitle>systemd.resource-control</refentrytitle><manvolnum>5</manvolnum></citerefentry>
        for the details about <varname>DevicePolicy=</varname> or <varname>DeviceAllow=</varname>. Also, see
        <varname>PrivateDevices=</varname> below, as it may change the setting of
        <varname>DevicePolicy=</varname>.</para>

        <xi:include href="system-only.xml" xpointer="singular"/>

        <xi:include href="version-info.xml" xpointer="v247"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>ExtensionImages=</varname></term>

        <listitem><para>This setting is similar to <varname>MountImages=</varname> in that it mounts a file
        system hierarchy from a block device node or loopback file, but instead of providing a destination
        path, an overlay will be set up. This option expects a whitespace separated list of mount
        definitions. Each definition consists of a source path, optionally followed by a colon and a list of
        mount options.</para>

        <para>A read-only OverlayFS will be set up on top of <filename>/usr/</filename> and
        <filename>/opt/</filename> hierarchies for sysext images and <filename>/etc/</filename>
        hierarchy for confext images. The order in which the images are listed will determine the
        order in which the overlay is laid down: images specified first to last will result in overlayfs
        layers bottom to top.</para>

        <para>Mount options may be defined as a single comma-separated list of options, in which case they
        will be implicitly applied to the root partition on the image, or a series of colon-separated tuples
        of partition name and mount options. Valid partition names and mount options are the same as for
        <varname>RootImageOptions=</varname> setting described above.</para>

        <para>Each mount definition may be prefixed with <literal>-</literal>, in which case it will be
        ignored when its source path does not exist. The source argument is a path to a block device node or
        regular file. If the source path contains a <literal>:</literal>, it needs to be escaped as
        <literal>\:</literal>. The device node or file system image file needs to follow the same rules as
        specified for <varname>RootImage=</varname>. Any mounts created with this option are specific to the
        unit, and are not visible in the host's mount table.</para>

        <para>These settings may be used more than once, each usage appends to the unit's list of image
        paths. If the empty string is assigned, the entire list of mount paths defined prior to this is
        reset.</para>

        <para>Each sysext image must carry a <filename>/usr/lib/extension-release.d/extension-release.IMAGE</filename>
        file while each confext image must carry a <filename>/etc/extension-release.d/extension-release.IMAGE</filename>
        file, with the appropriate metadata which matches <varname>RootImage=</varname>/<varname>RootDirectory=</varname>
        or the host. See:
        <citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
        To disable the safety check that the extension-release file name matches the image file name, the
        <varname>x-systemd.relax-extension-release-check</varname> mount option may be appended.</para>

        <para>When <varname>DevicePolicy=</varname> is set to <literal>closed</literal> or
        <literal>strict</literal>, or set to <literal>auto</literal> and <varname>DeviceAllow=</varname> is
        set, then this setting adds <filename>/dev/loop-control</filename> with <constant>rw</constant> mode,
        <literal>block-loop</literal> and <literal>block-blkext</literal> with <constant>rwm</constant> mode
        to <varname>DeviceAllow=</varname>. See
        <citerefentry><refentrytitle>systemd.resource-control</refentrytitle><manvolnum>5</manvolnum></citerefentry>
        for the details about <varname>DevicePolicy=</varname> or <varname>DeviceAllow=</varname>. Also, see
        <varname>PrivateDevices=</varname> below, as it may change the setting of
        <varname>DevicePolicy=</varname>.</para>

        <xi:include href="system-only.xml" xpointer="singular"/>

        <xi:include href="version-info.xml" xpointer="v248"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>ExtensionDirectories=</varname></term>

        <listitem><para>This setting is similar to <varname>BindReadOnlyPaths=</varname> in that it mounts a file
        system hierarchy from a directory, but instead of providing a destination path, an overlay will be set
        up. This option expects a whitespace separated list of source directories.</para>

        <para>A read-only OverlayFS will be set up on top of <filename>/usr/</filename> and
        <filename>/opt/</filename> hierarchies for sysext images and <filename>/etc/</filename>
        hierarchy for confext images. The order in which the directories are listed will determine
        the order in which the overlay is laid down: directories specified first to last will result in overlayfs
        layers bottom to top.</para>

        <para>Each directory listed in <varname>ExtensionDirectories=</varname> may be prefixed with <literal>-</literal>,
        in which case it will be ignored when its source path does not exist. Any mounts created with this option are
        specific to the unit, and are not visible in the host's mount table.</para>

        <para>These settings may be used more than once, each usage appends to the unit's list of directories
        paths. If the empty string is assigned, the entire list of mount paths defined prior to this is
        reset.</para>

        <para>Each sysext directory must contain a <filename>/usr/lib/extension-release.d/extension-release.IMAGE</filename>
        file while each confext directory must carry a <filename>/etc/extension-release.d/extension-release.IMAGE</filename>
        file, with the appropriate metadata which matches <varname>RootImage=</varname>/<varname>RootDirectory=</varname>
        or the host. See:
        <citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>

        <para>Note that usage from user units requires overlayfs support in unprivileged user namespaces,
        which was first introduced in kernel v5.11.</para>

        <xi:include href="system-or-user-ns.xml" xpointer="singular"/>

        <xi:include href="version-info.xml" xpointer="v251"/></listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>User/Group Identity</title>

    <xi:include href="system-only.xml" xpointer="plural"/>

    <variablelist class='unit-directives'>

      <varlistentry>
        <term><varname>User=</varname></term>
        <term><varname>Group=</varname></term>

        <listitem><para>Set the UNIX user or group that the processes are executed as, respectively. Takes a single
        user or group name, or a numeric ID as argument. For system services (services run by the system service
        manager, i.e. managed by PID 1) and for user services of the root user (services managed by root's instance of
        <command>systemd --user</command>), the default is <literal>root</literal>, but <varname>User=</varname> may be
        used to specify a different user. For user services of any other user, switching user identity is not
        permitted, hence the only valid setting is the same user the user's service manager is running as. If no group
        is set, the default group of the user is used. This setting does not affect commands whose command line is
        prefixed with <literal>+</literal>.</para>

        <para>Note that this enforces only weak restrictions on the user/group name syntax, but will generate
        warnings in many cases where user/group names do not adhere to the following rules: the specified
        name should consist only of the characters a-z, A-Z, 0-9, <literal>_</literal> and
        <literal>-</literal>, except for the first character which must be one of a-z, A-Z and
        <literal>_</literal> (i.e. digits and <literal>-</literal> are not permitted as first character). The
        user/group name must have at least one character, and at most 31. These restrictions are made in
        order to avoid ambiguities and to ensure user/group names and unit files remain portable among Linux
        systems. For further details on the names accepted and the names warned about see <ulink
        url="https://systemd.io/USER_NAMES">User/Group Name Syntax</ulink>.</para>

        <para>When used in conjunction with <varname>DynamicUser=</varname> the user/group name specified is
        dynamically allocated at the time the service is started, and released at the time the service is
        stopped — unless it is already allocated statically (see below). If <varname>DynamicUser=</varname>
        is not used the specified user and group must have been created statically in the user database no
        later than the moment the service is started, for example using the
        <citerefentry><refentrytitle>sysusers.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>
        facility, which is applied at boot or package install time. If the user does not exist by then
        program invocation will fail.</para>

        <para>If the <varname>User=</varname> setting is used the supplementary group list is initialized
        from the specified user's default group list, as defined in the system's user and group
        database. Additional groups may be configured through the <varname>SupplementaryGroups=</varname>
        setting (see below).</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>DynamicUser=</varname></term>

        <listitem><para>Takes a boolean parameter. If set, a UNIX user and group pair is allocated
        dynamically when the unit is started, and released as soon as it is stopped. The user and group will
        not be added to <filename>/etc/passwd</filename> or <filename>/etc/group</filename>, but are managed
        transiently during runtime. The
        <citerefentry><refentrytitle>nss-systemd</refentrytitle><manvolnum>8</manvolnum></citerefentry> glibc
        NSS module provides integration of these dynamic users/groups into the system's user and group
        databases. The user and group name to use may be configured via <varname>User=</varname> and
        <varname>Group=</varname> (see above). If these options are not used and dynamic user/group
        allocation is enabled for a unit, the name of the dynamic user/group is implicitly derived from the
        unit name. If the unit name without the type suffix qualifies as valid user name it is used directly,
        otherwise a name incorporating a hash of it is used. If a statically allocated user or group of the
        configured name already exists, it is used and no dynamic user/group is allocated. Note that if
        <varname>User=</varname> is specified and the static group with the name exists, then it is required
        that the static user with the name already exists. Similarly, if <varname>Group=</varname> is
        specified and the static user with the name exists, then it is required that the static group with
        the name already exists. Dynamic users/groups are allocated from the UID/GID range 61184…65519. It is
        recommended to avoid this range for regular system or login users. At any point in time each UID/GID
        from this range is only assigned to zero or one dynamically allocated users/groups in use. However,
        UID/GIDs are recycled after a unit is terminated. Care should be taken that any processes running as
        part of a unit for which dynamic users/groups are enabled do not leave files or directories owned by
        these users/groups around, as a different unit might get the same UID/GID assigned later on, and thus
        gain access to these files or directories. If <varname>DynamicUser=</varname> is enabled,
        <varname>RemoveIPC=</varname> and <varname>PrivateTmp=</varname> are implied (and cannot be turned
        off). This ensures that the lifetime of IPC objects and temporary files created by the executed
        processes is bound to the runtime of the service, and hence the lifetime of the dynamic
        user/group. Since <filename>/tmp/</filename> and <filename>/var/tmp/</filename> are usually the only
        world-writable directories on a system this ensures that a unit making use of dynamic user/group
        allocation cannot leave files around after unit termination. Furthermore
        <varname>NoNewPrivileges=</varname> and <varname>RestrictSUIDSGID=</varname> are implicitly enabled
        (and cannot be disabled), to ensure that processes invoked cannot take benefit or create SUID/SGID
        files or directories. Moreover <varname>ProtectSystem=strict</varname> and
        <varname>ProtectHome=read-only</varname> are implied, thus prohibiting the service to write to
        arbitrary file system locations. In order to allow the service to write to certain directories, they
        have to be allow-listed using <varname>ReadWritePaths=</varname>, but care must be taken so that
        UID/GID recycling doesn't create security issues involving files created by the service. Use
        <varname>RuntimeDirectory=</varname> (see below) in order to assign a writable runtime directory to a
        service, owned by the dynamic user/group and removed automatically when the unit is terminated. Use
        <varname>StateDirectory=</varname>, <varname>CacheDirectory=</varname> and
        <varname>LogsDirectory=</varname> in order to assign a set of writable directories for specific
        purposes to the service in a way that they are protected from vulnerabilities due to UID reuse (see
        below). If this option is enabled, care should be taken that the unit's processes do not get access
        to directories outside of these explicitly configured and managed ones. Specifically, do not use
        <varname>BindPaths=</varname> and be careful with <constant>AF_UNIX</constant> file descriptor
        passing for directory file descriptors, as this would permit processes to create files or directories
        owned by the dynamic user/group that are not subject to the lifecycle and access guarantees of the
        service. Note that this option is currently incompatible with D-Bus policies, thus a service using
        this option may currently not allocate a D-Bus service name (note that this does not affect calling
        into other D-Bus services). Defaults to off.</para>

        <xi:include href="version-info.xml" xpointer="v232"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>SupplementaryGroups=</varname></term>

        <listitem><para>Sets the supplementary Unix groups the processes are executed as. This takes a space-separated
        list of group names or IDs. This option may be specified more than once, in which case all listed groups are
        set as supplementary groups. When the empty string is assigned, the list of supplementary groups is reset, and
        all assignments prior to this one will have no effect. In any way, this option does not override, but extends
        the list of supplementary groups configured in the system group database for the user. This does not affect
        commands prefixed with <literal>+</literal>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>PAMName=</varname></term>

        <listitem><para>Sets the PAM service name to set up a session as. If set, the executed process will be
        registered as a PAM session under the specified service name. This is only useful in conjunction with the
        <varname>User=</varname> setting, and is otherwise ignored. If not set, no PAM session will be opened for the
        executed processes. See <citerefentry
        project='man-pages'><refentrytitle>pam</refentrytitle><manvolnum>8</manvolnum></citerefentry> for
        details.</para>

        <para>Note that for each unit making use of this option a PAM session handler process will be maintained as
        part of the unit and stays around as long as the unit is active, to ensure that appropriate actions can be
        taken when the unit and hence the PAM session terminates. This process is named <literal>(sd-pam)</literal> and
        is an immediate child process of the unit's main process.</para>

        <para>Note that when this option is used for a unit it is very likely (depending on PAM configuration) that the
        main unit process will be migrated to its own session scope unit when it is activated. This process will hence
        be associated with two units: the unit it was originally started from (and for which
        <varname>PAMName=</varname> was configured), and the session scope unit. Any child processes of that process
        will however be associated with the session scope unit only. This has implications when used in combination
        with <varname>NotifyAccess=</varname><option>all</option>, as these child processes will not be able to affect
        changes in the original unit through notification messages. These messages will be considered belonging to the
        session scope unit and not the original unit. It is hence not recommended to use <varname>PAMName=</varname> in
        combination with <varname>NotifyAccess=</varname><option>all</option>.</para>
        </listitem>
      </varlistentry>

    </variablelist>
  </refsect1>

  <refsect1>
    <title>Capabilities</title>

    <xi:include href="system-or-user-ns.xml" xpointer="plural"/>

    <variablelist class='unit-directives'>

      <varlistentry>
        <term><varname>CapabilityBoundingSet=</varname></term>

        <listitem><para>Controls which capabilities to include in the capability bounding set for the
        executed process. See <citerefentry
        project='man-pages'><refentrytitle>capabilities</refentrytitle><manvolnum>7</manvolnum></citerefentry>
        for details. Takes a whitespace-separated list of capability names,
        e.g. <constant>CAP_SYS_ADMIN</constant>, <constant>CAP_DAC_OVERRIDE</constant>,
        <constant>CAP_SYS_PTRACE</constant>. Capabilities listed will be included in the bounding set, all
        others are removed. If the list of capabilities is prefixed with <literal>~</literal>, all but the
        listed capabilities will be included, the effect of the assignment inverted. Note that this option
        also affects the respective capabilities in the effective, permitted and inheritable capability
        sets. If this option is not used, the capability bounding set is not modified on process execution,
        hence no limits on the capabilities of the process are enforced. This option may appear more than
        once, in which case the bounding sets are merged by <constant>OR</constant>, or by
        <constant>AND</constant> if the lines are prefixed with <literal>~</literal> (see below). If the
        empty string is assigned to this option, the bounding set is reset to the empty capability set, and
        all prior settings have no effect. If set to <literal>~</literal> (without any further argument),
        the bounding set is reset to the full set of available capabilities, also undoing any previous
        settings. This does not affect commands prefixed with <literal>+</literal>.</para>

        <para>Use
        <citerefentry><refentrytitle>systemd-analyze</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
        <command>capability</command> command to retrieve a list of capabilities defined on the local
        system.</para>

        <para>Example: if a unit has the following,
        <programlisting>CapabilityBoundingSet=CAP_A CAP_B
CapabilityBoundingSet=CAP_B CAP_C</programlisting>
        then <constant index='false'>CAP_A</constant>, <constant index='false'>CAP_B</constant>, and
        <constant index='false'>CAP_C</constant> are set. If the second line is prefixed with
        <literal>~</literal>, e.g.,
        <programlisting>CapabilityBoundingSet=CAP_A CAP_B
CapabilityBoundingSet=~CAP_B CAP_C</programlisting>
        then, only <constant index='false'>CAP_A</constant> is set.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>AmbientCapabilities=</varname></term>

        <listitem><para>Controls which capabilities to include in the ambient capability set for the executed
        process. Takes a whitespace-separated list of capability names, e.g. <constant>CAP_SYS_ADMIN</constant>,
        <constant>CAP_DAC_OVERRIDE</constant>, <constant>CAP_SYS_PTRACE</constant>. This option may appear more than
        once, in which case the ambient capability sets are merged (see the above examples in
        <varname>CapabilityBoundingSet=</varname>). If the list of capabilities is prefixed with <literal>~</literal>,
        all but the listed capabilities will be included, the effect of the assignment inverted. If the empty string is
        assigned to this option, the ambient capability set is reset to the empty capability set, and all prior
        settings have no effect. If set to <literal>~</literal> (without any further argument), the ambient capability
        set is reset to the full set of available capabilities, also undoing any previous settings. Note that adding
        capabilities to the ambient capability set adds them to the process's inherited capability set. </para><para>
        Ambient capability sets are useful if you want to execute a process as a non-privileged user but still want to
        give it some capabilities. Note that in this case option <constant>keep-caps</constant> is automatically added
        to <varname>SecureBits=</varname> to retain the capabilities over the user
        change. <varname>AmbientCapabilities=</varname> does not affect commands prefixed with
        <literal>+</literal>.</para>

        <xi:include href="version-info.xml" xpointer="v229"/></listitem>
      </varlistentry>

    </variablelist>
  </refsect1>

  <refsect1>
    <title>Security</title>

    <variablelist class='unit-directives'>

      <varlistentry>
        <term><varname>NoNewPrivileges=</varname></term>

        <listitem><para>Takes a boolean argument. If true, ensures that the service process and all its
        children can never gain new privileges through <function>execve()</function> (e.g. via setuid or
        setgid bits, or filesystem capabilities). This is the simplest and most effective way to ensure that
        a process and its children can never elevate privileges again. Defaults to false, but certain
        settings override this and ignore the value of this setting. This is the case when
        <varname>DynamicUser=</varname>, <varname>LockPersonality=</varname>,
        <varname>MemoryDenyWriteExecute=</varname>, <varname>PrivateDevices=</varname>,
        <varname>ProtectClock=</varname>, <varname>ProtectHostname=</varname>,
        <varname>ProtectKernelLogs=</varname>, <varname>ProtectKernelModules=</varname>,
        <varname>ProtectKernelTunables=</varname>, <varname>RestrictAddressFamilies=</varname>,
        <varname>RestrictNamespaces=</varname>, <varname>RestrictRealtime=</varname>,
        <varname>RestrictSUIDSGID=</varname>, <varname>SystemCallArchitectures=</varname>,
        <varname>SystemCallFilter=</varname>, or <varname>SystemCallLog=</varname> are specified. Note that
        even if this setting is overridden by them, <command>systemctl show</command> shows the original
        value of this setting. In case the service will be run in a new mount namespace anyway and SELinux is
        disabled, all file systems are mounted with <constant>MS_NOSUID</constant> flag. Also see <ulink
        url="https://docs.kernel.org/userspace-api/no_new_privs.html">No New Privileges
        Flag</ulink>.</para>

        <para>Note that this setting only has an effect on the unit's processes themselves (or any processes
        directly or indirectly forked off them). It has no effect on processes potentially invoked on request
        of them through tools such as <citerefentry
        project='man-pages'><refentrytitle>at</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
        <citerefentry
        project='man-pages'><refentrytitle>crontab</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>systemd-run</refentrytitle><manvolnum>1</manvolnum></citerefentry>, or
        arbitrary IPC services.</para>

        <xi:include href="version-info.xml" xpointer="v187"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>SecureBits=</varname></term>

        <listitem><para>Controls the secure bits set for the executed process. Takes a space-separated combination of
        options from the following list: <option>keep-caps</option>, <option>keep-caps-locked</option>,
        <option>no-setuid-fixup</option>, <option>no-setuid-fixup-locked</option>, <option>noroot</option>, and
        <option>noroot-locked</option>. This option may appear more than once, in which case the secure bits are
        ORed. If the empty string is assigned to this option, the bits are reset to 0. This does not affect commands
        prefixed with <literal>+</literal>. See <citerefentry
        project='man-pages'><refentrytitle>capabilities</refentrytitle><manvolnum>7</manvolnum></citerefentry> for
        details.</para></listitem>
      </varlistentry>

    </variablelist>
  </refsect1>

  <refsect1>
    <title>Mandatory Access Control</title>

    <xi:include href="system-only.xml" xpointer="plural"/>

    <variablelist class='unit-directives'>

      <varlistentry>
        <term><varname>SELinuxContext=</varname></term>

        <listitem><para>Set the SELinux security context of the executed process. If set, this will override the
        automated domain transition. However, the policy still needs to authorize the transition. This directive is
        ignored if SELinux is disabled. If prefixed by <literal>-</literal>, failing to set the SELinux
        security context will be ignored, but it's still possible that the subsequent
        <function>execve()</function> may fail if the policy doesn't allow the transition for the
        non-overridden context. This does not affect commands prefixed with <literal>+</literal>. See
        <citerefentry
        project='die-net'><refentrytitle>setexeccon</refentrytitle><manvolnum>3</manvolnum></citerefentry>
        for details.</para>

        <xi:include href="version-info.xml" xpointer="v209"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>AppArmorProfile=</varname></term>

        <listitem><para>Takes a profile name as argument. The process executed by the unit will switch to
        this profile when started. Profiles must already be loaded in the kernel, or the unit will fail. If
        prefixed by <literal>-</literal>, all errors will be ignored. This setting has no effect if AppArmor
        is not enabled. This setting does not affect commands prefixed with <literal>+</literal>.</para>

        <xi:include href="version-info.xml" xpointer="v210"/>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>SmackProcessLabel=</varname></term>

        <listitem><para>Takes a <option>SMACK64</option> security label as argument. The process executed by the unit
        will be started under this label and SMACK will decide whether the process is allowed to run or not, based on
        it. The process will continue to run under the label specified here unless the executable has its own
        <option>SMACK64EXEC</option> label, in which case the process will transition to run under that label. When not
        specified, the label that systemd is running under is used. This directive is ignored if SMACK is
        disabled.</para>

        <para>The value may be prefixed by <literal>-</literal>, in which case all errors will be ignored. An empty
        value may be specified to unset previous assignments. This does not affect commands prefixed with
        <literal>+</literal>.</para>

        <xi:include href="version-info.xml" xpointer="v218"/></listitem>
      </varlistentry>

    </variablelist>
  </refsect1>

  <refsect1>
    <title>Process Properties</title>

    <variablelist class='unit-directives'>

      <varlistentry>
        <term><varname>LimitCPU=</varname></term>
        <term><varname>LimitFSIZE=</varname></term>
        <term><varname>LimitDATA=</varname></term>
        <term><varname>LimitSTACK=</varname></term>
        <term><varname>LimitCORE=</varname></term>
        <term><varname>LimitRSS=</varname></term>
        <term><varname>LimitNOFILE=</varname></term>
        <term><varname>LimitAS=</varname></term>
        <term><varname>LimitNPROC=</varname></term>
        <term><varname>LimitMEMLOCK=</varname></term>
        <term><varname>LimitLOCKS=</varname></term>
        <term><varname>LimitSIGPENDING=</varname></term>
        <term><varname>LimitMSGQUEUE=</varname></term>
        <term><varname>LimitNICE=</varname></term>
        <term><varname>LimitRTPRIO=</varname></term>
        <term><varname>LimitRTTIME=</varname></term>

        <listitem><para>Set soft and hard limits on various resources for executed processes. See
        <citerefentry><refentrytitle>setrlimit</refentrytitle><manvolnum>2</manvolnum></citerefentry> for
        details on the process resource limit concept. Process resource limits may be specified in two formats:
        either as single value to set a specific soft and hard limit to the same value, or as colon-separated
        pair <option>soft:hard</option> to set both limits individually
        (e.g. <literal>LimitAS=4G:16G</literal>).  Use the string <option>infinity</option> to configure no
        limit on a specific resource. The multiplicative suffixes K, M, G, T, P and E (to the base 1024) may
        be used for resource limits measured in bytes (e.g. <literal>LimitAS=16G</literal>). For the limits
        referring to time values, the usual time units ms, s, min, h and so on may be used (see
        <citerefentry><refentrytitle>systemd.time</refentrytitle><manvolnum>7</manvolnum></citerefentry> for
        details). Note that if no time unit is specified for <varname>LimitCPU=</varname> the default unit of
        seconds is implied, while for <varname>LimitRTTIME=</varname> the default unit of microseconds is
        implied. Also, note that the effective granularity of the limits might influence their
        enforcement. For example, time limits specified for <varname>LimitCPU=</varname> will be rounded up
        implicitly to multiples of 1s. For <varname>LimitNICE=</varname> the value may be specified in two
        syntaxes: if prefixed with <literal>+</literal> or <literal>-</literal>, the value is understood as
        regular Linux nice value in the range -20…19. If not prefixed like this the value is understood as
        raw resource limit parameter in the range 0…40 (with 0 being equivalent to 1).</para>

        <para>Note that most process resource limits configured with these options are per-process, and
        processes may fork in order to acquire a new set of resources that are accounted independently of the
        original process, and may thus escape limits set. Also note that <varname>LimitRSS=</varname> is not
        implemented on Linux, and setting it has no effect. Often it is advisable to prefer the resource
        controls listed in
        <citerefentry><refentrytitle>systemd.resource-control</refentrytitle><manvolnum>5</manvolnum></citerefentry>
        over these per-process limits, as they apply to services as a whole, may be altered dynamically at
        runtime, and are generally more expressive. For example, <varname>MemoryMax=</varname> is a more
        powerful (and working) replacement for <varname>LimitRSS=</varname>.</para>

        <para>Note that <varname>LimitNPROC=</varname> will limit the number of processes from one (real) UID and
        not the number of processes started (forked) by the service. Therefore the limit is cumulative for all
        processes running under the same UID. Please also note that the <varname>LimitNPROC=</varname> will not be
        enforced if the service is running as root (and not dropping privileges). Due to these limitations,
        <varname>TasksMax=</varname> (see <citerefentry><refentrytitle>systemd.resource-control</refentrytitle>
        <manvolnum>5</manvolnum></citerefentry>) is typically a better choice than <varname>LimitNPROC=</varname>.
        </para>

        <para>Resource limits not configured explicitly for a unit default to the value configured in the various
        <varname>DefaultLimitCPU=</varname>, <varname>DefaultLimitFSIZE=</varname>, … options available in
        <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>, and –
        if not configured there – the kernel or per-user defaults, as defined by the OS (the latter only for user
        services, see below).</para>

        <para>For system units these resource limits may be chosen freely. When these settings are configured
        in a user service (i.e. a service run by the per-user instance of the service manager) they cannot be
        used to raise the limits above those set for the user manager itself when it was first invoked, as
        the user's service manager generally lacks the privileges to do so. In user context these
        configuration options are hence only useful to lower the limits passed in or to raise the soft limit
        to the maximum of the hard limit as configured for the user. To raise the user's limits further, the
        available configuration mechanisms differ between operating systems, but typically require
        privileges. In most cases it is possible to configure higher per-user resource limits via PAM or by
        setting limits on the system service encapsulating the user's service manager, i.e. the user's
        instance of <filename>user@.service</filename>. After making such changes, make sure to restart the
        user's service manager.</para>

        <table>
          <title>Resource limit directives, their equivalent <command>ulimit</command> shell commands and the unit used</title>

          <tgroup cols='4'>
            <colspec colname='directive' />
            <colspec colname='equivalent' />
            <colspec colname='unit' />
            <colspec colname='notes' />
            <thead>
              <row>
                <entry>Directive</entry>
                <entry><command>ulimit</command> equivalent</entry>
                <entry>Unit</entry>
                <entry>Notes</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry>LimitCPU=</entry>
                <entry>ulimit -t</entry>
                <entry>Seconds</entry>
                <entry>-</entry>
              </row>
              <row>
                <entry>LimitFSIZE=</entry>
                <entry>ulimit -f</entry>
                <entry>Bytes</entry>
                <entry>-</entry>
              </row>
              <row>
                <entry>LimitDATA=</entry>
                <entry>ulimit -d</entry>
                <entry>Bytes</entry>
                <entry>Don't use. This limits the allowed address range, not memory use! Defaults to unlimited and should not be lowered. To limit memory use, see <varname>MemoryMax=</varname> in <citerefentry><refentrytitle>systemd.resource-control</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</entry>
              </row>
              <row>
                <entry>LimitSTACK=</entry>
                <entry>ulimit -s</entry>
                <entry>Bytes</entry>
                <entry>-</entry>
              </row>
              <row>
                <entry>LimitCORE=</entry>
                <entry>ulimit -c</entry>
                <entry>Bytes</entry>
                <entry>-</entry>
              </row>
              <row>
                <entry>LimitRSS=</entry>
                <entry>ulimit -m</entry>
                <entry>Bytes</entry>
                <entry>Don't use. No effect on Linux.</entry>
              </row>
              <row>
                <entry>LimitNOFILE=</entry>
                <entry>ulimit -n</entry>
                <entry>Number of File Descriptors</entry>
                <entry>Don't use. Be careful when raising the soft limit above 1024, since <citerefentry project='man-pages'><refentrytitle>select</refentrytitle><manvolnum>2</manvolnum></citerefentry> cannot function with file descriptors above 1023 on Linux. Nowadays, the hard limit defaults to 524288, a very high value compared to historical defaults. Typically applications should increase their soft limit to the hard limit on their own, if they are OK with working with file descriptors above 1023, i.e. do not use <citerefentry project='man-pages'><refentrytitle>select</refentrytitle><manvolnum>2</manvolnum></citerefentry>. Note that file descriptors are nowadays accounted like any other form of memory, thus there should not be any need to lower the hard limit. Use <varname>MemoryMax=</varname> to control overall service memory use, including file descriptor memory.</entry>
              </row>
              <row>
                <entry>LimitAS=</entry>
                <entry>ulimit -v</entry>
                <entry>Bytes</entry>
                <entry>Don't use. This limits the allowed address range, not memory use! Defaults to unlimited and should not be lowered. To limit memory use, see <varname>MemoryMax=</varname> in <citerefentry><refentrytitle>systemd.resource-control</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</entry>
              </row>
              <row>
                <entry>LimitNPROC=</entry>
                <entry>ulimit -u</entry>
                <entry>Number of Processes</entry>
                <entry>This limit is enforced based on the number of processes belonging to the user. Typically it's better to track processes per service, i.e. use <varname>TasksMax=</varname>, see <citerefentry><refentrytitle>systemd.resource-control</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</entry>
              </row>
              <row>
                <entry>LimitMEMLOCK=</entry>
                <entry>ulimit -l</entry>
                <entry>Bytes</entry>
                <entry>-</entry>
              </row>
              <row>
                <entry>LimitLOCKS=</entry>
                <entry>ulimit -x</entry>
                <entry>Number of Locks</entry>
                <entry>-</entry>
              </row>
              <row>
                <entry>LimitSIGPENDING=</entry>
                <entry>ulimit -i</entry>
                <entry>Number of Queued Signals</entry>
                <entry>-</entry>
              </row>
              <row>
                <entry>LimitMSGQUEUE=</entry>
                <entry>ulimit -q</entry>
                <entry>Bytes</entry>
                <entry>-</entry>
              </row>
              <row>
                <entry>LimitNICE=</entry>
                <entry>ulimit -e</entry>
                <entry>Nice Level</entry>
                <entry>-</entry>
              </row>
              <row>
                <entry>LimitRTPRIO=</entry>
                <entry>ulimit -r</entry>
                <entry>Realtime Priority</entry>
                <entry>-</entry>
              </row>
              <row>
                <entry>LimitRTTIME=</entry>
                <entry>ulimit -R</entry>
                <entry>Microseconds</entry>
                <entry>-</entry>
              </row>
            </tbody>
          </tgroup>
        </table></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>UMask=</varname></term>

        <listitem><para>Controls the file mode creation mask. Takes an access mode in octal notation. See
        <citerefentry><refentrytitle>umask</refentrytitle><manvolnum>2</manvolnum></citerefentry> for
        details. Defaults to 0022 for system units. For user units the default value is inherited from the
        per-user service manager (whose default is in turn inherited from the system service manager, and
        thus typically also is 0022 — unless overridden by a PAM module). In order to change the per-user mask
        for all user services, consider setting the <varname>UMask=</varname> setting of the user's
        <filename>user@.service</filename> system service instance. The per-user umask may also be set via
        the <varname>umask</varname> field of a user's <ulink url="https://systemd.io/USER_RECORD">JSON User
        Record</ulink> (for users managed by
        <citerefentry><refentrytitle>systemd-homed.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
        this field may be controlled via <command>homectl --umask=</command>). It may also be set via a PAM
        module, such as <citerefentry
        project='man-pages'><refentrytitle>pam_umask</refentrytitle><manvolnum>8</manvolnum></citerefentry>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>CoredumpFilter=</varname></term>

        <listitem><para>Controls which types of memory mappings will be saved if the process dumps core
        (using the <filename>/proc/<replaceable>pid</replaceable>/coredump_filter</filename> file). Takes a
        whitespace-separated combination of mapping type names or numbers (with the default base 16). Mapping
        type names are <constant>private-anonymous</constant>, <constant>shared-anonymous</constant>,
        <constant>private-file-backed</constant>, <constant>shared-file-backed</constant>,
        <constant>elf-headers</constant>, <constant>private-huge</constant>,
        <constant>shared-huge</constant>, <constant>private-dax</constant>, <constant>shared-dax</constant>,
        and the special values <constant>all</constant> (all types) and <constant>default</constant> (the
        kernel default of <literal><constant>private-anonymous</constant>
        <constant>shared-anonymous</constant> <constant>elf-headers</constant>
        <constant>private-huge</constant></literal>). See
        <citerefentry project='man-pages'><refentrytitle>core</refentrytitle><manvolnum>5</manvolnum></citerefentry>
        for the meaning of the mapping types. When specified multiple times, all specified masks are
        ORed. When not set, or if the empty value is assigned, the inherited value is not changed.</para>

        <example>
          <title>Add DAX pages to the dump filter</title>

          <programlisting>CoredumpFilter=default private-dax shared-dax</programlisting>
        </example>

          <xi:include href="version-info.xml" xpointer="v246"/>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>KeyringMode=</varname></term>

        <listitem><para>Controls how the kernel session keyring is set up for the service (see <citerefentry
        project='man-pages'><refentrytitle>session-keyring</refentrytitle><manvolnum>7</manvolnum></citerefentry> for
        details on the session keyring). Takes one of <option>inherit</option>, <option>private</option>,
        <option>shared</option>. If set to <option>inherit</option> no special keyring setup is done, and the kernel's
        default behaviour is applied. If <option>private</option> is used a new session keyring is allocated when a
        service process is invoked, and it is not linked up with any user keyring. This is the recommended setting for
        system services, as this ensures that multiple services running under the same system user ID (in particular
        the root user) do not share their key material among each other. If <option>shared</option> is used a new
        session keyring is allocated as for <option>private</option>, but the user keyring of the user configured with
        <varname>User=</varname> is linked into it, so that keys assigned to the user may be requested by the unit's
        processes. In this mode multiple units running processes under the same user ID may share key material. Unless
        <option>inherit</option> is selected the unique invocation ID for the unit (see below) is added as a protected
        key by the name <literal>invocation_id</literal> to the newly created session keyring. Defaults to
        <option>private</option> for services of the system service manager and to <option>inherit</option> for
        non-service units and for services of the user service manager.</para>

        <xi:include href="version-info.xml" xpointer="v235"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>OOMScoreAdjust=</varname></term>

        <listitem><para>Sets the adjustment value for the Linux kernel's Out-Of-Memory (OOM) killer score for
        executed processes. Takes an integer between -1000 (to disable OOM killing of processes of this unit)
        and 1000 (to make killing of processes of this unit under memory pressure very likely). See <ulink
        url="https://docs.kernel.org/filesystems/proc.html">The /proc Filesystem</ulink> for
        details. If not specified defaults to the OOM score adjustment level of the service manager itself,
        which is normally at 0.</para>

        <para>Use the <varname>OOMPolicy=</varname> setting of service units to configure how the service
        manager shall react to the kernel OOM killer or <command>systemd-oomd</command> terminating a process of the service. See
        <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
        for details.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>TimerSlackNSec=</varname></term>
        <listitem><para>Sets the timer slack in nanoseconds for the executed processes. The timer slack controls the
        accuracy of wake-ups triggered by timers. See
        <citerefentry><refentrytitle>prctl</refentrytitle><manvolnum>2</manvolnum></citerefentry> for more
        information. Note that in contrast to most other time span definitions this parameter takes an integer value in
        nano-seconds if no unit is specified. The usual time units are understood too.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>Personality=</varname></term>

        <listitem><para>Controls which kernel architecture <citerefentry
        project='man-pages'><refentrytitle>uname</refentrytitle><manvolnum>2</manvolnum></citerefentry> shall
        report, when invoked by unit processes. Takes one of the architecture identifiers
        <constant>arm64</constant>, <constant>arm64-be</constant>, <constant>arm</constant>,
        <constant>arm-be</constant>, <constant>x86</constant>, <constant>x86-64</constant>,
        <constant>ppc</constant>, <constant>ppc-le</constant>, <constant>ppc64</constant>,
        <constant>ppc64-le</constant>, <constant>s390</constant> or <constant>s390x</constant>. Which
        personality architectures are supported depends on the kernel's native architecture. Usually the
        64-bit versions of the various system architectures support their immediate 32-bit personality
        architecture counterpart, but no others. For example, <constant>x86-64</constant> systems support the
        <constant>x86-64</constant> and <constant>x86</constant> personalities but no others. The personality
        feature is useful when running 32-bit services on a 64-bit host system. If not specified, the
        personality is left unmodified and thus reflects the personality of the host system's kernel. This
        option is not useful on architectures for which only one native word width was ever available, such
        as <constant>m68k</constant> (32-bit only) or <constant>alpha</constant> (64-bit only).</para>

        <xi:include href="version-info.xml" xpointer="v209"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>IgnoreSIGPIPE=</varname></term>

        <listitem><para>Takes a boolean argument. If true, causes <constant>SIGPIPE</constant> to be ignored in the
        executed process. Defaults to true because <constant>SIGPIPE</constant> generally is useful only in shell
        pipelines.</para></listitem>
      </varlistentry>

    </variablelist>
  </refsect1>

  <refsect1>
    <title>Scheduling</title>

    <variablelist class='unit-directives'>

      <varlistentry>
        <term><varname>Nice=</varname></term>

        <listitem><para>Sets the default nice level (scheduling priority) for executed processes. Takes an
        integer between -20 (highest priority) and 19 (lowest priority). In case of resource contention,
        smaller values mean more resources will be made available to the unit's processes, larger values mean
        less resources will be made available. See
        <citerefentry><refentrytitle>setpriority</refentrytitle><manvolnum>2</manvolnum></citerefentry> for
        details.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>CPUSchedulingPolicy=</varname></term>

        <listitem><para>Sets the CPU scheduling policy for executed processes. Takes one of <option>other</option>,
        <option>batch</option>, <option>idle</option>, <option>fifo</option> or <option>rr</option>. See
        <citerefentry project='man-pages'><refentrytitle>sched_setscheduler</refentrytitle><manvolnum>2</manvolnum></citerefentry> for
        details.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>CPUSchedulingPriority=</varname></term>

        <listitem><para>Sets the CPU scheduling priority for executed processes. The available priority range
        depends on the selected CPU scheduling policy (see above). For real-time scheduling policies an
        integer between 1 (lowest priority) and 99 (highest priority) can be used. In case of CPU resource
        contention, smaller values mean less CPU time is made available to the service, larger values mean
        more. See <citerefentry
        project='man-pages'><refentrytitle>sched_setscheduler</refentrytitle><manvolnum>2</manvolnum></citerefentry>
        for details. </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>CPUSchedulingResetOnFork=</varname></term>

        <listitem><para>Takes a boolean argument. If true, elevated CPU scheduling priorities and policies
        will be reset when the executed processes call
        <citerefentry project='man-pages'><refentrytitle>fork</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
        and can hence not leak into child processes. See
        <citerefentry project='man-pages'><refentrytitle>sched_setscheduler</refentrytitle><manvolnum>2</manvolnum></citerefentry>
        for details. Defaults to false.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>CPUAffinity=</varname></term>

        <listitem><para>Controls the CPU affinity of the executed processes. Takes a list of CPU indices or ranges
        separated by either whitespace or commas. Alternatively, takes a special "numa" value in which case systemd
        automatically derives allowed CPU range based on the value of <varname>NUMAMask=</varname> option. CPU ranges
        are specified by the lower and upper CPU indices separated by a dash. This option may be specified more than
        once, in which case the specified CPU affinity masks are merged. If the empty string is assigned, the mask
        is reset, all assignments prior to this will have no effect. See
        <citerefentry project='man-pages'><refentrytitle>sched_setaffinity</refentrytitle><manvolnum>2</manvolnum></citerefentry> for
        details.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>NUMAPolicy=</varname></term>

        <listitem><para>Controls the NUMA memory policy of the executed processes. Takes a policy type, one of:
        <option>default</option>, <option>preferred</option>, <option>bind</option>, <option>interleave</option> and
        <option>local</option>. A list of NUMA nodes that should be associated with the policy must be specified
        in <varname>NUMAMask=</varname>. For more details on each policy please see,
        <citerefentry><refentrytitle>set_mempolicy</refentrytitle><manvolnum>2</manvolnum></citerefentry>. For overall
        overview of NUMA support in Linux see,
        <citerefentry project='man-pages'><refentrytitle>numa</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
        </para>

        <xi:include href="version-info.xml" xpointer="v243"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>NUMAMask=</varname></term>

        <listitem><para>Controls the NUMA node list which will be applied alongside with selected NUMA policy.
        Takes a list of NUMA nodes and has the same syntax as a list of CPUs for <varname>CPUAffinity=</varname>
        option or special "all" value which will include all available NUMA nodes in the mask. Note that the list
        of NUMA nodes is not required for <option>default</option> and <option>local</option>
        policies and for <option>preferred</option> policy we expect a single NUMA node.</para>

        <xi:include href="version-info.xml" xpointer="v243"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>IOSchedulingClass=</varname></term>

        <listitem><para>Sets the I/O scheduling class for executed processes. Takes one of the strings
        <option>realtime</option>, <option>best-effort</option> or <option>idle</option>. The kernel's
        default scheduling class is <option>best-effort</option> at a priority of 4. If the empty string is
        assigned to this option, all prior assignments to both <varname>IOSchedulingClass=</varname> and
        <varname>IOSchedulingPriority=</varname> have no effect. See
        <citerefentry><refentrytitle>ioprio_set</refentrytitle><manvolnum>2</manvolnum></citerefentry> for
        details.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>IOSchedulingPriority=</varname></term>

        <listitem><para>Sets the I/O scheduling priority for executed processes. Takes an integer between 0
        (highest priority) and 7 (lowest priority). In case of I/O contention, smaller values mean more I/O
        bandwidth is made available to the unit's processes, larger values mean less bandwidth. The available
        priorities depend on the selected I/O scheduling class (see above). If the empty string is assigned
        to this option, all prior assignments to both <varname>IOSchedulingClass=</varname> and
        <varname>IOSchedulingPriority=</varname> have no effect. For the kernel's default scheduling class
        (<option>best-effort</option>) this defaults to 4. See
        <citerefentry><refentrytitle>ioprio_set</refentrytitle><manvolnum>2</manvolnum></citerefentry> for
        details.</para></listitem>
      </varlistentry>

    </variablelist>
  </refsect1>

  <refsect1>
    <title>Sandboxing</title>

    <para>The following sandboxing options are an effective way to limit the exposure of the system towards the unit's
    processes. It is recommended to turn on as many of these options for each unit as is possible without negatively
    affecting the process' ability to operate. Note that many of these sandboxing features are gracefully turned off on
    systems where the underlying security mechanism is not available. For example, <varname>ProtectSystem=</varname>
    has no effect if the kernel is built without file system namespacing or if the service manager runs in a container
    manager that makes file system namespacing unavailable to its payload. Similarly,
    <varname>RestrictRealtime=</varname> has no effect on systems that lack support for SECCOMP system call filtering,
    or in containers where support for this is turned off.</para>

    <para>Also note that some sandboxing functionality is generally not available in user services (i.e. services run
    by the per-user service manager). Specifically, the various settings requiring file system namespacing support
    (such as <varname>ProtectSystem=</varname>) are not available, as the underlying kernel functionality is only
    accessible to privileged processes. However, most namespacing settings, that will not work on their own in user
    services, will work when used in conjunction with <varname>PrivateUsers=</varname><option>true</option>.</para>

    <variablelist class='unit-directives'>

      <varlistentry>
        <term><varname>ProtectSystem=</varname></term>

        <listitem><para>Takes a boolean argument or the special values <literal>full</literal> or
        <literal>strict</literal>. If true, mounts the <filename>/usr/</filename> and the boot loader
        directories (<filename>/boot</filename> and <filename>/efi</filename>) read-only for processes
        invoked by this unit. If set to <literal>full</literal>, the <filename>/etc/</filename> directory is
        mounted read-only, too. If set to <literal>strict</literal> the entire file system hierarchy is
        mounted read-only, except for the API file system subtrees <filename>/dev/</filename>,
        <filename>/proc/</filename> and <filename>/sys/</filename> (protect these directories using
        <varname>PrivateDevices=</varname>, <varname>ProtectKernelTunables=</varname>,
        <varname>ProtectControlGroups=</varname>). This setting ensures that any modification of the vendor-supplied
        operating system (and optionally its configuration, and local mounts) is prohibited for the service. It is
        recommended to enable this setting for all long-running services, unless they are involved with system updates
        or need to modify the operating system in other ways. If this option is used,
        <varname>ReadWritePaths=</varname> may be used to exclude specific directories from being made read-only. This
        setting is implied if <varname>DynamicUser=</varname> is set. This setting cannot ensure protection in all
        cases. In general it has the same limitations as <varname>ReadOnlyPaths=</varname>, see below. Defaults to
        off.</para>

        <xi:include href="version-info.xml" xpointer="v214"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>ProtectHome=</varname></term>

        <listitem><para>Takes a boolean argument or the special values <literal>read-only</literal> or
        <literal>tmpfs</literal>. If true, the directories <filename>/home/</filename>,
        <filename>/root</filename>, and <filename>/run/user</filename> are made inaccessible and empty for
        processes invoked by this unit. If set to <literal>read-only</literal>, the three directories are
        made read-only instead. If set to <literal>tmpfs</literal>, temporary file systems are mounted on the
        three directories in read-only mode. The value <literal>tmpfs</literal> is useful to hide home
        directories not relevant to the processes invoked by the unit, while still allowing necessary
        directories to be made visible when listed in <varname>BindPaths=</varname> or
        <varname>BindReadOnlyPaths=</varname>.</para>

        <para>Setting this to <literal>yes</literal> is mostly equivalent to setting the three directories in
        <varname>InaccessiblePaths=</varname>. Similarly, <literal>read-only</literal> is mostly equivalent to
        <varname>ReadOnlyPaths=</varname>, and <literal>tmpfs</literal> is mostly equivalent to
        <varname>TemporaryFileSystem=</varname> with <literal>:ro</literal>.</para>

        <para>It is recommended to enable this setting for all long-running services (in particular
        network-facing ones), to ensure they cannot get access to private user data, unless the services
        actually require access to the user's private data. This setting is implied if
        <varname>DynamicUser=</varname> is set. This setting cannot ensure protection in all cases. In
        general it has the same limitations as <varname>ReadOnlyPaths=</varname>, see below.</para>

        <xi:include href="system-or-user-ns.xml" xpointer="singular"/>

        <xi:include href="version-info.xml" xpointer="v214"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>RuntimeDirectory=</varname></term>
        <term><varname>StateDirectory=</varname></term>
        <term><varname>CacheDirectory=</varname></term>
        <term><varname>LogsDirectory=</varname></term>
        <term><varname>ConfigurationDirectory=</varname></term>

        <listitem><para>These options take a whitespace-separated list of directory names. The specified
        directory names must be relative, and may not include <literal>..</literal>. If set, when the unit is
        started, one or more directories by the specified names will be created (including their parents)
        below the locations defined in the following table. Also, the corresponding environment variable will
        be defined with the full paths of the directories. If multiple directories are set, then in the
        environment variable the paths are concatenated with colon (<literal>:</literal>).</para>
        <table>
          <title>Automatic directory creation and environment variables</title>
          <tgroup cols='4'>
            <thead>
              <row>
                <entry>Directory</entry>
                <entry>Below path for system units</entry>
                <entry>Below path for user units</entry>
                <entry>Environment variable set</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry><varname>RuntimeDirectory=</varname></entry>
                <entry><filename>/run/</filename></entry>
                <entry><varname>$XDG_RUNTIME_DIR</varname></entry>
                <entry><varname>$RUNTIME_DIRECTORY</varname></entry>
              </row>
              <row>
                <entry><varname>StateDirectory=</varname></entry>
                <entry><filename>/var/lib/</filename></entry>
                <entry><varname>$XDG_STATE_HOME</varname></entry>
                <entry><varname>$STATE_DIRECTORY</varname></entry>
              </row>
              <row>
                <entry><varname>CacheDirectory=</varname></entry>
                <entry><filename>/var/cache/</filename></entry>
                <entry><varname>$XDG_CACHE_HOME</varname></entry>
                <entry><varname>$CACHE_DIRECTORY</varname></entry>
              </row>
              <row>
                <entry><varname>LogsDirectory=</varname></entry>
                <entry><filename>/var/log/</filename></entry>
                <entry><varname>$XDG_STATE_HOME</varname><filename>/log/</filename></entry>
                <entry><varname>$LOGS_DIRECTORY</varname></entry>
              </row>
              <row>
                <entry><varname>ConfigurationDirectory=</varname></entry>
                <entry><filename>/etc/</filename></entry>
                <entry><varname>$XDG_CONFIG_HOME</varname></entry>
                <entry><varname>$CONFIGURATION_DIRECTORY</varname></entry>
              </row>
            </tbody>
          </tgroup>
        </table>

        <para>In case of <varname>RuntimeDirectory=</varname> the innermost subdirectories are removed when
        the unit is stopped. It is possible to preserve the specified directories in this case if
        <varname>RuntimeDirectoryPreserve=</varname> is configured to <option>restart</option> or
        <option>yes</option> (see below). The directories specified with <varname>StateDirectory=</varname>,
        <varname>CacheDirectory=</varname>, <varname>LogsDirectory=</varname>,
        <varname>ConfigurationDirectory=</varname> are not removed when the unit is stopped.</para>

        <para>Except in case of <varname>ConfigurationDirectory=</varname>, the innermost specified directories will be
        owned by the user and group specified in <varname>User=</varname> and <varname>Group=</varname>. If the
        specified directories already exist and their owning user or group do not match the configured ones, all files
        and directories below the specified directories as well as the directories themselves will have their file
        ownership recursively changed to match what is configured. As an optimization, if the specified directories are
        already owned by the right user and group, files and directories below of them are left as-is, even if they do
        not match what is requested. The innermost specified directories will have their access mode adjusted to the
        what is specified in <varname>RuntimeDirectoryMode=</varname>, <varname>StateDirectoryMode=</varname>,
        <varname>CacheDirectoryMode=</varname>, <varname>LogsDirectoryMode=</varname> and
        <varname>ConfigurationDirectoryMode=</varname>.</para>

        <para>These options imply <varname>BindPaths=</varname> for the specified paths. When combined with
        <varname>RootDirectory=</varname> or <varname>RootImage=</varname> these paths always reside on the host and
        are mounted from there into the unit's file system namespace.</para>

        <para>If <varname>DynamicUser=</varname> is used, the logic for <varname>CacheDirectory=</varname>,
        <varname>LogsDirectory=</varname> and <varname>StateDirectory=</varname> is slightly altered: the directories are created below
        <filename>/var/cache/private</filename>, <filename>/var/log/private</filename> and <filename>/var/lib/private</filename>,
        respectively, which are host directories made inaccessible to
        unprivileged users, which ensures that access to these directories cannot be gained through dynamic
        user ID recycling. Symbolic links are created to hide this difference in behaviour. Both from
        perspective of the host and from inside the unit, the relevant directories hence always appear
        directly below <filename>/var/cache</filename>, <filename>/var/log</filename> and
        <filename>/var/lib</filename>.</para>

        <para>Use <varname>RuntimeDirectory=</varname> to manage one or more runtime directories for the unit and bind
        their lifetime to the daemon runtime. This is particularly useful for unprivileged daemons that cannot create
        runtime directories in <filename>/run/</filename> due to lack of privileges, and to make sure the runtime
        directory is cleaned up automatically after use. For runtime directories that require more complex or different
        configuration or lifetime guarantees, please consider using
        <citerefentry><refentrytitle>tmpfiles.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>

        <para><varname>RuntimeDirectory=</varname>, <varname>StateDirectory=</varname>, <varname>CacheDirectory=</varname>
        and <varname>LogsDirectory=</varname>  optionally support a second parameter, separated by <literal>:</literal>.
        The second parameter will be interpreted as a destination path that will be created as a symlink to the directory.
        The symlinks will be created after any <varname>BindPaths=</varname> or <varname>TemporaryFileSystem=</varname>
        options have been set up, to make ephemeral symlinking possible. The same source can have multiple symlinks, by
        using the same first parameter, but a different second parameter.</para>

        <para>The directories defined by these options are always created under the standard paths used by systemd
        (<filename>/var/</filename>, <filename>/run/</filename>, <filename>/etc/</filename>, …). If the service needs
        directories in a different location, a different mechanism has to be used to create them.</para>

        <para><citerefentry><refentrytitle>tmpfiles.d</refentrytitle><manvolnum>5</manvolnum></citerefentry> provides
        functionality that overlaps with these options. Using these options is recommended, because the lifetime of
        the directories is tied directly to the lifetime of the unit, and it is not necessary to ensure that the
        <filename>tmpfiles.d</filename> configuration is executed before the unit is started.</para>

        <para>To remove any of the directories created by these settings, use the <command>systemctl clean
        …</command> command on the relevant units, see
        <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
        details.</para>

        <para>Example: if a system service unit has the following,
        <programlisting>RuntimeDirectory=foo/bar baz</programlisting>
        the service manager creates <filename index='false'>/run/foo</filename> (if it does not exist),

        <filename index='false'>/run/foo/bar</filename>, and <filename index='false'>/run/baz</filename>. The
        directories <filename index='false'>/run/foo/bar</filename> and
        <filename index='false'>/run/baz</filename> except <filename index='false'>/run/foo</filename> are
        owned by the user and group specified in <varname>User=</varname> and <varname>Group=</varname>, and removed
        when the service is stopped.</para>

        <para>Example: if a system service unit has the following,
        <programlisting>RuntimeDirectory=foo/bar
StateDirectory=aaa/bbb ccc</programlisting>
        then the environment variable <literal>RUNTIME_DIRECTORY</literal> is set with <literal>/run/foo/bar</literal>, and
        <literal>STATE_DIRECTORY</literal> is set with <literal>/var/lib/aaa/bbb:/var/lib/ccc</literal>.</para>

        <para>Example: if a system service unit has the following,
        <programlisting>RuntimeDirectory=foo:bar foo:baz</programlisting>
        the service manager creates <filename index='false'>/run/foo</filename> (if it does not exist), and
        <filename index='false'>/run/bar</filename> plus <filename index='false'>/run/baz</filename> as symlinks to
        <filename index='false'>/run/foo</filename>.</para>

        <xi:include href="version-info.xml" xpointer="v211"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>RuntimeDirectoryMode=</varname></term>
        <term><varname>StateDirectoryMode=</varname></term>
        <term><varname>CacheDirectoryMode=</varname></term>
        <term><varname>LogsDirectoryMode=</varname></term>
        <term><varname>ConfigurationDirectoryMode=</varname></term>

        <listitem><para>Specifies the access mode of the directories specified in <varname>RuntimeDirectory=</varname>,
        <varname>StateDirectory=</varname>, <varname>CacheDirectory=</varname>, <varname>LogsDirectory=</varname>, or
        <varname>ConfigurationDirectory=</varname>, respectively, as an octal number. Defaults to
        <constant>0755</constant>. See "Permissions" in <citerefentry
        project='man-pages'><refentrytitle>path_resolution</refentrytitle><manvolnum>7</manvolnum></citerefentry> for a
        discussion of the meaning of permission bits.</para>

        <xi:include href="version-info.xml" xpointer="v234"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>RuntimeDirectoryPreserve=</varname></term>

        <listitem><para>Takes a boolean argument or <option>restart</option>. If set to <option>no</option> (the
        default), the directories specified in <varname>RuntimeDirectory=</varname> are always removed when the service
        stops. If set to <option>restart</option> the directories are preserved when the service is both automatically
        and manually restarted. Here, the automatic restart means the operation specified in
        <varname>Restart=</varname>, and manual restart means the one triggered by <command>systemctl restart
        foo.service</command>. If set to <option>yes</option>, then the directories are not removed when the service is
        stopped. Note that since the runtime directory <filename>/run/</filename> is a mount point of
        <literal>tmpfs</literal>, then for system services the directories specified in
        <varname>RuntimeDirectory=</varname> are removed when the system is rebooted.</para>

        <xi:include href="version-info.xml" xpointer="v235"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>TimeoutCleanSec=</varname></term>
        <listitem><para>Configures a timeout on the clean-up operation requested through <command>systemctl
        clean …</command>, see
        <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
        details. Takes the usual time values and defaults to <constant>infinity</constant>, i.e. by default
        no timeout is applied. If a timeout is configured the clean operation will be aborted forcibly when
        the timeout is reached, potentially leaving resources on disk.</para>

        <xi:include href="version-info.xml" xpointer="v244"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>ReadWritePaths=</varname></term>
        <term><varname>ReadOnlyPaths=</varname></term>
        <term><varname>InaccessiblePaths=</varname></term>
        <term><varname>ExecPaths=</varname></term>
        <term><varname>NoExecPaths=</varname></term>

        <listitem><para>Sets up a new file system namespace for executed processes. These options may be used
        to limit access a process has to the file system. Each setting takes a space-separated list of paths
        relative to the host's root directory (i.e. the system running the service manager). Note that if
        paths contain symlinks, they are resolved relative to the root directory set with
        <varname>RootDirectory=</varname>/<varname>RootImage=</varname>.</para>

        <para>Paths listed in <varname>ReadWritePaths=</varname> are accessible from within the namespace
        with the same access modes as from outside of it. Paths listed in <varname>ReadOnlyPaths=</varname>
        are accessible for reading only, writing will be refused even if the usual file access controls would
        permit this. Nest <varname>ReadWritePaths=</varname> inside of <varname>ReadOnlyPaths=</varname> in
        order to provide writable subdirectories within read-only directories. Use
        <varname>ReadWritePaths=</varname> in order to allow-list specific paths for write access if
        <varname>ProtectSystem=strict</varname> is used.</para>

        <para>Paths listed in <varname>InaccessiblePaths=</varname> will be made inaccessible for processes inside
        the namespace along with everything below them in the file system hierarchy. This may be more restrictive than
        desired, because it is not possible to nest <varname>ReadWritePaths=</varname>, <varname>ReadOnlyPaths=</varname>,
        <varname>BindPaths=</varname>, or <varname>BindReadOnlyPaths=</varname> inside it. For a more flexible option,
        see <varname>TemporaryFileSystem=</varname>.</para>

        <para>Content in paths listed in <varname>NoExecPaths=</varname> are not executable even if the usual
        file access controls would permit this. Nest <varname>ExecPaths=</varname> inside of
        <varname>NoExecPaths=</varname> in order to provide executable content within non-executable
        directories.</para>

        <para>Non-directory paths may be specified as well. These options may be specified more than once,
        in which case all paths listed will have limited access from within the namespace. If the empty string is
        assigned to this option, the specific list is reset, and all prior assignments have no effect.</para>

        <para>Paths in <varname>ReadWritePaths=</varname>, <varname>ReadOnlyPaths=</varname>,
        <varname>InaccessiblePaths=</varname>, <varname>ExecPaths=</varname> and
        <varname>NoExecPaths=</varname> may be prefixed with <literal>-</literal>, in which case they will be
        ignored when they do not exist. If prefixed with <literal>+</literal> the paths are taken relative to the root
        directory of the unit, as configured with <varname>RootDirectory=</varname>/<varname>RootImage=</varname>,
        instead of relative to the root directory of the host (see above). When combining <literal>-</literal> and
        <literal>+</literal> on the same path make sure to specify <literal>-</literal> first, and <literal>+</literal>
        second.</para>

        <para>Note that these settings will disconnect propagation of mounts from the unit's processes to the
        host. This means that this setting may not be used for services which shall be able to install mount points in
        the main mount namespace. For <varname>ReadWritePaths=</varname> and <varname>ReadOnlyPaths=</varname>,
        propagation in the other direction is not affected, i.e. mounts created on the host generally appear in the
        unit processes' namespace, and mounts removed on the host also disappear there too. In particular, note that
        mount propagation from host to unit will result in unmodified mounts to be created in the unit's namespace,
        i.e. writable mounts appearing on the host will be writable in the unit's namespace too, even when propagated
        below a path marked with <varname>ReadOnlyPaths=</varname>! Restricting access with these options hence does
        not extend to submounts of a directory that are created later on. This means the lock-down offered by that
        setting is not complete, and does not offer full protection.</para>

        <para>Note that the effect of these settings may be undone by privileged processes. In order to set up an
        effective sandboxed environment for a unit it is thus recommended to combine these settings with either
        <varname>CapabilityBoundingSet=~CAP_SYS_ADMIN</varname> or <varname>SystemCallFilter=~@mount</varname>.</para>

        <para>Please be extra careful when applying these options to API file systems (a list of them could be
        found in <varname>MountAPIVPS=</varname>), since they may be required for basic system functionalities.
        Moreover, <filename>/run/</filename> needs to be writable for setting up mount namespace and propagation.</para>

        <para>Simple allow-list example using these directives:
        <programlisting>[Service]
ReadOnlyPaths=/
ReadWritePaths=/var /run
InaccessiblePaths=-/lost+found
NoExecPaths=/
ExecPaths=/usr/sbin/my_daemon /usr/lib /usr/lib64
</programlisting></para>

        <xi:include href="system-or-user-ns.xml" xpointer="plural"/>

        <xi:include href="version-info.xml" xpointer="v231"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>TemporaryFileSystem=</varname></term>

        <listitem><para>Takes a space-separated list of mount points for temporary file systems (tmpfs). If set, a new file
        system namespace is set up for executed processes, and a temporary file system is mounted on each mount point.
        This option may be specified more than once, in which case temporary file systems are mounted on all listed mount
        points. If the empty string is assigned to this option, the list is reset, and all prior assignments have no effect.
        Each mount point may optionally be suffixed with a colon (<literal>:</literal>) and mount options such as
        <literal>size=10%</literal> or <literal>ro</literal>. By default, each temporary file system is mounted
        with <literal>nodev,strictatime,mode=0755</literal>. These can be disabled by explicitly specifying the corresponding
        mount options, e.g., <literal>dev</literal> or <literal>nostrictatime</literal>.</para>

        <para>This is useful to hide files or directories not relevant to the processes invoked by the unit, while necessary
        files or directories can be still accessed by combining with <varname>BindPaths=</varname> or
        <varname>BindReadOnlyPaths=</varname>:</para>

        <para>Example: if a unit has the following,
        <programlisting>TemporaryFileSystem=/var:ro
BindReadOnlyPaths=/var/lib/systemd</programlisting>
        then the invoked processes by the unit cannot see any files or directories under <filename>/var/</filename> except for
        <filename>/var/lib/systemd</filename> or its contents.</para>

        <xi:include href="system-or-user-ns.xml" xpointer="singular"/>

        <xi:include href="version-info.xml" xpointer="v238"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>PrivateTmp=</varname></term>

        <listitem><para>Takes a boolean argument. If true, sets up a new file system namespace for the
        executed processes and mounts private <filename>/tmp/</filename> and <filename>/var/tmp/</filename>
        directories inside it that are not shared by processes outside of the namespace. This is useful to
        secure access to temporary files of the process, but makes sharing between processes via
        <filename>/tmp/</filename> or <filename>/var/tmp/</filename> impossible. If true, all temporary files
        created by a service in these directories will be removed after the service is stopped. Defaults to
        false. It is possible to run two or more units within the same private <filename>/tmp/</filename> and
        <filename>/var/tmp/</filename> namespace by using the <varname>JoinsNamespaceOf=</varname> directive,
        see <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
        for details. This setting is implied if <varname>DynamicUser=</varname> is set. For this setting, the
        same restrictions regarding mount propagation and privileges apply as for
        <varname>ReadOnlyPaths=</varname> and related calls, see above. Enabling this setting has the side
        effect of adding <varname>Requires=</varname> and <varname>After=</varname> dependencies on all mount
        units necessary to access <filename>/tmp/</filename> and <filename>/var/tmp/</filename>. Moreover an
        implicitly <varname>After=</varname> ordering on
        <citerefentry><refentrytitle>systemd-tmpfiles-setup.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
        is added.</para>

        <para>Note that the implementation of this setting might be impossible (for example if mount namespaces are not
        available), and the unit should be written in a way that does not solely rely on this setting for
        security.</para>

        <xi:include href="system-or-user-ns.xml" xpointer="singular"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>PrivateDevices=</varname></term>

        <listitem><para>Takes a boolean argument. If true, sets up a new <filename>/dev/</filename> mount for
        the executed processes and only adds API pseudo devices such as <filename>/dev/null</filename>,
        <filename>/dev/zero</filename> or <filename>/dev/random</filename> (as well as the pseudo TTY
        subsystem) to it, but no physical devices such as <filename>/dev/sda</filename>, system memory
        <filename>/dev/mem</filename>, system ports <filename>/dev/port</filename> and others. This is useful
        to turn off physical device access by the executed process. Defaults to false.</para>

        <para>Enabling this option will install a system call filter to block low-level I/O system calls that
        are grouped in the <varname>@raw-io</varname> set, remove <constant>CAP_MKNOD</constant> and
        <constant>CAP_SYS_RAWIO</constant> from the capability bounding set for the unit, and set
        <varname>DevicePolicy=closed</varname> (see
        <citerefentry><refentrytitle>systemd.resource-control</refentrytitle><manvolnum>5</manvolnum></citerefentry>
        for details). Note that using this setting will disconnect propagation of mounts from the service to
        the host (propagation in the opposite direction continues to work). This means that this setting may
        not be used for services which shall be able to install mount points in the main mount namespace. The
        new <filename>/dev/</filename> will be mounted read-only and 'noexec'. The latter may break old
        programs which try to set up executable memory by using
        <citerefentry><refentrytitle>mmap</refentrytitle><manvolnum>2</manvolnum></citerefentry> of
        <filename>/dev/zero</filename> instead of using <constant>MAP_ANON</constant>. For this setting the
        same restrictions regarding mount propagation and privileges apply as for
        <varname>ReadOnlyPaths=</varname> and related calls, see above. If turned on and if running in user
        mode, or in system mode, but without the <constant>CAP_SYS_ADMIN</constant> capability (e.g. setting
        <varname>User=</varname>), <varname>NoNewPrivileges=yes</varname> is implied.</para>

        <para>Note that the implementation of this setting might be impossible (for example if mount
        namespaces are not available), and the unit should be written in a way that does not solely rely on
        this setting for security.</para>

        <xi:include href="system-or-user-ns.xml" xpointer="singular"/>

        <para>When access to some but not all devices must be possible, the <varname>DeviceAllow=</varname>
        setting might be used instead. See
        <citerefentry><refentrytitle>systemd.resource-control</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
        </para>

        <xi:include href="version-info.xml" xpointer="v209"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>PrivateNetwork=</varname></term>

        <listitem><para>Takes a boolean argument. If true, sets up a new network namespace for the executed processes
        and configures only the loopback network device <literal>lo</literal> inside it. No other network devices will
        be available to the executed process. This is useful to turn off network access by the executed process.
        Defaults to false. It is possible to run two or more units within the same private network namespace by using
        the <varname>JoinsNamespaceOf=</varname> directive, see
        <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
        details. Note that this option will disconnect all socket families from the host, including
        <constant>AF_NETLINK</constant> and <constant>AF_UNIX</constant>. Effectively, for
        <constant>AF_NETLINK</constant> this means that device configuration events received from
        <citerefentry><refentrytitle>systemd-udevd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> are
        not delivered to the unit's processes. And for <constant>AF_UNIX</constant> this has the effect that
        <constant>AF_UNIX</constant> sockets in the abstract socket namespace of the host will become unavailable to
        the unit's processes (however, those located in the file system will continue to be accessible).</para>

        <para>Note that the implementation of this setting might be impossible (for example if network namespaces are
        not available), and the unit should be written in a way that does not solely rely on this setting for
        security.</para>

        <para>When this option is enabled, <varname>PrivateMounts=</varname> is implied unless it is
        explicitly disabled, and <filename>/sys</filename> will be remounted to associate it with the new
        network namespace.</para>

        <para>When this option is used on a socket unit any sockets bound on behalf of this unit will be
        bound within a private network namespace. This may be combined with
        <varname>JoinsNamespaceOf=</varname> to listen on sockets inside of network namespaces of other
        services.</para>

        <xi:include href="system-or-user-ns.xml" xpointer="singular"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>NetworkNamespacePath=</varname></term>

        <listitem><para>Takes an absolute file system path referring to a Linux network namespace
        pseudo-file (i.e. a file like <filename>/proc/$PID/ns/net</filename> or a bind mount or symlink to
        one). When set the invoked processes are added to the network namespace referenced by that path. The
        path has to point to a valid namespace file at the moment the processes are forked off. If this
        option is used <varname>PrivateNetwork=</varname> has no effect. If this option is used together with
        <varname>JoinsNamespaceOf=</varname> then it only has an effect if this unit is started before any of
        the listed units that have <varname>PrivateNetwork=</varname> or
        <varname>NetworkNamespacePath=</varname> configured, as otherwise the network namespace of those
        units is reused.</para>

        <para>When this option is enabled, <varname>PrivateMounts=</varname> is implied unless it is
        explicitly disabled, and <filename>/sys</filename> will be remounted to associate it with the new
        network namespace.</para>

        <para>When this option is used on a socket unit any sockets bound on behalf of this unit will be
        bound within the specified network namespace.</para>

        <xi:include href="system-or-user-ns.xml" xpointer="singular"/>

        <xi:include href="version-info.xml" xpointer="v242"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>PrivateIPC=</varname></term>

        <listitem><para>Takes a boolean argument. If true, sets up a new IPC namespace for the executed processes.
        Each IPC namespace has its own set of System V IPC identifiers and its own POSIX message queue file system.
        This is useful to avoid name clash of IPC identifiers. Defaults to false. It is possible to run two or
        more units within the same private IPC namespace by using the <varname>JoinsNamespaceOf=</varname> directive,
        see <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
        details.</para>

        <para>Note that IPC namespacing does not have an effect on
        <constant>AF_UNIX</constant> sockets, which are the most common
        form of IPC used on Linux. Instead, <constant>AF_UNIX</constant>
        sockets in the file system are subject to mount namespacing, and
        those in the abstract namespace are subject to network namespacing.
        IPC namespacing only has an effect on SysV IPC (which is mostly
        legacy) as well as POSIX message queues (for which
        <constant>AF_UNIX</constant>/<constant>SOCK_SEQPACKET</constant>
        sockets are typically a better replacement). IPC namespacing also
        has no effect on POSIX shared memory (which is subject to mount
        namespacing) either. See
        <citerefentry project='man-pages'><refentrytitle>ipc_namespaces</refentrytitle><manvolnum>7</manvolnum></citerefentry> for
        the details.</para>

        <para>Note that the implementation of this setting might be impossible (for example if IPC namespaces are
        not available), and the unit should be written in a way that does not solely rely on this setting for
        security.</para>

        <xi:include href="system-or-user-ns.xml" xpointer="singular"/>

        <xi:include href="version-info.xml" xpointer="v248"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>IPCNamespacePath=</varname></term>

        <listitem><para>Takes an absolute file system path referring to a Linux IPC namespace
        pseudo-file (i.e. a file like <filename>/proc/$PID/ns/ipc</filename> or a bind mount or symlink to
        one). When set the invoked processes are added to the network namespace referenced by that path. The
        path has to point to a valid namespace file at the moment the processes are forked off. If this
        option is used <varname>PrivateIPC=</varname> has no effect. If this option is used together with
        <varname>JoinsNamespaceOf=</varname> then it only has an effect if this unit is started before any of
        the listed units that have <varname>PrivateIPC=</varname> or
        <varname>IPCNamespacePath=</varname> configured, as otherwise the network namespace of those
        units is reused.</para>

        <xi:include href="system-or-user-ns.xml" xpointer="singular"/>

        <xi:include href="version-info.xml" xpointer="v248"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>MemoryKSM=</varname></term>

        <listitem><para>Takes a boolean argument. When set, it enables KSM (kernel samepage merging) for
        the processes. KSM is a memory-saving de-duplication feature. Anonymous memory pages with identical
        content can be replaced by a single write-protected page. This feature should only be enabled for
        jobs that share the same security domain. For details, see
        <ulink url="https://docs.kernel.org/admin-guide/mm/ksm.html">Kernel Samepage Merging</ulink> in the
        kernel documentation.</para>

        <para>Note that this functionality might not be available, for example if KSM is disabled in the
        kernel, or the kernel doesn't support controlling KSM at the process level through
        <function>prctl()</function>.</para>

        <xi:include href="version-info.xml" xpointer="v254"/>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>PrivateUsers=</varname></term>

        <listitem><para>Takes a boolean argument. If true, sets up a new user namespace for the executed processes and
        configures a minimal user and group mapping, that maps the <literal>root</literal> user and group as well as
        the unit's own user and group to themselves and everything else to the <literal>nobody</literal> user and
        group. This is useful to securely detach the user and group databases used by the unit from the rest of the
        system, and thus to create an effective sandbox environment. All files, directories, processes, IPC objects and
        other resources owned by users/groups not equaling <literal>root</literal> or the unit's own will stay visible
        from within the unit but appear owned by the <literal>nobody</literal> user and group. If this mode is enabled,
        all unit processes are run without privileges in the host user namespace (regardless if the unit's own
        user/group is <literal>root</literal> or not). Specifically this means that the process will have zero process
        capabilities on the host's user namespace, but full capabilities within the service's user namespace. Settings
        such as <varname>CapabilityBoundingSet=</varname> will affect only the latter, and there's no way to acquire
        additional capabilities in the host's user namespace. Defaults to off.</para>

        <para>When this setting is set up by a per-user instance of the service manager, the mapping of the
        <literal>root</literal> user and group to itself is omitted (unless the user manager is root).
        Additionally, in the per-user instance manager case, the
        user namespace will be set up before most other namespaces. This means that combining
        <varname>PrivateUsers=</varname><option>true</option> with other namespaces will enable use of features not
        normally supported by the per-user instances of the service manager.</para>

        <para>This setting is particularly useful in conjunction with
        <varname>RootDirectory=</varname>/<varname>RootImage=</varname>, as the need to synchronize the user and group
        databases in the root directory and on the host is reduced, as the only users and groups who need to be matched
        are <literal>root</literal>, <literal>nobody</literal> and the unit's own user and group.</para>

        <para>Note that the implementation of this setting might be impossible (for example if user namespaces are not
        available), and the unit should be written in a way that does not solely rely on this setting for
        security.</para>

        <xi:include href="version-info.xml" xpointer="v232"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>ProtectHostname=</varname></term>

        <listitem><para>Takes a boolean argument. When set, sets up a new UTS namespace for the executed
        processes. In addition, changing hostname or domainname is prevented. Defaults to off.</para>

        <para>Note that the implementation of this setting might be impossible (for example if UTS namespaces
        are not available), and the unit should be written in a way that does not solely rely on this setting
        for security.</para>

        <para>Note that when this option is enabled for a service hostname changes no longer propagate from
        the system into the service, it is hence not suitable for services that need to take notice of system
        hostname changes dynamically.</para>

        <para>If this setting is on, but the unit doesn't have the <constant>CAP_SYS_ADMIN</constant>
        capability (e.g. services for which <varname>User=</varname> is set),
        <varname>NoNewPrivileges=yes</varname> is implied.</para>

        <xi:include href="system-or-user-ns.xml" xpointer="singular"/>

        <xi:include href="version-info.xml" xpointer="v242"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>ProtectClock=</varname></term>

        <listitem><para>Takes a boolean argument. If set, writes to the hardware clock or system clock will
        be denied. Defaults to off. Enabling this option removes <constant>CAP_SYS_TIME</constant> and
        <constant>CAP_WAKE_ALARM</constant> from the capability bounding set for this unit, installs a system
        call filter to block calls that can set the clock, and <varname>DeviceAllow=char-rtc r</varname> is
        implied. Note that the system calls are blocked altogether, the filter does not take into account
        that some of the calls can be used to read the clock state with some parameter combinations.
        Effectively, <filename>/dev/rtc0</filename>, <filename>/dev/rtc1</filename>, etc. are made read-only
        to the service. See
        <citerefentry><refentrytitle>systemd.resource-control</refentrytitle><manvolnum>5</manvolnum></citerefentry>
        for the details about <varname>DeviceAllow=</varname>. If this setting is on, but the unit doesn't
        have the <constant>CAP_SYS_ADMIN</constant> capability (e.g. services for which
        <varname>User=</varname> is set), <varname>NoNewPrivileges=yes</varname> is implied.</para>

        <para>It is recommended to turn this on for most services that do not need modify the clock or check
        its state.</para>

        <xi:include href="system-or-user-ns.xml" xpointer="singular"/>

        <xi:include href="version-info.xml" xpointer="v245"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>ProtectKernelTunables=</varname></term>

        <listitem><para>Takes a boolean argument. If true, kernel variables accessible through
        <filename>/proc/sys/</filename>, <filename>/sys/</filename>, <filename>/proc/sysrq-trigger</filename>,
        <filename>/proc/latency_stats</filename>, <filename>/proc/acpi</filename>,
        <filename>/proc/timer_stats</filename>, <filename>/proc/fs</filename> and <filename>/proc/irq</filename> will
        be made read-only to all processes of the unit. Usually, tunable kernel variables should be initialized only at
        boot-time, for example with the
        <citerefentry><refentrytitle>sysctl.d</refentrytitle><manvolnum>5</manvolnum></citerefentry> mechanism. Few
        services need to write to these at runtime; it is hence recommended to turn this on for most services. For this
        setting the same restrictions regarding mount propagation and privileges apply as for
        <varname>ReadOnlyPaths=</varname> and related calls, see above. Defaults to off. If this
        setting is on, but the unit doesn't have the <constant>CAP_SYS_ADMIN</constant> capability
        (e.g. services for which <varname>User=</varname> is set),
        <varname>NoNewPrivileges=yes</varname> is implied. Note that this option does not prevent
        indirect changes to kernel tunables effected by IPC calls to other processes. However,
        <varname>InaccessiblePaths=</varname> may be used to make relevant IPC file system objects
        inaccessible. If <varname>ProtectKernelTunables=</varname> is set,
        <varname>MountAPIVFS=yes</varname> is implied.</para>

        <xi:include href="system-or-user-ns.xml" xpointer="singular"/>

        <xi:include href="version-info.xml" xpointer="v232"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>ProtectKernelModules=</varname></term>

        <listitem><para>Takes a boolean argument. If true, explicit module loading will be denied. This allows
        module load and unload operations to be turned off on modular kernels. It is recommended to turn this on for most services
        that do not need special file systems or extra kernel modules to work. Defaults to off. Enabling this option
        removes <constant>CAP_SYS_MODULE</constant> from the capability bounding set for the unit, and installs a
        system call filter to block module system calls, also <filename>/usr/lib/modules</filename> is made
        inaccessible. For this setting the same restrictions regarding mount propagation and privileges apply as for
        <varname>ReadOnlyPaths=</varname> and related calls, see above. Note that limited automatic module loading due
        to user configuration or kernel mapping tables might still happen as side effect of requested user operations,
        both privileged and unprivileged. To disable module auto-load feature please see
        <citerefentry><refentrytitle>sysctl.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>
        <constant>kernel.modules_disabled</constant> mechanism and
        <filename>/proc/sys/kernel/modules_disabled</filename> documentation. If this setting is on,
        but the unit doesn't have the <constant>CAP_SYS_ADMIN</constant> capability (e.g. services for
        which <varname>User=</varname> is set), <varname>NoNewPrivileges=yes</varname> is implied.</para>

        <xi:include href="system-or-user-ns.xml" xpointer="singular"/>

        <xi:include href="version-info.xml" xpointer="v232"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>ProtectKernelLogs=</varname></term>

        <listitem><para>Takes a boolean argument. If true, access to the kernel log ring buffer will be denied. It is
        recommended to turn this on for most services that do not need to read from or write to the kernel log ring
        buffer. Enabling this option removes <constant>CAP_SYSLOG</constant> from the capability bounding set for this
        unit, and installs a system call filter to block the
        <citerefentry project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>2</manvolnum></citerefentry>
        system call (not to be confused with the libc API
        <citerefentry project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
        for userspace logging). The kernel exposes its log buffer to userspace via <filename>/dev/kmsg</filename> and
        <filename>/proc/kmsg</filename>. If enabled, these are made inaccessible to all the processes in the unit.
        If this setting is on, but the unit doesn't have the <constant>CAP_SYS_ADMIN</constant>
        capability (e.g. services for which <varname>User=</varname> is set),
        <varname>NoNewPrivileges=yes</varname> is implied.</para>

        <xi:include href="system-or-user-ns.xml" xpointer="singular"/>

        <xi:include href="version-info.xml" xpointer="v244"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>ProtectControlGroups=</varname></term>

        <listitem><para>Takes a boolean argument. If true, the Linux Control Groups (<citerefentry
        project='man-pages'><refentrytitle>cgroups</refentrytitle><manvolnum>7</manvolnum></citerefentry>) hierarchies
        accessible through <filename>/sys/fs/cgroup/</filename> will be made read-only to all processes of the
        unit. Except for container managers no services should require write access to the control groups hierarchies;
        it is hence recommended to turn this on for most services. For this setting the same restrictions regarding
        mount propagation and privileges apply as for <varname>ReadOnlyPaths=</varname> and related calls, see
        above. Defaults to off. If <varname>ProtectControlGroups=</varname> is set, <varname>MountAPIVFS=yes</varname>
        is implied.</para>

        <xi:include href="system-only.xml" xpointer="singular"/>

        <xi:include href="version-info.xml" xpointer="v232"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>RestrictAddressFamilies=</varname></term>

        <listitem><para>Restricts the set of socket address families accessible to the processes of this
        unit. Takes <literal>none</literal>, or a space-separated list of address family names to
        allow-list, such as <constant>AF_UNIX</constant>, <constant>AF_INET</constant> or
        <constant>AF_INET6</constant>. When <literal>none</literal> is specified, then all address
        families will be denied. When prefixed with <literal>~</literal> the listed address
        families will be applied as deny list, otherwise as allow list. Note that this restricts access
        to the
        <citerefentry project='man-pages'><refentrytitle>socket</refentrytitle><manvolnum>2</manvolnum></citerefentry>
        system call only. Sockets passed into the process by other means (for example, by using socket
        activation with socket units, see
        <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>)
        are unaffected. Also, sockets created with <function>socketpair()</function> (which creates connected
        AF_UNIX sockets only) are unaffected. Note that this option has no effect on 32-bit x86, s390, s390x,
        mips, mips-le, ppc, ppc-le, ppc64, ppc64-le and is ignored (but works correctly on other ABIs,
        including x86-64). Note that on systems supporting multiple ABIs (such as x86/x86-64) it is
        recommended to turn off alternative ABIs for services, so that they cannot be used to circumvent the
        restrictions of this option. Specifically, it is recommended to combine this option with
        <varname>SystemCallArchitectures=native</varname> or similar. If running in user mode, or in system
        mode, but without the <constant>CAP_SYS_ADMIN</constant> capability (e.g. setting
        <varname>User=</varname>), <varname>NoNewPrivileges=yes</varname> is implied. By default, no
        restrictions apply, all address families are accessible to processes. If assigned the empty string,
        any previous address family restriction changes are undone. This setting does not affect commands
        prefixed with <literal>+</literal>.</para>

        <para>Use this option to limit exposure of processes to remote access, in particular via exotic and sensitive
        network protocols, such as <constant>AF_PACKET</constant>. Note that in most cases, the local
        <constant>AF_UNIX</constant> address family should be included in the configured allow list as it is frequently
        used for local communication, including for
        <citerefentry><refentrytitle>syslog</refentrytitle><manvolnum>2</manvolnum></citerefentry>
        logging.</para>

        <xi:include href="version-info.xml" xpointer="v211"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>RestrictFileSystems=</varname></term>

        <listitem><para>Restricts the set of filesystems processes of this unit can open files on. Takes a space-separated
        list of filesystem names. Any filesystem listed is made accessible to the unit's processes, access to filesystem
        types not listed is prohibited (allow-listing). If the first character of the list is <literal>~</literal>, the
        effect is inverted: access to the filesystems listed is prohibited (deny-listing). If the empty string is assigned,
        access to filesystems is not restricted.</para>

        <para>If you specify both types of this option (i.e. allow-listing and deny-listing), the first encountered will take
        precedence and will dictate the default action (allow access to the filesystem or deny it). Then the next occurrences
        of this option will add or delete the listed filesystems from the set of the restricted filesystems, depending on its
        type and the default action.</para>

        <para>Example: if a unit has the following,
        <programlisting>RestrictFileSystems=ext4 tmpfs
RestrictFileSystems=ext2 ext4</programlisting>
        then access to <constant>ext4</constant>, <constant>tmpfs</constant>, and <constant>ext2</constant> is allowed
        and access to other filesystems is denied.</para>

        <para>Example: if a unit has the following,
        <programlisting>RestrictFileSystems=ext4 tmpfs
RestrictFileSystems=~ext4</programlisting>
        then only access <constant>tmpfs</constant> is allowed.</para>

        <para>Example: if a unit has the following,
        <programlisting>RestrictFileSystems=~ext4 tmpfs
RestrictFileSystems=ext4</programlisting>
        then only access to <constant>tmpfs</constant> is denied.</para>

        <para>As the number of possible filesystems is large, predefined sets of filesystems are provided. A set
        starts with <literal>@</literal> character, followed by name of the set.</para>

        <table>
          <title>Currently predefined filesystem sets</title>

          <tgroup cols='2'>
            <colspec colname='set' />
            <colspec colname='description' />
            <thead>
              <row>
                <entry>Set</entry>
                <entry>Description</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry>@basic-api</entry>
                <entry>Basic filesystem API.</entry>
              </row>
              <row>
                <entry>@auxiliary-api</entry>
                <entry>Auxiliary filesystem API.</entry>
              </row>
              <row>
                <entry>@common-block</entry>
                <entry>Common block device filesystems.</entry>
              </row>
              <row>
                <entry>@historical-block</entry>
                <entry>Historical block device filesystems.</entry>
              </row>
              <row>
                <entry>@network</entry>
                <entry>Well-known network filesystems.</entry>
              </row>
              <row>
                <entry>@privileged-api</entry>
                <entry>Privileged filesystem API.</entry>
              </row>
              <row>
                <entry>@temporary</entry>
                <entry>Temporary filesystems: tmpfs, ramfs.</entry>
              </row>
              <row>
                <entry>@known</entry>
                <entry>All known filesystems defined by the kernel. This list is defined statically in systemd based on a kernel version that was available when this systemd version was released. It will become progressively more out-of-date as the kernel is updated.</entry>
              </row>
            </tbody>
          </tgroup>
        </table>

        <para>Use
        <citerefentry><refentrytitle>systemd-analyze</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
        <command>filesystems</command> command to retrieve a list of filesystems defined on the local
        system.</para>

        <para>Note that this setting might not be supported on some systems (for example if the LSM eBPF hook is
        not enabled in the underlying kernel or if not using the unified control group hierarchy). In that case this setting
        has no effect.</para>

        <xi:include href="cgroup-sandboxing.xml" xpointer="singular"/>

        <xi:include href="version-info.xml" xpointer="v250"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>RestrictNamespaces=</varname></term>

        <listitem><para>Restricts access to Linux namespace functionality for the processes of this unit. For details
        about Linux namespaces, see <citerefentry
        project='man-pages'><refentrytitle>namespaces</refentrytitle><manvolnum>7</manvolnum></citerefentry>. Either
        takes a boolean argument, or a space-separated list of namespace type identifiers. If false (the default), no
        restrictions on namespace creation and switching are made. If true, access to any kind of namespacing is
        prohibited. Otherwise, a space-separated list of namespace type identifiers must be specified, consisting of
        any combination of: <constant>cgroup</constant>, <constant>ipc</constant>, <constant>net</constant>,
        <constant>mnt</constant>, <constant>pid</constant>, <constant>user</constant> and <constant>uts</constant>. Any
        namespace type listed is made accessible to the unit's processes, access to namespace types not listed is
        prohibited (allow-listing). By prepending the list with a single tilde character (<literal>~</literal>) the
        effect may be inverted: only the listed namespace types will be made inaccessible, all unlisted ones are
        permitted (deny-listing). If the empty string is assigned, the default namespace restrictions are applied,
        which is equivalent to false. This option may appear more than once, in which case the namespace types are
        merged by <constant>OR</constant>, or by <constant>AND</constant> if the lines are prefixed with
        <literal>~</literal> (see examples below). Internally, this setting limits access to the
        <citerefentry><refentrytitle>unshare</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>clone</refentrytitle><manvolnum>2</manvolnum></citerefentry> and
        <citerefentry><refentrytitle>setns</refentrytitle><manvolnum>2</manvolnum></citerefentry> system calls, taking
        the specified flags parameters into account. Note that — if this option is used — in addition to restricting
        creation and switching of the specified types of namespaces (or all of them, if true) access to the
        <function>setns()</function> system call with a zero flags parameter is prohibited. This setting is only
        supported on x86, x86-64, mips, mips-le, mips64, mips64-le, mips64-n32, mips64-le-n32, ppc64, ppc64-le, s390
        and s390x, and enforces no restrictions on other architectures. If running in user mode, or in system mode, but
        without the <constant>CAP_SYS_ADMIN</constant> capability (e.g. setting <varname>User=</varname>),
        <varname>NoNewPrivileges=yes</varname> is implied.</para>

        <para>Example: if a unit has the following,
        <programlisting>RestrictNamespaces=cgroup ipc
RestrictNamespaces=cgroup net</programlisting>
        then <constant>cgroup</constant>, <constant>ipc</constant>, and <constant>net</constant> are set.
        If the second line is prefixed with <literal>~</literal>, e.g.,
        <programlisting>RestrictNamespaces=cgroup ipc
RestrictNamespaces=~cgroup net</programlisting>
        then, only <constant>ipc</constant> is set.</para>

        <xi:include href="version-info.xml" xpointer="v233"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>LockPersonality=</varname></term>

        <listitem><para>Takes a boolean argument. If set, locks down the <citerefentry
        project='man-pages'><refentrytitle>personality</refentrytitle><manvolnum>2</manvolnum></citerefentry> system
        call so that the kernel execution domain may not be changed from the default or the personality selected with
        <varname>Personality=</varname> directive. This may be useful to improve security, because odd personality
        emulations may be poorly tested and source of vulnerabilities. If running in user mode, or in system mode, but
        without the <constant>CAP_SYS_ADMIN</constant> capability (e.g. setting <varname>User=</varname>),
        <varname>NoNewPrivileges=yes</varname> is implied.</para>

        <xi:include href="version-info.xml" xpointer="v235"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>MemoryDenyWriteExecute=</varname></term>

        <listitem><para>Takes a boolean argument. If set, attempts to create memory mappings that are writable and
        executable at the same time, or to change existing memory mappings to become executable, or mapping shared
        memory segments as executable, are prohibited. Specifically, a system call filter is added (or
        preferably, an equivalent kernel check is enabled with
        <citerefentry><refentrytitle>prctl</refentrytitle><manvolnum>2</manvolnum></citerefentry>) that
        rejects <citerefentry><refentrytitle>mmap</refentrytitle><manvolnum>2</manvolnum></citerefentry>
        system calls with both <constant>PROT_EXEC</constant> and <constant>PROT_WRITE</constant> set,
        <citerefentry><refentrytitle>mprotect</refentrytitle><manvolnum>2</manvolnum></citerefentry> or
        <citerefentry><refentrytitle>pkey_mprotect</refentrytitle><manvolnum>2</manvolnum></citerefentry> system calls
        with <constant>PROT_EXEC</constant> set and
        <citerefentry><refentrytitle>shmat</refentrytitle><manvolnum>2</manvolnum></citerefentry> system calls with
        <constant>SHM_EXEC</constant> set. Note that this option is incompatible with programs and libraries that
        generate program code dynamically at runtime, including JIT execution engines, executable stacks, and code
        "trampoline" feature of various C compilers. This option improves service security, as it makes harder for
        software exploits to change running code dynamically. However, the protection can be circumvented, if
        the service can write to a filesystem, which is not mounted with <constant>noexec</constant> (such as
        <filename>/dev/shm</filename>), or it can use <function>memfd_create()</function>. This can be
        prevented by making such file systems inaccessible to the service
        (e.g. <varname>InaccessiblePaths=/dev/shm</varname>) and installing further system call filters
        (<varname>SystemCallFilter=~memfd_create</varname>). Note that this feature is fully available on
        x86-64, and partially on x86. Specifically, the <function>shmat()</function> protection is not
        available on x86. Note that on systems supporting multiple ABIs (such as x86/x86-64) it is
        recommended to turn off alternative ABIs for services, so that they cannot be used to circumvent the
        restrictions of this option. Specifically, it is recommended to combine this option with
        <varname>SystemCallArchitectures=native</varname> or similar. If running in user mode, or in system
        mode, but without the <constant>CAP_SYS_ADMIN</constant> capability (e.g. setting
        <varname>User=</varname>), <varname>NoNewPrivileges=yes</varname> is implied.</para>

        <xi:include href="version-info.xml" xpointer="v231"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>RestrictRealtime=</varname></term>

        <listitem><para>Takes a boolean argument. If set, any attempts to enable realtime scheduling in a process of
        the unit are refused. This restricts access to realtime task scheduling policies such as
        <constant>SCHED_FIFO</constant>, <constant>SCHED_RR</constant> or <constant>SCHED_DEADLINE</constant>. See
        <citerefentry project='man-pages'><refentrytitle>sched</refentrytitle><manvolnum>7</manvolnum></citerefentry>
        for details about these scheduling policies. If running in user mode, or in system mode, but without the
        <constant>CAP_SYS_ADMIN</constant> capability (e.g. setting <varname>User=</varname>),
        <varname>NoNewPrivileges=yes</varname> is implied. Realtime scheduling policies may be used to monopolize CPU
        time for longer periods of time, and may hence be used to lock up or otherwise trigger Denial-of-Service
        situations on the system. It is hence recommended to restrict access to realtime scheduling to the few programs
        that actually require them. Defaults to off.</para>

        <xi:include href="version-info.xml" xpointer="v231"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>RestrictSUIDSGID=</varname></term>

        <listitem><para>Takes a boolean argument. If set, any attempts to set the set-user-ID (SUID) or
        set-group-ID (SGID) bits on files or directories will be denied (for details on these bits see
        <citerefentry
        project='man-pages'><refentrytitle>inode</refentrytitle><manvolnum>7</manvolnum></citerefentry>). If
        running in user mode, or in system mode, but without the <constant>CAP_SYS_ADMIN</constant>
        capability (e.g. setting <varname>User=</varname>), <varname>NoNewPrivileges=yes</varname> is
        implied. As the SUID/SGID bits are mechanisms to elevate privileges, and allow users to acquire the
        identity of other users, it is recommended to restrict creation of SUID/SGID files to the few
        programs that actually require them. Note that this restricts marking of any type of file system
        object with these bits, including both regular files and directories (where the SGID is a different
        meaning than for files, see documentation). This option is implied if <varname>DynamicUser=</varname>
        is enabled. Defaults to off.</para>

        <xi:include href="version-info.xml" xpointer="v242"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>RemoveIPC=</varname></term>

        <listitem><para>Takes a boolean parameter. If set, all System V and POSIX IPC objects owned by the user and
        group the processes of this unit are run as are removed when the unit is stopped. This setting only has an
        effect if at least one of <varname>User=</varname>, <varname>Group=</varname> and
        <varname>DynamicUser=</varname> are used. It has no effect on IPC objects owned by the root user. Specifically,
        this removes System V semaphores, as well as System V and POSIX shared memory segments and message queues. If
        multiple units use the same user or group the IPC objects are removed when the last of these units is
        stopped. This setting is implied if <varname>DynamicUser=</varname> is set.</para>

        <xi:include href="system-only.xml" xpointer="singular"/>

        <xi:include href="version-info.xml" xpointer="v232"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>PrivateMounts=</varname></term>

        <listitem><para>Takes a boolean parameter. If set, the processes of this unit will be run in their own private
        file system (mount) namespace with all mount propagation from the processes towards the host's main file system
        namespace turned off. This means any file system mount points established or removed by the unit's processes
        will be private to them and not be visible to the host. However, file system mount points established or
        removed on the host will be propagated to the unit's processes. See <citerefentry
        project='man-pages'><refentrytitle>mount_namespaces</refentrytitle><manvolnum>7</manvolnum></citerefentry> for
        details on file system namespaces. Defaults to off.</para>

        <para>When turned on, this executes three operations for each invoked process: a new
        <constant>CLONE_NEWNS</constant> namespace is created, after which all existing mounts are remounted to
        <constant>MS_SLAVE</constant> to disable propagation from the unit's processes to the host (but leaving
        propagation in the opposite direction in effect). Finally, the mounts are remounted again to the propagation
        mode configured with <varname>MountFlags=</varname>, see below.</para>

        <para>File system namespaces are set up individually for each process forked off by the service manager. Mounts
        established in the namespace of the process created by <varname>ExecStartPre=</varname> will hence be cleaned
        up automatically as soon as that process exits and will not be available to subsequent processes forked off for
        <varname>ExecStart=</varname> (and similar applies to the various other commands configured for
        units). Similarly, <varname>JoinsNamespaceOf=</varname> does not permit sharing kernel mount namespaces between
        units, it only enables sharing of the <filename>/tmp/</filename> and <filename>/var/tmp/</filename>
        directories.</para>

        <para>Other file system namespace unit settings — <varname>PrivateMounts=</varname>,
        <varname>PrivateTmp=</varname>, <varname>PrivateDevices=</varname>, <varname>ProtectSystem=</varname>,
        <varname>ProtectHome=</varname>, <varname>ReadOnlyPaths=</varname>, <varname>InaccessiblePaths=</varname>,
        <varname>ReadWritePaths=</varname>, … — also enable file system namespacing in a fashion equivalent to this
        option. Hence it is primarily useful to explicitly request this behaviour if none of the other settings are
        used.</para>

        <xi:include href="system-or-user-ns.xml" xpointer="singular"/>

        <xi:include href="version-info.xml" xpointer="v239"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>MountFlags=</varname></term>

        <listitem><para>Takes a mount propagation setting: <option>shared</option>, <option>slave</option> or
        <option>private</option>, which controls whether file system mount points in the file system namespaces set up
        for this unit's processes will receive or propagate mounts and unmounts from other file system namespaces. See
        <citerefentry project='man-pages'><refentrytitle>mount</refentrytitle><manvolnum>2</manvolnum></citerefentry>
        for details on mount propagation, and the three propagation flags in particular.</para>

        <para>This setting only controls the <emphasis>final</emphasis> propagation setting in effect on all mount
        points of the file system namespace created for each process of this unit. Other file system namespacing unit
        settings (see the discussion in <varname>PrivateMounts=</varname> above) will implicitly disable mount and
        unmount propagation from the unit's processes towards the host by changing the propagation setting of all mount
        points in the unit's file system namespace to <option>slave</option> first. Setting this option to
        <option>shared</option> does not reestablish propagation in that case.</para>

        <para>If not set – but file system namespaces are enabled through another file system namespace unit setting –
        <option>shared</option> mount propagation is used, but — as mentioned — as <option>slave</option> is applied
        first, propagation from the unit's processes to the host is still turned off.</para>

        <para>It is not recommended to use <option>private</option> mount propagation for units, as this means
        temporary mounts (such as removable media) of the host will stay mounted and thus indefinitely busy in forked
        off processes, as unmount propagation events won't be received by the file system namespace of the unit.</para>

        <para>Usually, it is best to leave this setting unmodified, and use higher level file system namespacing
        options instead, in particular <varname>PrivateMounts=</varname>, see above.</para>

        <xi:include href="system-or-user-ns.xml" xpointer="singular"/></listitem>
      </varlistentry>

    </variablelist>
  </refsect1>

  <refsect1>
    <title>System Call Filtering</title>
    <variablelist class='unit-directives'>

      <varlistentry>
        <term><varname>SystemCallFilter=</varname></term>

        <listitem><para>Takes a space-separated list of system call names. If this setting is used, all
        system calls executed by the unit processes except for the listed ones will result in immediate
        process termination with the <constant>SIGSYS</constant> signal (allow-listing). (See
        <varname>SystemCallErrorNumber=</varname> below for changing the default action). If the first
        character of the list is <literal>~</literal>, the effect is inverted: only the listed system calls
        will result in immediate process termination (deny-listing). Deny-listed system calls and system call
        groups may optionally be suffixed with a colon (<literal>:</literal>) and <literal>errno</literal>
        error number (between 0 and 4095) or errno name such as <constant>EPERM</constant>,
        <constant>EACCES</constant> or <constant>EUCLEAN</constant> (see <citerefentry
        project='man-pages'><refentrytitle>errno</refentrytitle><manvolnum>3</manvolnum></citerefentry> for a
        full list). This value will be returned when a deny-listed system call is triggered, instead of
        terminating the processes immediately. Special setting <literal>kill</literal> can be used to
        explicitly specify killing. This value takes precedence over the one given in
        <varname>SystemCallErrorNumber=</varname>, see below. If running in user mode, or in system mode,
        but without the <constant>CAP_SYS_ADMIN</constant> capability (e.g. setting
        <varname>User=</varname>), <varname>NoNewPrivileges=yes</varname> is implied. This feature
        makes use of the Secure Computing Mode 2 interfaces of the kernel ('seccomp filtering') and is useful
        for enforcing a minimal sandboxing environment. Note that the <function>execve()</function>,
        <function>exit()</function>, <function>exit_group()</function>, <function>getrlimit()</function>,
        <function>rt_sigreturn()</function>, <function>sigreturn()</function> system calls and the system calls
        for querying time and sleeping are implicitly allow-listed and do not need to be listed
        explicitly. This option may be specified more than once, in which case the filter masks are
        merged. If the empty string is assigned, the filter is reset, all prior assignments will have no
        effect. This does not affect commands prefixed with <literal>+</literal>.</para>

        <para>Note that on systems supporting multiple ABIs (such as x86/x86-64) it is recommended to turn off
        alternative ABIs for services, so that they cannot be used to circumvent the restrictions of this
        option. Specifically, it is recommended to combine this option with
        <varname>SystemCallArchitectures=native</varname> or similar.</para>

        <para>Note that strict system call filters may impact execution and error handling code paths of the service
        invocation. Specifically, access to the <function>execve()</function> system call is required for the execution
        of the service binary — if it is blocked service invocation will necessarily fail. Also, if execution of the
        service binary fails for some reason (for example: missing service executable), the error handling logic might
        require access to an additional set of system calls in order to process and log this failure correctly. It
        might be necessary to temporarily disable system call filters in order to simplify debugging of such
        failures.</para>

        <para>If you specify both types of this option (i.e. allow-listing and deny-listing), the first
        encountered will take precedence and will dictate the default action (termination or approval of a
        system call). Then the next occurrences of this option will add or delete the listed system calls
        from the set of the filtered system calls, depending of its type and the default action. (For
        example, if you have started with an allow list rule for <function>read()</function> and
        <function>write()</function>, and right after it add a deny list rule for <function>write()</function>,
        then <function>write()</function> will be removed from the set.)</para>

        <para>As the number of possible system calls is large, predefined sets of system calls are provided. A set
        starts with <literal>@</literal> character, followed by name of the set.

        <table>
          <title>Currently predefined system call sets</title>

          <tgroup cols='2'>
            <colspec colname='set' />
            <colspec colname='description' />
            <thead>
              <row>
                <entry>Set</entry>
                <entry>Description</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry>@aio</entry>
                <entry>Asynchronous I/O (<citerefentry project='man-pages'><refentrytitle>io_setup</refentrytitle><manvolnum>2</manvolnum></citerefentry>, <citerefentry project='man-pages'><refentrytitle>io_submit</refentrytitle><manvolnum>2</manvolnum></citerefentry>, and related calls)</entry>
              </row>
              <row>
                <entry>@basic-io</entry>
                <entry>System calls for basic I/O: reading, writing, seeking, file descriptor duplication and closing (<citerefentry project='man-pages'><refentrytitle>read</refentrytitle><manvolnum>2</manvolnum></citerefentry>, <citerefentry project='man-pages'><refentrytitle>write</refentrytitle><manvolnum>2</manvolnum></citerefentry>, and related calls)</entry>
              </row>
              <row>
                <entry>@chown</entry>
                <entry>Changing file ownership (<citerefentry project='man-pages'><refentrytitle>chown</refentrytitle><manvolnum>2</manvolnum></citerefentry>, <citerefentry project='man-pages'><refentrytitle>fchownat</refentrytitle><manvolnum>2</manvolnum></citerefentry>, and related calls)</entry>
              </row>
              <row>
                <entry>@clock</entry>
                <entry>System calls for changing the system clock (<citerefentry project='man-pages'><refentrytitle>adjtimex</refentrytitle><manvolnum>2</manvolnum></citerefentry>, <citerefentry project='man-pages'><refentrytitle>settimeofday</refentrytitle><manvolnum>2</manvolnum></citerefentry>, and related calls)</entry>
              </row>
              <row>
                <entry>@cpu-emulation</entry>
                <entry>System calls for CPU emulation functionality (<citerefentry project='man-pages'><refentrytitle>vm86</refentrytitle><manvolnum>2</manvolnum></citerefentry> and related calls)</entry>
              </row>
              <row>
                <entry>@debug</entry>
                <entry>Debugging, performance monitoring and tracing functionality (<citerefentry project='man-pages'><refentrytitle>ptrace</refentrytitle><manvolnum>2</manvolnum></citerefentry>, <citerefentry project='man-pages'><refentrytitle>perf_event_open</refentrytitle><manvolnum>2</manvolnum></citerefentry> and related calls)</entry>
              </row>
              <row>
                <entry>@file-system</entry>
                <entry>File system operations: opening, creating files and directories for read and write, renaming and removing them, reading file properties, or creating hard and symbolic links</entry>
              </row>
              <row>
                <entry>@io-event</entry>
                <entry>Event loop system calls (<citerefentry project='man-pages'><refentrytitle>poll</refentrytitle><manvolnum>2</manvolnum></citerefentry>, <citerefentry project='man-pages'><refentrytitle>select</refentrytitle><manvolnum>2</manvolnum></citerefentry>, <citerefentry project='man-pages'><refentrytitle>epoll</refentrytitle><manvolnum>7</manvolnum></citerefentry>, <citerefentry project='man-pages'><refentrytitle>eventfd</refentrytitle><manvolnum>2</manvolnum></citerefentry> and related calls)</entry>
              </row>
              <row>
                <entry>@ipc</entry>
                <entry>Pipes, SysV IPC, POSIX Message Queues and other IPC (<citerefentry project='man-pages'><refentrytitle>mq_overview</refentrytitle><manvolnum>7</manvolnum></citerefentry>, <citerefentry project='man-pages'><refentrytitle>svipc</refentrytitle><manvolnum>7</manvolnum></citerefentry>)</entry>
              </row>
              <row>
                <entry>@keyring</entry>
                <entry>Kernel keyring access (<citerefentry project='man-pages'><refentrytitle>keyctl</refentrytitle><manvolnum>2</manvolnum></citerefentry> and related calls)</entry>
              </row>
              <row>
                <entry>@memlock</entry>
                <entry>Locking of memory in RAM (<citerefentry project='man-pages'><refentrytitle>mlock</refentrytitle><manvolnum>2</manvolnum></citerefentry>, <citerefentry project='man-pages'><refentrytitle>mlockall</refentrytitle><manvolnum>2</manvolnum></citerefentry> and related calls)</entry>
              </row>
              <row>
                <entry>@module</entry>
                <entry>Loading and unloading of kernel modules (<citerefentry project='man-pages'><refentrytitle>init_module</refentrytitle><manvolnum>2</manvolnum></citerefentry>, <citerefentry project='man-pages'><refentrytitle>delete_module</refentrytitle><manvolnum>2</manvolnum></citerefentry> and related calls)</entry>
              </row>
              <row>
                <entry>@mount</entry>
                <entry>Mounting and unmounting of file systems (<citerefentry project='man-pages'><refentrytitle>mount</refentrytitle><manvolnum>2</manvolnum></citerefentry>, <citerefentry project='man-pages'><refentrytitle>chroot</refentrytitle><manvolnum>2</manvolnum></citerefentry>, and related calls)</entry>
              </row>
              <row>
                <entry>@network-io</entry>
                <entry>Socket I/O (including local AF_UNIX): <citerefentry project='man-pages'><refentrytitle>socket</refentrytitle><manvolnum>7</manvolnum></citerefentry>, <citerefentry project='man-pages'><refentrytitle>unix</refentrytitle><manvolnum>7</manvolnum></citerefentry></entry>
              </row>
              <row>
                <entry>@obsolete</entry>
                <entry>Unusual, obsolete or unimplemented (<citerefentry project='man-pages'><refentrytitle>create_module</refentrytitle><manvolnum>2</manvolnum></citerefentry>, <citerefentry project='man-pages'><refentrytitle>gtty</refentrytitle><manvolnum>2</manvolnum></citerefentry>, …)</entry>
              </row>
              <row>
                <entry>@pkey</entry>
                <entry>System calls that deal with memory protection keys (<citerefentry project='man-pages'><refentrytitle>pkeys</refentrytitle><manvolnum>7</manvolnum></citerefentry>)</entry>
              </row>
              <row>
                <entry>@privileged</entry>
                <entry>All system calls which need super-user capabilities (<citerefentry project='man-pages'><refentrytitle>capabilities</refentrytitle><manvolnum>7</manvolnum></citerefentry>)</entry>
              </row>
              <row>
                <entry>@process</entry>
                <entry>Process control, execution, namespacing operations (<citerefentry project='man-pages'><refentrytitle>clone</refentrytitle><manvolnum>2</manvolnum></citerefentry>, <citerefentry project='man-pages'><refentrytitle>kill</refentrytitle><manvolnum>2</manvolnum></citerefentry>, <citerefentry project='man-pages'><refentrytitle>namespaces</refentrytitle><manvolnum>7</manvolnum></citerefentry>, …)</entry>
              </row>
              <row>
                <entry>@raw-io</entry>
                <entry>Raw I/O port access (<citerefentry project='man-pages'><refentrytitle>ioperm</refentrytitle><manvolnum>2</manvolnum></citerefentry>, <citerefentry project='man-pages'><refentrytitle>iopl</refentrytitle><manvolnum>2</manvolnum></citerefentry>, <function>pciconfig_read()</function>, …)</entry>
              </row>
              <row>
                <entry>@reboot</entry>
                <entry>System calls for rebooting and reboot preparation (<citerefentry project='man-pages'><refentrytitle>reboot</refentrytitle><manvolnum>2</manvolnum></citerefentry>, <function>kexec()</function>, …)</entry>
              </row>
              <row>
                <entry>@resources</entry>
                <entry>System calls for changing resource limits, memory and scheduling parameters (<citerefentry project='man-pages'><refentrytitle>setrlimit</refentrytitle><manvolnum>2</manvolnum></citerefentry>, <citerefentry project='man-pages'><refentrytitle>setpriority</refentrytitle><manvolnum>2</manvolnum></citerefentry>, …)</entry>
              </row>
              <row>
                <entry>@sandbox</entry>
                <entry>System calls for sandboxing programs (<citerefentry project='man-pages'><refentrytitle>seccomp</refentrytitle><manvolnum>2</manvolnum></citerefentry>, Landlock system calls, …)</entry>
              </row>
              <row>
                <entry>@setuid</entry>
                <entry>System calls for changing user ID and group ID credentials, (<citerefentry project='man-pages'><refentrytitle>setuid</refentrytitle><manvolnum>2</manvolnum></citerefentry>, <citerefentry project='man-pages'><refentrytitle>setgid</refentrytitle><manvolnum>2</manvolnum></citerefentry>, <citerefentry project='man-pages'><refentrytitle>setresuid</refentrytitle><manvolnum>2</manvolnum></citerefentry>, …)</entry>
              </row>
              <row>
                <entry>@signal</entry>
                <entry>System calls for manipulating and handling process signals (<citerefentry project='man-pages'><refentrytitle>signal</refentrytitle><manvolnum>2</manvolnum></citerefentry>, <citerefentry project='man-pages'><refentrytitle>sigprocmask</refentrytitle><manvolnum>2</manvolnum></citerefentry>, …)</entry>
              </row>
              <row>
                <entry>@swap</entry>
                <entry>System calls for enabling/disabling swap devices (<citerefentry project='man-pages'><refentrytitle>swapon</refentrytitle><manvolnum>2</manvolnum></citerefentry>, <citerefentry project='man-pages'><refentrytitle>swapoff</refentrytitle><manvolnum>2</manvolnum></citerefentry>)</entry>
              </row>
              <row>
                <entry>@sync</entry>
                <entry>Synchronizing files and memory to disk (<citerefentry project='man-pages'><refentrytitle>fsync</refentrytitle><manvolnum>2</manvolnum></citerefentry>, <citerefentry project='man-pages'><refentrytitle>msync</refentrytitle><manvolnum>2</manvolnum></citerefentry>, and related calls)</entry>
              </row>
              <row>
                <entry>@system-service</entry>
                <entry>A reasonable set of system calls used by common system services, excluding any special purpose calls. This is the recommended starting point for allow-listing system calls for system services, as it contains what is typically needed by system services, but excludes overly specific interfaces. For example, the following APIs are excluded: <literal>@clock</literal>, <literal>@mount</literal>, <literal>@swap</literal>, <literal>@reboot</literal>.</entry>
              </row>
              <row>
                <entry>@timer</entry>
                <entry>System calls for scheduling operations by time (<citerefentry project='man-pages'><refentrytitle>alarm</refentrytitle><manvolnum>2</manvolnum></citerefentry>, <citerefentry project='man-pages'><refentrytitle>timer_create</refentrytitle><manvolnum>2</manvolnum></citerefentry>, …)</entry>
              </row>
              <row>
                <entry>@known</entry>
                <entry>All system calls defined by the kernel. This list is defined statically in systemd based on a kernel version that was available when this systemd version was released. It will become progressively more out-of-date as the kernel is updated.</entry>
              </row>
            </tbody>
          </tgroup>
        </table>

        Note, that as new system calls are added to the kernel, additional system calls might be added to the groups
        above. Contents of the sets may also change between systemd versions. In addition, the list of system calls
        depends on the kernel version and architecture for which systemd was compiled. Use
        <command>systemd-analyze syscall-filter</command> to list the actual list of system calls in each
        filter.</para>

        <para>Generally, allow-listing system calls (rather than deny-listing) is the safer mode of
        operation. It is recommended to enforce system call allow lists for all long-running system
        services. Specifically, the following lines are a relatively safe basic choice for the majority of
        system services:</para>

        <programlisting>[Service]
SystemCallFilter=@system-service
SystemCallErrorNumber=EPERM</programlisting>

        <para>Note that various kernel system calls are defined redundantly: there are multiple system calls
        for executing the same operation. For example, the <function>pidfd_send_signal()</function> system
        call may be used to execute operations similar to what can be done with the older
        <function>kill()</function> system call, hence blocking the latter without the former only provides
        weak protection. Since new system calls are added regularly to the kernel as development progresses,
        keeping system call deny lists comprehensive requires constant work. It is thus recommended to use
        allow-listing instead, which offers the benefit that new system calls are by default implicitly
        blocked until the allow list is updated.</para>

        <para>Also note that a number of system calls are required to be accessible for the dynamic linker to
        work. The dynamic linker is required for running most regular programs (specifically: all dynamic ELF
        binaries, which is how most distributions build packaged programs). This means that blocking these
        system calls (which include <function>open()</function>, <function>openat()</function> or
        <function>mmap()</function>) will make most programs typically shipped with generic distributions
        unusable.</para>

        <para>It is recommended to combine the file system namespacing related options with
        <varname>SystemCallFilter=~@mount</varname>, in order to prohibit the unit's processes to undo the
        mappings. Specifically these are the options <varname>PrivateTmp=</varname>,
        <varname>PrivateDevices=</varname>, <varname>ProtectSystem=</varname>, <varname>ProtectHome=</varname>,
        <varname>ProtectKernelTunables=</varname>, <varname>ProtectControlGroups=</varname>,
        <varname>ProtectKernelLogs=</varname>, <varname>ProtectClock=</varname>, <varname>ReadOnlyPaths=</varname>,
        <varname>InaccessiblePaths=</varname> and <varname>ReadWritePaths=</varname>.</para>

        <xi:include href="version-info.xml" xpointer="v187"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>SystemCallErrorNumber=</varname></term>

        <listitem><para>Takes an <literal>errno</literal> error number (between 1 and 4095) or errno name
        such as <constant>EPERM</constant>, <constant>EACCES</constant> or <constant>EUCLEAN</constant>, to
        return when the system call filter configured with <varname>SystemCallFilter=</varname> is triggered,
        instead of terminating the process immediately. See <citerefentry
        project='man-pages'><refentrytitle>errno</refentrytitle><manvolnum>3</manvolnum></citerefentry> for a
        full list of error codes. When this setting is not used, or when the empty string or the special
        setting <literal>kill</literal> is assigned, the process will be terminated immediately when the
        filter is triggered.</para>

        <xi:include href="version-info.xml" xpointer="v209"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>SystemCallArchitectures=</varname></term>

        <listitem><para>Takes a space-separated list of architecture identifiers to include in the system call
        filter. The known architecture identifiers are the same as for <varname>ConditionArchitecture=</varname>
        described in <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
        as well as <constant>x32</constant>, <constant>mips64-n32</constant>, <constant>mips64-le-n32</constant>, and
        the special identifier <constant>native</constant>. The special identifier <constant>native</constant>
        implicitly maps to the native architecture of the system (or more precisely: to the architecture the system
        manager is compiled for). If running in user mode, or in system mode, but without the
        <constant>CAP_SYS_ADMIN</constant> capability (e.g. setting <varname>User=</varname>),
        <varname>NoNewPrivileges=yes</varname> is implied. By default, this option is set to the empty list, i.e. no
        filtering is applied.</para>

        <para>If this setting is used, processes of this unit will only be permitted to call native system calls, and
        system calls of the specified architectures. For the purposes of this option, the x32 architecture is treated
        as including x86-64 system calls. However, this setting still fulfills its purpose, as explained below, on
        x32.</para>

        <para>System call filtering is not equally effective on all architectures. For example, on x86
        filtering of network socket-related calls is not possible, due to ABI limitations — a limitation that x86-64
        does not have, however. On systems supporting multiple ABIs at the same time — such as x86/x86-64 — it is hence
        recommended to limit the set of permitted system call architectures so that secondary ABIs may not be used to
        circumvent the restrictions applied to the native ABI of the system. In particular, setting
        <varname>SystemCallArchitectures=native</varname> is a good choice for disabling non-native ABIs.</para>

        <para>System call architectures may also be restricted system-wide via the
        <varname>SystemCallArchitectures=</varname> option in the global configuration. See
        <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
        details.</para>

        <xi:include href="version-info.xml" xpointer="v209"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>SystemCallLog=</varname></term>

        <listitem><para>Takes a space-separated list of system call names. If this setting is used, all
        system calls executed by the unit processes for the listed ones will be logged. If the first
        character of the list is <literal>~</literal>, the effect is inverted: all system calls except the
        listed system calls will be logged. If running in user mode, or in system mode, but without the
        <constant>CAP_SYS_ADMIN</constant> capability (e.g. setting <varname>User=</varname>),
        <varname>NoNewPrivileges=yes</varname> is implied. This feature makes use of the Secure Computing
        Mode 2 interfaces of the kernel ('seccomp filtering') and is useful for auditing or setting up a
        minimal sandboxing environment. This option may be specified more than once, in which case the filter
        masks are merged. If the empty string is assigned, the filter is reset, all prior assignments will
        have no effect. This does not affect commands prefixed with <literal>+</literal>.</para>

        <xi:include href="version-info.xml" xpointer="v247"/></listitem>
      </varlistentry>

    </variablelist>
  </refsect1>

  <refsect1>
    <title>Environment</title>

    <variablelist class='unit-directives'>

      <varlistentry>
        <term><varname>Environment=</varname></term>

        <listitem><para>Sets environment variables for executed processes. Each line is unquoted using the
        rules described in "Quoting" section in
        <citerefentry><refentrytitle>systemd.syntax</refentrytitle><manvolnum>7</manvolnum></citerefentry>
        and becomes a list of variable assignments. If you need to assign a value containing spaces or the
        equals sign to a variable, put quotes around the whole assignment. Variable expansion is not
        performed inside the strings and the <literal>$</literal> character has no special meaning. Specifier
        expansion is performed, see the "Specifiers" section in
        <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
        </para>

        <para>This option may be specified more than once, in which case all listed variables will be set. If
        the same variable is listed twice, the later setting will override the earlier setting. If the empty
        string is assigned to this option, the list of environment variables is reset, all prior assignments
        have no effect.</para>

        <para>The names of the variables can contain ASCII letters, digits, and the underscore character.
        Variable names cannot be empty or start with a digit. In variable values, most characters are
        allowed, but non-printable characters are currently rejected.</para>

        <para>Example:
        <programlisting>Environment="VAR1=word1 word2" VAR2=word3 "VAR3=$word 5 6"</programlisting>
        gives three variables <literal>VAR1</literal>,
        <literal>VAR2</literal>, <literal>VAR3</literal>
        with the values <literal>word1 word2</literal>,
        <literal>word3</literal>, <literal>$word 5 6</literal>.
        </para>

        <para>See <citerefentry
        project='man-pages'><refentrytitle>environ</refentrytitle><manvolnum>7</manvolnum></citerefentry> for
        details about environment variables.</para>

        <para>Note that environment variables are not suitable for passing secrets (such as passwords, key
        material, …)  to service processes. Environment variables set for a unit are exposed to unprivileged
        clients via D-Bus IPC, and generally not understood as being data that requires protection. Moreover,
        environment variables are propagated down the process tree, including across security boundaries
        (such as setuid/setgid executables), and hence might leak to processes that should not have access to
        the secret data. Use <varname>LoadCredential=</varname>, <varname>LoadCredentialEncrypted=</varname>
        or <varname>SetCredentialEncrypted=</varname> (see below) to pass data to unit processes
        securely.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>EnvironmentFile=</varname></term>

        <listitem><para>Similar to <varname>Environment=</varname>, but reads the environment variables from
        a text file. The text file should contain newline-separated variable assignments. Empty lines, lines
        without an <literal>=</literal> separator, or lines starting with <literal>;</literal> or
        <literal>#</literal> will be ignored, which may be used for commenting. The file must be UTF-8
        encoded. Valid characters are <ulink
        url="https://www.unicode.org/glossary/#unicode_scalar_value">unicode scalar values</ulink> other than
        <ulink url="https://www.unicode.org/glossary/#noncharacter">noncharacters</ulink>, U+0000 NUL, and
        U+FEFF <ulink url="https://www.unicode.org/glossary/#byte_order_mark">byte order mark</ulink>.
        Control codes other than NUL are allowed.</para>

        <para>In the file, an unquoted value after the <literal>=</literal> is parsed with the same backslash-escape
        rules as <ulink
        url="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_02_01">unquoted
        text</ulink> in a POSIX shell, but unlike in a shell, interior whitespace is preserved and quotes after the
        first non-whitespace character are preserved. Leading and trailing whitespace (space, tab, carriage return) is
        discarded, but interior whitespace within the line is preserved verbatim. A line ending with a backslash will be
        continued to the following one, with the newline itself discarded. A backslash
        <literal>\</literal> followed by any character other than newline will preserve the following character, so that
        <literal>\\</literal> will become the value <literal>\</literal>.</para>

        <para>In the file, a <literal>'</literal>-quoted value after the <literal>=</literal> can span multiple lines
        and contain any character verbatim other than single quote, like <ulink
        url="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_02_02">single-quoted
        text</ulink> in a POSIX shell. No backslash-escape sequences are recognized. Leading and trailing whitespace
        outside of the single quotes is discarded.</para>

        <para>In the file, a <literal>"</literal>-quoted value after the <literal>=</literal> can span multiple lines,
        and the same escape sequences are recognized as in <ulink
        url="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_02_03">double-quoted
        text</ulink> of a POSIX shell. Backslash (<literal>\</literal>) followed by any of <literal>"\`$</literal> will
        preserve that character. A backslash followed by newline is a line continuation, and the newline itself is
        discarded. A backslash followed by any other character is ignored; both the backslash and the following
        character are preserved verbatim. Leading and trailing whitespace outside of the double quotes is
        discarded.</para>

        <para>The argument passed should be an absolute filename or wildcard expression, optionally prefixed with
        <literal>-</literal>, which indicates that if the file does not exist, it will not be read and no error or
        warning message is logged. This option may be specified more than once in which case all specified files are
        read. If the empty string is assigned to this option, the list of file to read is reset, all prior assignments
        have no effect.</para>

        <para>The files listed with this directive will be read shortly before the process is executed (more
        specifically, after all processes from a previous unit state terminated. This means you can generate these
        files in one unit state, and read it with this option in the next. The files are read from the file
        system of the service manager, before any file system changes like bind mounts take place).</para>

        <para>Settings from these files override settings made with <varname>Environment=</varname>. If the same
        variable is set twice from these files, the files will be read in the order they are specified and the later
        setting will override the earlier setting.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>PassEnvironment=</varname></term>

        <listitem><para>Pass environment variables set for the system service manager to executed processes. Takes a
        space-separated list of variable names. This option may be specified more than once, in which case all listed
        variables will be passed. If the empty string is assigned to this option, the list of environment variables to
        pass is reset, all prior assignments have no effect. Variables specified that are not set for the system
        manager will not be passed and will be silently ignored. Note that this option is only relevant for the system
        service manager, as system services by default do not automatically inherit any environment variables set for
        the service manager itself. However, in case of the user service manager all environment variables are passed
        to the executed processes anyway, hence this option is without effect for the user service manager.</para>

        <para>Variables set for invoked processes due to this setting are subject to being overridden by those
        configured with <varname>Environment=</varname> or <varname>EnvironmentFile=</varname>.</para>

        <para>Example:
        <programlisting>PassEnvironment=VAR1 VAR2 VAR3</programlisting>
        passes three variables <literal>VAR1</literal>,
        <literal>VAR2</literal>, <literal>VAR3</literal>
        with the values set for those variables in PID1.</para>

        <para>
        See <citerefentry
        project='man-pages'><refentrytitle>environ</refentrytitle><manvolnum>7</manvolnum></citerefentry> for details
        about environment variables.</para>

        <xi:include href="version-info.xml" xpointer="v228"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>UnsetEnvironment=</varname></term>

        <listitem><para>Explicitly unset environment variable assignments that would normally be passed from the
        service manager to invoked processes of this unit. Takes a space-separated list of variable names or variable
        assignments. This option may be specified more than once, in which case all listed variables/assignments will
        be unset. If the empty string is assigned to this option, the list of environment variables/assignments to
        unset is reset. If a variable assignment is specified (that is: a variable name, followed by
        <literal>=</literal>, followed by its value), then any environment variable matching this precise assignment is
        removed. If a variable name is specified (that is a variable name without any following <literal>=</literal> or
        value), then any assignment matching the variable name, regardless of its value is removed. Note that the
        effect of <varname>UnsetEnvironment=</varname> is applied as final step when the environment list passed to
        executed processes is compiled. That means it may undo assignments from any configuration source, including
        assignments made through <varname>Environment=</varname> or <varname>EnvironmentFile=</varname>, inherited from
        the system manager's global set of environment variables, inherited via <varname>PassEnvironment=</varname>,
        set by the service manager itself (such as <varname>$NOTIFY_SOCKET</varname> and such), or set by a PAM module
        (in case <varname>PAMName=</varname> is used).</para>

        <para>See "Environment Variables in Spawned Processes" below for a description of how those
        settings combine to form the inherited environment. See <citerefentry
        project='man-pages'><refentrytitle>environ</refentrytitle><manvolnum>7</manvolnum></citerefentry> for general
        information about environment variables.</para>

        <xi:include href="version-info.xml" xpointer="v235"/></listitem>
      </varlistentry>

    </variablelist>
  </refsect1>

  <refsect1>
    <title>Logging and Standard Input/Output</title>

    <variablelist class='unit-directives'>
      <varlistentry>

        <term><varname>StandardInput=</varname></term>

        <listitem><para>Controls where file descriptor 0 (STDIN) of the executed processes is connected to. Takes one
        of <option>null</option>, <option>tty</option>, <option>tty-force</option>, <option>tty-fail</option>,
        <option>data</option>, <option>file:<replaceable>path</replaceable></option>, <option>socket</option> or
        <option>fd:<replaceable>name</replaceable></option>.</para>

        <para>If <option>null</option> is selected, standard input will be connected to <filename>/dev/null</filename>,
        i.e. all read attempts by the process will result in immediate EOF.</para>

        <para>If <option>tty</option> is selected, standard input is connected to a TTY (as configured by
        <varname>TTYPath=</varname>, see below) and the executed process becomes the controlling process of the
        terminal. If the terminal is already being controlled by another process, the executed process waits until the
        current controlling process releases the terminal.</para>

        <para><option>tty-force</option> is similar to <option>tty</option>, but the executed process is forcefully and
        immediately made the controlling process of the terminal, potentially removing previous controlling processes
        from the terminal.</para>

        <para><option>tty-fail</option> is similar to <option>tty</option>, but if the terminal already has a
        controlling process start-up of the executed process fails.</para>

        <para>The <option>data</option> option may be used to configure arbitrary textual or binary data to pass via
        standard input to the executed process. The data to pass is configured via
        <varname>StandardInputText=</varname>/<varname>StandardInputData=</varname> (see below). Note that the actual
        file descriptor type passed (memory file, regular file, UNIX pipe, …) might depend on the kernel and available
        privileges. In any case, the file descriptor is read-only, and when read returns the specified data followed by
        EOF.</para>

        <para>The <option>file:<replaceable>path</replaceable></option> option may be used to connect a specific file
        system object to standard input. An absolute path following the <literal>:</literal> character is expected,
        which may refer to a regular file, a FIFO or special file. If an <constant>AF_UNIX</constant> socket in the
        file system is specified, a stream socket is connected to it. The latter is useful for connecting standard
        input of processes to arbitrary system services.</para>

        <para>The <option>socket</option> option is valid in socket-activated services only, and requires the relevant
        socket unit file (see
        <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry> for details)
        to have <varname>Accept=yes</varname> set, or to specify a single socket only. If this option is set, standard
        input will be connected to the socket the service was activated from, which is primarily useful for
        compatibility with daemons designed for use with the traditional <citerefentry
        project='freebsd'><refentrytitle>inetd</refentrytitle><manvolnum>8</manvolnum></citerefentry> socket activation
        daemon.</para>

        <para>The <option>fd:<replaceable>name</replaceable></option> option connects standard input to a specific,
        named file descriptor provided by a socket unit. The name may be specified as part of this option, following a
        <literal>:</literal> character (e.g. <literal>fd:foobar</literal>). If no name is specified, the name
        <literal>stdin</literal> is implied (i.e. <literal>fd</literal> is equivalent to <literal>fd:stdin</literal>).
        At least one socket unit defining the specified name must be provided via the <varname>Sockets=</varname>
        option, and the file descriptor name may differ from the name of its containing socket unit. If multiple
        matches are found, the first one will be used. See <varname>FileDescriptorName=</varname> in
        <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry> for more
        details about named file descriptors and their ordering.</para>

        <para>This setting defaults to <option>null</option>, unless
        <varname>StandardInputText=</varname>/<varname>StandardInputData=</varname> are set, in which case it
        defaults to <option>data</option>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>StandardOutput=</varname></term>

        <listitem><para>Controls where file descriptor 1 (stdout) of the executed processes is connected
        to. Takes one of <option>inherit</option>, <option>null</option>, <option>tty</option>,
        <option>journal</option>, <option>kmsg</option>, <option>journal+console</option>,
        <option>kmsg+console</option>, <option>file:<replaceable>path</replaceable></option>,
        <option>append:<replaceable>path</replaceable></option>, <option>truncate:<replaceable>path</replaceable></option>,
        <option>socket</option> or <option>fd:<replaceable>name</replaceable></option>.</para>

        <para><option>inherit</option> duplicates the file descriptor of standard input for standard output.</para>

        <para><option>null</option> connects standard output to <filename>/dev/null</filename>, i.e. everything written
        to it will be lost.</para>

        <para><option>tty</option> connects standard output to a tty (as configured via <varname>TTYPath=</varname>,
        see below). If the TTY is used for output only, the executed process will not become the controlling process of
        the terminal, and will not fail or wait for other processes to release the terminal.</para>

        <para><option>journal</option> connects standard output with the journal, which is accessible via
        <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>. Note
        that everything that is written to kmsg (see below) is implicitly stored in the journal as well, the
        specific option listed below is hence a superset of this one. (Also note that any external,
        additional syslog daemons receive their log data from the journal, too, hence this is the option to
        use when logging shall be processed with such a daemon.)</para>

        <para><option>kmsg</option> connects standard output with the kernel log buffer which is accessible via
        <citerefentry project='man-pages'><refentrytitle>dmesg</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
        in addition to the journal. The journal daemon might be configured to send all logs to kmsg anyway, in which
        case this option is no different from <option>journal</option>.</para>

        <para><option>journal+console</option> and <option>kmsg+console</option> work in a similar way as the
        two options above but copy the output to the system console as well.</para>

        <para>The <option>file:<replaceable>path</replaceable></option> option may be used to connect a specific file
        system object to standard output. The semantics are similar to the same option of
        <varname>StandardInput=</varname>, see above. If <replaceable>path</replaceable> refers to a regular file
        on the filesystem, it is opened (created if it doesn't exist yet) for writing at the beginning of the file,
        but without truncating it.
        If standard input and output are directed to the same file path, it is opened only once — for reading as well
        as writing — and duplicated. This is particularly useful when the specified path refers to an
        <constant>AF_UNIX</constant> socket in the file system, as in that case only a
        single stream connection is created for both input and output.</para>

        <para><option>append:<replaceable>path</replaceable></option> is similar to
        <option>file:<replaceable>path</replaceable></option> above, but it opens the file in append mode.
        </para>

        <para><option>truncate:<replaceable>path</replaceable></option> is similar to
        <option>file:<replaceable>path</replaceable></option> above, but it truncates the file when opening
        it. For units with multiple command lines, e.g. <varname>Type=oneshot</varname> services with
        multiple <varname>ExecStart=</varname>, or services with <varname>ExecCondition=</varname>,
        <varname>ExecStartPre=</varname> or <varname>ExecStartPost=</varname>, the output file is reopened
        and therefore re-truncated for each command line. If the output file is truncated while another
        process still has the file open, e.g. by an <varname>ExecReload=</varname> running concurrently with
        an <varname>ExecStart=</varname>, and the other process continues writing to the file without
        adjusting its offset, then the space between the file pointers of the two processes may be filled
        with <constant>NUL</constant> bytes, producing a sparse file. Thus,
        <option>truncate:<replaceable>path</replaceable></option> is typically only useful for units where
        only one process runs at a time, such as services with a single <varname>ExecStart=</varname> and no
        <varname>ExecStartPost=</varname>, <varname>ExecReload=</varname>, <varname>ExecStop=</varname> or
        similar.</para>

        <para><option>socket</option> connects standard output to a socket acquired via socket activation. The
        semantics are similar to the same option of <varname>StandardInput=</varname>, see above.</para>

        <para>The <option>fd:<replaceable>name</replaceable></option> option connects standard output to a
        specific, named file descriptor provided by a socket unit. A name may be specified as part of this
        option, following a <literal>:</literal> character
        (e.g. <literal>fd:<replaceable>foobar</replaceable></literal>). If no name is specified, the name
        <literal>stdout</literal> is implied (i.e. <literal>fd</literal> is equivalent to
        <literal>fd:stdout</literal>). At least one socket unit defining the specified name must be provided
        via the <varname>Sockets=</varname> option, and the file descriptor name may differ from the name of
        its containing socket unit. If multiple matches are found, the first one will be used. See
        <varname>FileDescriptorName=</varname> in
        <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>
        for more details about named descriptors and their ordering.</para>

        <para>If the standard output (or error output, see below) of a unit is connected to the journal or
        the kernel log buffer, the unit will implicitly gain a dependency of type <varname>After=</varname>
        on <filename>systemd-journald.socket</filename> (also see the "Implicit Dependencies" section
        above). Also note that in this case stdout (or stderr, see below) will be an
        <constant>AF_UNIX</constant> stream socket, and not a pipe or FIFO that can be re-opened. This means
        when executing shell scripts the construct <command>echo "hello" &gt; /dev/stderr</command> for
        writing text to stderr will not work. To mitigate this use the construct <command>echo "hello"
        >&amp;2</command> instead, which is mostly equivalent and avoids this pitfall.</para>

        <para>If <varname>StandardInput=</varname> is set to one of <option>tty</option>, <option>tty-force</option>,
        <option>tty-fail</option>, <option>socket</option>, or <option>fd:<replaceable>name</replaceable></option>, this
        setting defaults to <option>inherit</option>.</para>

        <para>In other cases, this setting defaults to the value set with <varname>DefaultStandardOutput=</varname> in
        <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>, which
        defaults to <option>journal</option>. Note that setting this parameter might result in additional dependencies
        to be added to the unit (see above).</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>StandardError=</varname></term>

        <listitem><para>Controls where file descriptor 2 (stderr) of the executed processes is connected to. The
        available options are identical to those of <varname>StandardOutput=</varname>, with some exceptions: if set to
        <option>inherit</option> the file descriptor used for standard output is duplicated for standard error, while
        <option>fd:<replaceable>name</replaceable></option> will use a default file descriptor name of
        <literal>stderr</literal>.</para>

        <para>This setting defaults to the value set with <varname>DefaultStandardError=</varname> in
        <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>, which
        defaults to <option>inherit</option>. Note that setting this parameter might result in additional dependencies
        to be added to the unit (see above).</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>StandardInputText=</varname></term>
        <term><varname>StandardInputData=</varname></term>

        <listitem><para>Configures arbitrary textual or binary data to pass via file descriptor 0 (STDIN) to
        the executed processes. These settings have no effect unless <varname>StandardInput=</varname> is set
        to <option>data</option> (which is the default if <varname>StandardInput=</varname> is not set
        otherwise, but <varname>StandardInputText=</varname>/<varname>StandardInputData=</varname> is). Use
        this option to embed process input data directly in the unit file.</para>

        <para><varname>StandardInputText=</varname> accepts arbitrary textual data. C-style escapes for special
        characters as well as the usual <literal>%</literal>-specifiers are resolved. Each time this setting is used
        the specified text is appended to the per-unit data buffer, followed by a newline character (thus every use
        appends a new line to the end of the buffer). Note that leading and trailing whitespace of lines configured
        with this option is removed. If an empty line is specified the buffer is cleared (hence, in order to insert an
        empty line, add an additional <literal>\n</literal> to the end or beginning of a line).</para>

        <para><varname>StandardInputData=</varname> accepts arbitrary binary data, encoded in <ulink
        url="https://tools.ietf.org/html/rfc2045#section-6.8">Base64</ulink>. No escape sequences or specifiers are
        resolved. Any whitespace in the encoded version is ignored during decoding.</para>

        <para>Note that <varname>StandardInputText=</varname> and <varname>StandardInputData=</varname> operate on the
        same data buffer, and may be mixed in order to configure both binary and textual data for the same input
        stream. The textual or binary data is joined strictly in the order the settings appear in the unit
        file. Assigning an empty string to either will reset the data buffer.</para>

        <para>Please keep in mind that in order to maintain readability long unit file settings may be split into
        multiple lines, by suffixing each line (except for the last) with a <literal>\</literal> character (see
        <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
        details). This is particularly useful for large data configured with these two options. Example:</para>

        <programlisting>…
StandardInput=data
StandardInputData=V2XigLJyZSBubyBzdHJhbmdlcnMgdG8gbG92ZQpZb3Uga25vdyB0aGUgcnVsZXMgYW5kIHNvIGRv \
                  IEkKQSBmdWxsIGNvbW1pdG1lbnQncyB3aGF0IEnigLJtIHRoaW5raW5nIG9mCllvdSB3b3VsZG4n \
                  dCBnZXQgdGhpcyBmcm9tIGFueSBvdGhlciBndXkKSSBqdXN0IHdhbm5hIHRlbGwgeW91IGhvdyBJ \
                  J20gZmVlbGluZwpHb3R0YSBtYWtlIHlvdSB1bmRlcnN0YW5kCgpOZXZlciBnb25uYSBnaXZlIHlv \
                  dSB1cApOZXZlciBnb25uYSBsZXQgeW91IGRvd24KTmV2ZXIgZ29ubmEgcnVuIGFyb3VuZCBhbmQg \
                  ZGVzZXJ0IHlvdQpOZXZlciBnb25uYSBtYWtlIHlvdSBjcnkKTmV2ZXIgZ29ubmEgc2F5IGdvb2Ri \
                  eWUKTmV2ZXIgZ29ubmEgdGVsbCBhIGxpZSBhbmQgaHVydCB5b3UK
…</programlisting>

        <xi:include href="version-info.xml" xpointer="v236"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>LogLevelMax=</varname></term>

        <listitem><para>Configures filtering by log level of log messages generated by this unit. Takes a
        <command>syslog</command> log level, one of <option>emerg</option> (lowest log level, only highest priority
        messages), <option>alert</option>, <option>crit</option>, <option>err</option>, <option>warning</option>,
        <option>notice</option>, <option>info</option>, <option>debug</option> (highest log level, also lowest priority
        messages). See <citerefentry
        project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry> for
        details. By default no filtering is applied (i.e. the default maximum log level is <option>debug</option>). Use
        this option to configure the logging system to drop log messages of a specific service above the specified
        level. For example, set <varname>LogLevelMax=</varname><option>info</option> in order to turn off debug logging
        of a particularly chatty unit. Note that the configured level is applied to any log messages written by any
        of the processes belonging to this unit, as well as any log messages written by the system manager process
        (PID 1) in reference to this unit, sent via any supported logging protocol. The filtering is applied
        early in the logging pipeline, before any kind of further processing is done. Moreover, messages which pass
        through this filter successfully might still be dropped by filters applied at a later stage in the logging
        subsystem. For example, <varname>MaxLevelStore=</varname> configured in
        <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> might
        prohibit messages of higher log levels to be stored on disk, even though the per-unit
        <varname>LogLevelMax=</varname> permitted it to be processed.</para>

        <xi:include href="version-info.xml" xpointer="v236"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>LogExtraFields=</varname></term>

        <listitem><para>Configures additional log metadata fields to include in all log records generated by
        processes associated with this unit, including systemd. This setting takes one or more journal field
        assignments in the format <literal>FIELD=VALUE</literal> separated by whitespace. See
        <citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>
        for details on the journal field concept. Even though the underlying journal implementation permits
        binary field values, this setting accepts only valid UTF-8 values. To include space characters in a
        journal field value, enclose the assignment in double quotes ("). <!-- " fake closing quote for emacs-->
        The usual specifiers are expanded in all assignments (see below). Note that this setting is not only
        useful for attaching additional metadata to log records of a unit, but given that all fields and
        values are indexed may also be used to implement cross-unit log record matching. Assign an empty
        string to reset the list.</para>

        <xi:include href="version-info.xml" xpointer="v236"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>LogRateLimitIntervalSec=</varname></term>
        <term><varname>LogRateLimitBurst=</varname></term>

        <listitem><para>Configures the rate limiting that is applied to log messages generated by this unit.
        If, in the time interval defined by <varname>LogRateLimitIntervalSec=</varname>, more messages than
        specified in <varname>LogRateLimitBurst=</varname> are logged by a service, all further messages
        within the interval are dropped until the interval is over. A message about the number of dropped
        messages is generated. The time specification for <varname>LogRateLimitIntervalSec=</varname> may be
        specified in the following units: "s", "min", "h", "ms", "us". See
        <citerefentry><refentrytitle>systemd.time</refentrytitle><manvolnum>7</manvolnum></citerefentry> for
        details. The default settings are set by <varname>RateLimitIntervalSec=</varname> and
        <varname>RateLimitBurst=</varname> configured in
        <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
        Note that this only applies to log messages that are processed by the logging subsystem, i.e. by
        <citerefentry><refentrytitle>systemd-journald.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
        This means that if you connect a service's stderr directly to a file via
        <varname>StandardOutput=file:…</varname> or a similar setting, the rate limiting will not be applied
        to messages written that way (but it will be enforced for messages generated via
        <citerefentry project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
        and similar functions).</para>

        <xi:include href="version-info.xml" xpointer="v240"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>LogFilterPatterns=</varname></term>

        <listitem><para>Define an extended regular expression to filter log messages based on the
        <varname>MESSAGE=</varname> field of the structured message. If the first character of the pattern is
        <literal>~</literal>, log entries matching the pattern should be discarded. This option takes a single
        pattern as an argument but can be used multiple times to create a list of allowed and denied patterns.
        If the empty string is assigned, the filter is reset, and all prior assignments will have no effect.</para>

        <para>Because the <literal>~</literal> character is used to define denied patterns, it must be replaced
        with <literal>\x7e</literal> to allow a message starting with <literal>~</literal>. For example,
        <literal>~foobar</literal> would add a pattern matching <literal>foobar</literal> to the deny list, while
        <literal>\x7efoobar</literal> would add a pattern matching <literal>~foobar</literal> to the allow list.</para>

        <para>Log messages are tested against denied patterns (if any), then against allowed patterns
        (if any). If a log message matches any of the denied patterns, it will be discarded, whatever the
        allowed patterns. Then, remaining log messages are tested against allowed patterns. Messages matching
        against none of the allowed pattern are discarded. If no allowed patterns are defined, then all
        messages are processed directly after going through denied filters.</para>

        <para>Filtering is based on the unit for which <varname>LogFilterPatterns=</varname> is defined, meaning log
        messages coming from
        <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> about the
        unit are not taken into account. Filtered log messages won't be forwarded to traditional syslog daemons,
        the kernel log buffer (kmsg), the systemd console, or sent as wall messages to all logged-in
        users.</para>

        <xi:include href="version-info.xml" xpointer="v253"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>LogNamespace=</varname></term>

        <listitem><para>Run the unit's processes in the specified journal namespace. Expects a short
        user-defined string identifying the namespace. If not used the processes of the service are run in
        the default journal namespace, i.e. their log stream is collected and processed by
        <filename>systemd-journald.service</filename>. If this option is used any log data generated by
        processes of this unit (regardless if via the <function>syslog()</function>, journal native logging
        or stdout/stderr logging) is collected and processed by an instance of the
        <filename>systemd-journald@.service</filename> template unit, which manages the specified
        namespace. The log data is stored in a data store independent from the default log namespace's data
        store. See
        <citerefentry><refentrytitle>systemd-journald.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
        for details about journal namespaces.</para>

        <para>Internally, journal namespaces are implemented through Linux mount namespacing and
        over-mounting the directory that contains the relevant <constant>AF_UNIX</constant> sockets used for
        logging in the unit's mount namespace. Since mount namespaces are used this setting disconnects
        propagation of mounts from the unit's processes to the host, similarly to how
        <varname>ReadOnlyPaths=</varname> and similar settings describe above work. Journal namespaces may hence
        not be used for services that need to establish mount points on the host.</para>

        <para>When this option is used the unit will automatically gain ordering and requirement dependencies
        on the two socket units associated with the <filename>systemd-journald@.service</filename> instance
        so that they are automatically established prior to the unit starting up. Note that when this option
        is used log output of this service does not appear in the regular
        <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
        output, unless the <option>--namespace=</option> option is used.</para>

        <xi:include href="system-only.xml" xpointer="singular"/>

        <xi:include href="version-info.xml" xpointer="v245"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>SyslogIdentifier=</varname></term>

        <listitem><para>Sets the process name ("<command>syslog</command> tag") to prefix log lines sent to
        the logging system or the kernel log buffer with. If not set, defaults to the process name of the
        executed process. This option is only useful when <varname>StandardOutput=</varname> or
        <varname>StandardError=</varname> are set to <option>journal</option> or <option>kmsg</option> (or to
        the same settings in combination with <option>+console</option>) and only applies to log messages
        written to stdout or stderr.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>SyslogFacility=</varname></term>

        <listitem><para>Sets the <command>syslog</command> facility identifier to use when logging. One of
        <option>kern</option>, <option>user</option>, <option>mail</option>, <option>daemon</option>,
        <option>auth</option>, <option>syslog</option>, <option>lpr</option>, <option>news</option>,
        <option>uucp</option>, <option>cron</option>, <option>authpriv</option>, <option>ftp</option>,
        <option>local0</option>, <option>local1</option>, <option>local2</option>, <option>local3</option>,
        <option>local4</option>, <option>local5</option>, <option>local6</option> or
        <option>local7</option>. See <citerefentry
        project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry> for
        details. This option is only useful when <varname>StandardOutput=</varname> or
        <varname>StandardError=</varname> are set to <option>journal</option> or <option>kmsg</option> (or to
        the same settings in combination with <option>+console</option>), and only applies to log messages
        written to stdout or stderr. Defaults to <option>daemon</option>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>SyslogLevel=</varname></term>

        <listitem><para>The default <command>syslog</command> log level to use when logging to the logging system or
        the kernel log buffer. One of <option>emerg</option>, <option>alert</option>, <option>crit</option>,
        <option>err</option>, <option>warning</option>, <option>notice</option>, <option>info</option>,
        <option>debug</option>. See <citerefentry
        project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry> for
        details. This option is only useful when <varname>StandardOutput=</varname> or
        <varname>StandardError=</varname> are set to <option>journal</option> or
        <option>kmsg</option> (or to the same settings in combination with <option>+console</option>), and only applies
        to log messages written to stdout or stderr. Note that individual lines output by executed processes may be
        prefixed with a different log level which can be used to override the default log level specified here. The
        interpretation of these prefixes may be disabled with <varname>SyslogLevelPrefix=</varname>, see below. For
        details, see <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
        Defaults to <option>info</option>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>SyslogLevelPrefix=</varname></term>

        <listitem><para>Takes a boolean argument. If true and <varname>StandardOutput=</varname> or
        <varname>StandardError=</varname> are set to <option>journal</option> or <option>kmsg</option> (or to
        the same settings in combination with <option>+console</option>), log lines written by the executed
        process that are prefixed with a log level will be processed with this log level set but the prefix
        removed. If set to false, the interpretation of these prefixes is disabled and the logged lines are
        passed on as-is. This only applies to log messages written to stdout or stderr. For details about
        this prefixing see
        <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
        Defaults to true.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>TTYPath=</varname></term>

        <listitem><para>Sets the terminal device node to use if standard input, output, or error are connected to a TTY
        (see above). Defaults to <filename>/dev/console</filename>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>TTYReset=</varname></term>

        <listitem><para>Reset the terminal device specified with <varname>TTYPath=</varname> before and after
        execution. Defaults to <literal>no</literal>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>TTYVHangup=</varname></term>

        <listitem><para>Disconnect all clients which have opened the terminal device specified with
        <varname>TTYPath=</varname> before and after execution. Defaults to <literal>no</literal>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>TTYRows=</varname></term>
        <term><varname>TTYColumns=</varname></term>

        <listitem><para>Configure the size of the TTY specified with <varname>TTYPath=</varname>. If unset or
        set to the empty string, the kernel default is used.</para>

        <xi:include href="version-info.xml" xpointer="v250"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>TTYVTDisallocate=</varname></term>

        <listitem><para>If the terminal device specified with <varname>TTYPath=</varname> is a virtual console
        terminal, try to deallocate the TTY before and after execution. This ensures that the screen and scrollback
        buffer is cleared. Defaults to <literal>no</literal>.</para></listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>Credentials</title>

    <variablelist class='unit-directives'>

      <varlistentry>
        <term><varname>LoadCredential=</varname><replaceable>ID</replaceable><optional>:<replaceable>PATH</replaceable></optional></term>
        <term><varname>LoadCredentialEncrypted=</varname><replaceable>ID</replaceable><optional>:<replaceable>PATH</replaceable></optional></term>

        <listitem><para>Pass a credential to the unit. Credentials are limited-size binary or textual objects
        that may be passed to unit processes. They are primarily used for passing cryptographic keys (both
        public and private) or certificates, user account information or identity information from host to
        services. The data is accessible from the unit's processes via the file system, at a read-only
        location that (if possible and permitted) is backed by non-swappable memory. The data is only
        accessible to the user associated with the unit, via the
        <varname>User=</varname>/<varname>DynamicUser=</varname> settings (as well as the superuser). When
        available, the location of credentials is exported as the <varname>$CREDENTIALS_DIRECTORY</varname>
        environment variable to the unit's processes.</para>

        <para>The <varname>LoadCredential=</varname> setting takes a textual ID to use as name for a
        credential plus a file system path, separated by a colon. The ID must be a short ASCII string
        suitable as filename in the filesystem, and may be chosen freely by the user. If the specified path
        is absolute it is opened as regular file and the credential data is read from it. If the absolute
        path refers to an <constant>AF_UNIX</constant> stream socket in the file system a connection is made
        to it (only once at unit start-up) and the credential data read from the connection, providing an
        easy IPC integration point for dynamically transferring credentials from other services.</para>

        <para>If the specified path is not absolute and itself qualifies as valid credential identifier it is
        attempted to find a credential that the service manager itself received under the specified name —
        which may be used to propagate credentials from an invoking environment (e.g. a container manager
        that invoked the service manager) into a service. If no matching system credential is found, the
        directories <filename>/etc/credstore/</filename>, <filename>/run/credstore/</filename> and
        <filename>/usr/lib/credstore/</filename> are searched for files under the credential's name — which
        hence are recommended locations for credential data on disk. If
        <varname>LoadCredentialEncrypted=</varname> is used <filename>/run/credstore.encrypted/</filename>,
        <filename>/etc/credstore.encrypted/</filename>, and
        <filename>/usr/lib/credstore.encrypted/</filename> are searched as well.</para>

        <para>If the file system path is omitted it is chosen identical to the credential name, i.e. this is
        a terse way to declare credentials to inherit from the service manager into a service. This option
        may be used multiple times, each time defining an additional credential to pass to the unit.</para>

        <para>If an absolute path referring to a directory is specified, every file in that directory
        (recursively) will be loaded as a separate credential. The ID for each credential will be the
        provided ID suffixed with <literal>_$FILENAME</literal> (e.g., <literal>Key_file1</literal>). When
        loading from a directory, symlinks will be ignored.</para>

        <para>The contents of the file/socket may be arbitrary binary or textual data, including newline
        characters and <constant>NUL</constant> bytes.</para>

        <para>The <varname>LoadCredentialEncrypted=</varname> setting is identical to
        <varname>LoadCredential=</varname>, except that the credential data is decrypted and authenticated
        before being passed on to the executed processes. Specifically, the referenced path should refer to a
        file or socket with an encrypted credential, as implemented by
        <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry>. This
        credential is loaded, decrypted, authenticated and then passed to the application in plaintext form,
        in the same way a regular credential specified via <varname>LoadCredential=</varname> would be. A
        credential configured this way may be symmetrically encrypted/authenticated with a secret key derived
        from the system's TPM2 security chip, or with a secret key stored in
        <filename>/var/lib/systemd/credentials.secret</filename>, or with both. Using encrypted and
        authenticated credentials improves security as credentials are not stored in plaintext and only
        authenticated and decrypted into plaintext the moment a service requiring them is started. Moreover,
        credentials may be bound to the local hardware and installations, so that they cannot easily be
        analyzed offline, or be generated externally. When <varname>DevicePolicy=</varname> is set to
        <literal>closed</literal> or <literal>strict</literal>, or set to <literal>auto</literal> and
        <varname>DeviceAllow=</varname> is set, or <varname>PrivateDevices=</varname> is set, then this
        setting adds <filename>/dev/tpmrm0</filename> with <constant>rw</constant> mode to
        <varname>DeviceAllow=</varname>. See
        <citerefentry><refentrytitle>systemd.resource-control</refentrytitle><manvolnum>5</manvolnum></citerefentry>
        for the details about <varname>DevicePolicy=</varname> or <varname>DeviceAllow=</varname>.</para>

        <para>The credential files/IPC sockets must be accessible to the service manager, but don't have to
        be directly accessible to the unit's processes: the credential data is read and copied into separate,
        read-only copies for the unit that are accessible to appropriately privileged processes. This is
        particularly useful in combination with <varname>DynamicUser=</varname> as this way privileged data
        can be made available to processes running under a dynamic UID (i.e. not a previously known one)
        without having to open up access to all users.</para>

        <para>In order to reference the path a credential may be read from within a
        <varname>ExecStart=</varname> command line use <literal>${CREDENTIALS_DIRECTORY}/mycred</literal>,
        e.g. <literal>ExecStart=cat ${CREDENTIALS_DIRECTORY}/mycred</literal>. In order to reference the path
        a credential may be read from within a <varname>Environment=</varname> line use
        <literal>%d/mycred</literal>, e.g. <literal>Environment=MYCREDPATH=%d/mycred</literal>.</para>

        <para>Currently, an accumulated credential size limit of 1 MB per unit is enforced.</para>

        <para>The service manager itself may receive system credentials that can be propagated to services
        from a hosting container manager or VM hypervisor. See the <ulink
        url="https://systemd.io/CONTAINER_INTERFACE">Container Interface</ulink> documentation for details
        about the former. For the latter, pass <ulink
        url="https://www.dmtf.org/standards/smbios">DMI/SMBIOS</ulink> OEM string table entries (field type
        11) with a prefix of <literal>io.systemd.credential:</literal> or
        <literal>io.systemd.credential.binary:</literal>. In both cases a key/value pair separated by
        <literal>=</literal> is expected, in the latter case the right-hand side is Base64 decoded when
        parsed (thus permitting binary data to be passed in). Example <ulink
        url="https://www.qemu.org/docs/master/system/index.html">qemu</ulink> switch: <literal>-smbios
        type=11,value=io.systemd.credential:xx=yy</literal>, or <literal>-smbios
        type=11,value=io.systemd.credential.binary:rick=TmV2ZXIgR29ubmEgR2l2ZSBZb3UgVXA=</literal>. Alternatively,
        use the <command>qemu</command> <literal>fw_cfg</literal> node
        <literal>opt/io.systemd.credentials/</literal>. Example <command>qemu</command> switch:
        <literal>-fw_cfg name=opt/io.systemd.credentials/mycred,string=supersecret</literal>. They may also
        be passed from the UEFI firmware environment via
        <citerefentry><refentrytitle>systemd-stub</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
        from the initrd (see
        <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>), or be
        specified on the kernel command line using the <literal>systemd.set_credential=</literal> and
        <literal>systemd.set_credential_binary=</literal> switches (see
        <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> – this is
        not recommended since unprivileged userspace can read the kernel command line). </para>

        <para>If referencing an <constant>AF_UNIX</constant> stream socket to connect to, the connection will
        originate from an abstract namespace socket, that includes information about the unit and the
        credential ID in its socket name. Use <citerefentry
        project='man-pages'><refentrytitle>getpeername</refentrytitle><manvolnum>2</manvolnum></citerefentry>
        to query this information. The returned socket name is formatted as <constant>NUL</constant>
        <replaceable>RANDOM</replaceable> <literal>/unit/</literal> <replaceable>UNIT</replaceable>
        <literal>/</literal> <replaceable>ID</replaceable>, i.e. a <constant>NUL</constant> byte (as required
        for abstract namespace socket names), followed by a random string (consisting of alphadecimal
        characters), followed by the literal string <literal>/unit/</literal>, followed by the requesting
        unit name, followed by the literal character <literal>/</literal>, followed by the textual credential
        ID requested. Example: <literal>\0adf9d86b6eda275e/unit/foobar.service/credx</literal> in case the
        credential <literal>credx</literal> is requested for a unit <literal>foobar.service</literal>. This
        functionality is useful for using a single listening socket to serve credentials to multiple
        consumers.</para>

        <para>For further information see <ulink url="https://systemd.io/CREDENTIALS">System and Service
        Credentials</ulink> documentation.</para>

        <xi:include href="version-info.xml" xpointer="v247"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>ImportCredential=</varname><replaceable>GLOB</replaceable></term>

        <listitem><para>Pass one or more credentials to the unit. Takes a credential name for which we'll
        attempt to find a credential that the service manager itself received under the specified name —
        which may be used to propagate credentials from an invoking environment (e.g. a container manager
        that invoked the service manager) into a service. If the credential name is a glob, all credentials
        matching the glob are passed to the unit. Matching credentials are searched for in the system
        credentials, the encrypted system credentials, and under <filename>/etc/credstore/</filename>,
        <filename>/run/credstore/</filename>, <filename>/usr/lib/credstore/</filename>,
        <filename>/run/credstore.encrypted/</filename>, <filename>/etc/credstore.encrypted/</filename>, and
        <filename>/usr/lib/credstore.encrypted/</filename> in that order. When multiple credentials of the
        same name are found, the first one found is used.</para>

        <para>The globbing expression implements a restrictive subset of <citerefentry
        project='man-pages'><refentrytitle>glob</refentrytitle><manvolnum>7</manvolnum></citerefentry>: only
        a single trailing <literal>*</literal> wildcard may be specified. Both <literal>?</literal> and
        <literal>[]</literal> wildcards are not permitted, nor are <literal>*</literal> wildcards anywhere
        except at the end of the glob expression.</para>

        <para>When multiple credentials of the same name are found, credentials found by
        <varname>LoadCredential=</varname> and <varname>LoadCredentialEncrypted=</varname> take priority over
        credentials found by <varname>ImportCredential=</varname>.</para>

        <xi:include href="version-info.xml" xpointer="v254"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>SetCredential=</varname><replaceable>ID</replaceable>:<replaceable>VALUE</replaceable></term>
        <term><varname>SetCredentialEncrypted=</varname><replaceable>ID</replaceable>:<replaceable>VALUE</replaceable></term>

        <listitem><para>The <varname>SetCredential=</varname> setting is similar to
        <varname>LoadCredential=</varname> but accepts a literal value to use as data for the credential,
        instead of a file system path to read the data from. Do not use this option for data that is supposed
        to be secret, as it is accessible to unprivileged processes via IPC. It's only safe to use this for
        user IDs, public key material and similar non-sensitive data. For everything else use
        <varname>LoadCredential=</varname>. In order to embed binary data into the credential data use
        C-style escaping (i.e. <literal>\n</literal> to embed a newline, or <literal>\x00</literal> to embed
        a <constant>NUL</constant> byte).</para>

        <para>The <varname>SetCredentialEncrypted=</varname> setting is identical to
        <varname>SetCredential=</varname> but expects an encrypted credential in literal form as value. This
        allows embedding confidential credentials securely directly in unit files. Use
        <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry>'
        <option>-p</option> switch to generate suitable <varname>SetCredentialEncrypted=</varname> lines
        directly from plaintext credentials. For further details see
        <varname>LoadCredentialEncrypted=</varname> above.</para>

        <para>When multiple credentials of the same name are found, credentials found by
        <varname>LoadCredential=</varname>, <varname>LoadCredentialEncrypted=</varname> and
        <varname>ImportCredential=</varname> take priority over credentials found by
        <varname>SetCredential=</varname>. As such, <varname>SetCredential=</varname> will act as default if
        no credentials are found by any of the former. In this case not being able to retrieve the credential
        from the path specified in <varname>LoadCredential=</varname> or
        <varname>LoadCredentialEncrypted=</varname> is not considered fatal.</para>

        <xi:include href="version-info.xml" xpointer="v247"/></listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>System V Compatibility</title>
    <variablelist class='unit-directives'>

      <varlistentry>
        <term><varname>UtmpIdentifier=</varname></term>

        <listitem><para>Takes a four character identifier string for an <citerefentry
        project='man-pages'><refentrytitle>utmp</refentrytitle><manvolnum>5</manvolnum></citerefentry> and wtmp entry
        for this service. This should only be set for services such as <command>getty</command> implementations (such
        as <citerefentry
        project='die-net'><refentrytitle>agetty</refentrytitle><manvolnum>8</manvolnum></citerefentry>) where utmp/wtmp
        entries must be created and cleared before and after execution, or for services that shall be executed as if
        they were run by a <command>getty</command> process (see below). If the configured string is longer than four
        characters, it is truncated and the terminal four characters are used. This setting interprets %I style string
        replacements. This setting is unset by default, i.e. no utmp/wtmp entries are created or cleaned up for this
        service.</para></listitem>
      </varlistentry>

      <varlistentry>
         <term><varname>UtmpMode=</varname></term>

         <listitem><para>Takes one of <literal>init</literal>, <literal>login</literal> or <literal>user</literal>. If
         <varname>UtmpIdentifier=</varname> is set, controls which type of <citerefentry
         project='man-pages'><refentrytitle>utmp</refentrytitle><manvolnum>5</manvolnum></citerefentry>/wtmp entries
         for this service are generated. This setting has no effect unless <varname>UtmpIdentifier=</varname> is set
         too. If <literal>init</literal> is set, only an <constant>INIT_PROCESS</constant> entry is generated and the
         invoked process must implement a <command>getty</command>-compatible utmp/wtmp logic. If
         <literal>login</literal> is set, first an <constant>INIT_PROCESS</constant> entry, followed by a
         <constant>LOGIN_PROCESS</constant> entry is generated. In this case, the invoked process must implement a
         <citerefentry
         project='die-net'><refentrytitle>login</refentrytitle><manvolnum>1</manvolnum></citerefentry>-compatible
         utmp/wtmp logic. If <literal>user</literal> is set, first an <constant>INIT_PROCESS</constant> entry, then a
         <constant>LOGIN_PROCESS</constant> entry and finally a <constant>USER_PROCESS</constant> entry is
         generated. In this case, the invoked process may be any process that is suitable to be run as session
         leader. Defaults to <literal>init</literal>.</para>

         <xi:include href="version-info.xml" xpointer="v225"/></listitem>
      </varlistentry>

    </variablelist>
  </refsect1>

  <refsect1>
    <title>Environment Variables in Spawned Processes</title>

    <para>Processes started by the service manager are executed with an environment variable block assembled from
    multiple sources. Processes started by the system service manager generally do not inherit environment variables
    set for the service manager itself (but this may be altered via <varname>PassEnvironment=</varname>), but processes
    started by the user service manager instances generally do inherit all environment variables set for the service
    manager itself.</para>

    <para>For each invoked process the list of environment variables set is compiled from the following sources:</para>

    <itemizedlist>
      <listitem><para>Variables globally configured for the service manager, using the
      <varname>DefaultEnvironment=</varname> setting in
      <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
      the kernel command line option <varname>systemd.setenv=</varname> understood by
      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, or via
      <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
      <command>set-environment</command> verb.</para></listitem>

      <listitem><para>Variables defined by the service manager itself (see the list below).</para></listitem>

      <listitem><para>Variables set in the service manager's own environment variable block (subject to
      <varname>PassEnvironment=</varname> for the system service manager).</para></listitem>

      <listitem><para>Variables set via <varname>Environment=</varname> in the unit file.</para></listitem>

      <listitem><para>Variables read from files specified via <varname>EnvironmentFile=</varname> in the unit
      file.</para></listitem>

      <listitem><para>Variables set by any PAM modules in case <varname>PAMName=</varname> is in effect,
      cf. <citerefentry
      project='man-pages'><refentrytitle>pam_env</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
      </para></listitem>
    </itemizedlist>

    <para>If the same environment variable is set by multiple of these sources, the later source — according
    to the order of the list above — wins. Note that as the final step all variables listed in
    <varname>UnsetEnvironment=</varname> are removed from the compiled environment variable list, immediately
    before it is passed to the executed process.</para>

    <para>The general philosophy is to expose a small curated list of environment variables to processes.
    Services started by the system manager (PID 1) will be started, without additional service-specific
    configuration, with just a few environment variables. The user manager inherits environment variables as
    any other system service, but in addition may receive additional environment variables from PAM, and,
    typically, additional imported variables when the user starts a graphical session. It is recommended to
    keep the environment blocks in both the system and user managers lean. Importing all variables
    inherited by the graphical session or by one of the user shells is strongly discouraged.</para>

    <para>Hint: <command>systemd-run -P env</command> and <command>systemd-run --user -P env</command> print
    the effective system and user service environment blocks.</para>

    <refsect2>
      <title>Environment Variables Set or Propagated by the Service Manager</title>

      <para>The following environment variables are propagated by the service manager or generated internally
      for each invoked process:</para>

      <variablelist class='environment-variables'>
        <varlistentry>
          <term><varname>$PATH</varname></term>

          <listitem><para>Colon-separated list of directories to use when launching
          executables. <command>systemd</command> uses a fixed value of
          <literal><filename>/usr/local/sbin</filename>:<filename>/usr/local/bin</filename>:<filename>/usr/sbin</filename>:<filename>/usr/bin</filename></literal>
          in the system manager. In case of the user manager, a different path may be configured by the
          distribution. It is recommended to not rely on the order of entries, and have only one program
          with a given name in <varname>$PATH</varname>.</para>

          <xi:include href="version-info.xml" xpointer="v208"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>$LANG</varname></term>

          <listitem><para>Locale. Can be set in <citerefentry
          project='man-pages'><refentrytitle>locale.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
          or on the kernel command line (see
          <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> and
          <citerefentry><refentrytitle>kernel-command-line</refentrytitle><manvolnum>7</manvolnum></citerefentry>).
          </para>

          <xi:include href="version-info.xml" xpointer="v208"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>$USER</varname></term>
          <term><varname>$LOGNAME</varname></term>
          <term><varname>$HOME</varname></term>
          <term><varname>$SHELL</varname></term>

          <listitem><para>User name (twice), home directory, and the
          login shell. The variables are set for the units that have
          <varname>User=</varname> set, which includes user
          <command>systemd</command> instances. See
          <citerefentry project='die-net'><refentrytitle>passwd</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
          </para>

          <xi:include href="version-info.xml" xpointer="v208"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>$INVOCATION_ID</varname></term>

          <listitem><para>Contains a randomized, unique 128-bit ID identifying each runtime cycle of the unit, formatted
          as 32 character hexadecimal string. A new ID is assigned each time the unit changes from an inactive state into
          an activating or active state, and may be used to identify this specific runtime cycle, in particular in data
          stored offline, such as the journal. The same ID is passed to all processes run as part of the
          unit.</para>

          <xi:include href="version-info.xml" xpointer="v232"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>$XDG_RUNTIME_DIR</varname></term>

          <listitem><para>The directory to use for runtime objects (such as IPC objects) and volatile state. Set for all
          services run by the user <command>systemd</command> instance, as well as any system services that use
          <varname>PAMName=</varname> with a PAM stack that includes <command>pam_systemd</command>. See below and
          <citerefentry><refentrytitle>pam_systemd</refentrytitle><manvolnum>8</manvolnum></citerefentry> for more
          information.</para>

          <xi:include href="version-info.xml" xpointer="v208"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>$RUNTIME_DIRECTORY</varname></term>
          <term><varname>$STATE_DIRECTORY</varname></term>
          <term><varname>$CACHE_DIRECTORY</varname></term>
          <term><varname>$LOGS_DIRECTORY</varname></term>
          <term><varname>$CONFIGURATION_DIRECTORY</varname></term>

          <listitem><para>Absolute paths to the directories defined with
          <varname>RuntimeDirectory=</varname>, <varname>StateDirectory=</varname>,
          <varname>CacheDirectory=</varname>, <varname>LogsDirectory=</varname>, and
          <varname>ConfigurationDirectory=</varname> when those settings are used.</para>

          <xi:include href="version-info.xml" xpointer="v244"/>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>$CREDENTIALS_DIRECTORY</varname></term>

          <listitem><para>An absolute path to the per-unit directory with credentials configured via
          <varname>ImportCredential=</varname>/<varname>LoadCredential=</varname>/<varname>SetCredential=</varname>.
          The directory is marked read-only and is placed in unswappable memory (if supported and permitted),
          and is only accessible to the UID associated with the unit via <varname>User=</varname> or
          <varname>DynamicUser=</varname> (and the superuser).</para>

          <xi:include href="version-info.xml" xpointer="v247"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>$MAINPID</varname></term>

          <listitem><para>The PID of the unit's main process if it is
          known. This is only set for control processes as invoked by
          <varname>ExecReload=</varname> and similar.</para>

          <xi:include href="version-info.xml" xpointer="v209"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>$MANAGERPID</varname></term>

          <listitem><para>The PID of the user <command>systemd</command>
          instance, set for processes spawned by it.</para>

          <xi:include href="version-info.xml" xpointer="v208"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>$LISTEN_FDS</varname></term>
          <term><varname>$LISTEN_PID</varname></term>
          <term><varname>$LISTEN_FDNAMES</varname></term>

          <listitem><para>Information about file descriptors passed to a
          service for socket activation. See
          <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
          </para>

          <xi:include href="version-info.xml" xpointer="v208"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>$NOTIFY_SOCKET</varname></term>

          <listitem><para>The socket <function>sd_notify()</function> talks to. See
          <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
          </para>

          <xi:include href="version-info.xml" xpointer="v229"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>$WATCHDOG_PID</varname></term>
          <term><varname>$WATCHDOG_USEC</varname></term>

          <listitem><para>Information about watchdog keep-alive notifications. See
          <citerefentry><refentrytitle>sd_watchdog_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
          </para>

          <xi:include href="version-info.xml" xpointer="v229"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>$SYSTEMD_EXEC_PID</varname></term>

          <listitem><para>The PID of the unit process (e.g. process invoked by
          <varname>ExecStart=</varname>). The child process can use this information to determine
          whether the process is directly invoked by the service manager or indirectly as a child of
          another process by comparing this value with the current PID (similarly to the scheme used in
          <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>
          with <varname>$LISTEN_PID</varname> and <varname>$LISTEN_FDS</varname>).</para>

          <xi:include href="version-info.xml" xpointer="v248"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>$TERM</varname></term>

          <listitem><para>Terminal type, set only for units connected to
          a terminal (<varname>StandardInput=tty</varname>,
          <varname>StandardOutput=tty</varname>, or
          <varname>StandardError=tty</varname>). See
          <citerefentry project='man-pages'><refentrytitle>termcap</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
          </para>

          <xi:include href="version-info.xml" xpointer="v209"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>$LOG_NAMESPACE</varname></term>

          <listitem><para>Contains the name of the selected logging namespace when the
          <varname>LogNamespace=</varname> service setting is used.</para>

          <xi:include href="version-info.xml" xpointer="v246"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>$JOURNAL_STREAM</varname></term>

          <listitem><para>If the standard output or standard error output of the executed processes are connected to the
          journal (for example, by setting <varname>StandardError=journal</varname>) <varname>$JOURNAL_STREAM</varname>
          contains the device and inode numbers of the connection file descriptor, formatted in decimal, separated by a
          colon (<literal>:</literal>). This permits invoked processes to safely detect whether their standard output or
          standard error output are connected to the journal. The device and inode numbers of the file descriptors should
          be compared with the values set in the environment variable to determine whether the process output is still
          connected to the journal. Note that it is generally not sufficient to only check whether
          <varname>$JOURNAL_STREAM</varname> is set at all as services might invoke external processes replacing their
          standard output or standard error output, without unsetting the environment variable.</para>

          <para>If both standard output and standard error of the executed processes are connected to the journal via a
          stream socket, this environment variable will contain information about the standard error stream, as that's
          usually the preferred destination for log data. (Note that typically the same stream is used for both standard
          output and standard error, hence very likely the environment variable contains device and inode information
          matching both stream file descriptors.)</para>

          <para>This environment variable is primarily useful to allow services to optionally upgrade their used log
          protocol to the native journal protocol (using
          <citerefentry><refentrytitle>sd_journal_print</refentrytitle><manvolnum>3</manvolnum></citerefentry> and other
          functions) if their standard output or standard error output is connected to the journal anyway, thus enabling
          delivery of structured metadata along with logged messages.</para>

          <xi:include href="version-info.xml" xpointer="v231"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>$SERVICE_RESULT</varname></term>

          <listitem><para>Only used for the service unit type. This environment variable is passed to all
          <varname>ExecStop=</varname> and <varname>ExecStopPost=</varname> processes, and encodes the service
          "result". Currently, the following values are defined:</para>

          <table>
            <title>Defined <varname>$SERVICE_RESULT</varname> values</title>
            <tgroup cols='2'>
              <colspec colname='result'/>
              <colspec colname='meaning'/>
              <thead>
                <row>
                  <entry>Value</entry>
                  <entry>Meaning</entry>
                </row>
              </thead>

              <tbody>
                <row>
                  <entry><literal>success</literal></entry>
                  <entry>The service ran successfully and exited cleanly.</entry>
                </row>
                <row>
                  <entry><literal>protocol</literal></entry>
                  <entry>A protocol violation occurred: the service did not take the steps required by its unit configuration (specifically what is configured in its <varname>Type=</varname> setting).</entry>
                </row>
                <row>
                  <entry><literal>timeout</literal></entry>
                  <entry>One of the steps timed out.</entry>
                </row>
                <row>
                  <entry><literal>exit-code</literal></entry>
                  <entry>Service process exited with a non-zero exit code; see <varname>$EXIT_CODE</varname> below for the actual exit code returned.</entry>
                </row>
                <row>
                  <entry><literal>signal</literal></entry>
                  <entry>A service process was terminated abnormally by a signal, without dumping core. See <varname>$EXIT_CODE</varname> below for the actual signal causing the termination.</entry>
                </row>
                <row>
                  <entry><literal>core-dump</literal></entry>
                  <entry>A service process terminated abnormally with a signal and dumped core. See <varname>$EXIT_CODE</varname> below for the signal causing the termination.</entry>
                </row>
                <row>
                  <entry><literal>watchdog</literal></entry>
                  <entry>Watchdog keep-alive ping was enabled for the service, but the deadline was missed.</entry>
                </row>
                <row>
                  <entry><literal>start-limit-hit</literal></entry>
                  <entry>A start limit was defined for the unit and it was hit, causing the unit to fail to start. See <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>'s <varname>StartLimitIntervalSec=</varname> and <varname>StartLimitBurst=</varname> for details.</entry>
                </row>
                <row>
                  <entry><literal>resources</literal></entry>
                  <entry>A catch-all condition in case a system operation failed.</entry>
                </row>
              </tbody>
            </tgroup>
          </table>

          <para>This environment variable is useful to monitor failure or successful termination of a service. Even
          though this variable is available in both <varname>ExecStop=</varname> and <varname>ExecStopPost=</varname>, it
          is usually a better choice to place monitoring tools in the latter, as the former is only invoked for services
          that managed to start up correctly, and the latter covers both services that failed during their start-up and
          those which failed during their runtime.</para>

          <xi:include href="version-info.xml" xpointer="v232"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>$EXIT_CODE</varname></term>
          <term><varname>$EXIT_STATUS</varname></term>

          <listitem><para>Only defined for the service unit type. These environment variables are passed to all
          <varname>ExecStop=</varname>, <varname>ExecStopPost=</varname> processes and contain exit status/code
          information of the main process of the service. For the precise definition of the exit code and status, see
          <citerefentry><refentrytitle>wait</refentrytitle><manvolnum>2</manvolnum></citerefentry>. <varname>$EXIT_CODE</varname>
          is one of <literal>exited</literal>, <literal>killed</literal>,
          <literal>dumped</literal>. <varname>$EXIT_STATUS</varname> contains the numeric exit code formatted as string
          if <varname>$EXIT_CODE</varname> is <literal>exited</literal>, and the signal name in all other cases. Note
          that these environment variables are only set if the service manager succeeded to start and identify the main
          process of the service.</para>

          <table>
            <title>Summary of possible service result variable values</title>
            <tgroup cols='3'>
              <colspec colname='result' />
              <colspec colname='code' />
              <colspec colname='status' />
              <thead>
                <row>
                  <entry><varname>$SERVICE_RESULT</varname></entry>
                  <entry><varname>$EXIT_CODE</varname></entry>
                  <entry><varname>$EXIT_STATUS</varname></entry>
                </row>
              </thead>

              <tbody>
                <row>
                  <entry morerows="1" valign="top"><literal>success</literal></entry>
                  <entry valign="top"><literal>killed</literal></entry>
                  <entry><literal>HUP</literal>, <literal>INT</literal>, <literal>TERM</literal>, <literal>PIPE</literal></entry>
                </row>
                <row>
                  <entry valign="top"><literal>exited</literal></entry>
                  <entry><literal>0</literal></entry>
                </row>
                <row>
                  <entry morerows="1" valign="top"><literal>protocol</literal></entry>
                  <entry valign="top">not set</entry>
                  <entry>not set</entry>
                </row>
                <row>
                  <entry><literal>exited</literal></entry>
                  <entry><literal>0</literal></entry>
                </row>
                <row>
                  <entry morerows="1" valign="top"><literal>timeout</literal></entry>
                  <entry valign="top"><literal>killed</literal></entry>
                  <entry><literal>TERM</literal>, <literal>KILL</literal></entry>
                </row>
                <row>
                  <entry valign="top"><literal>exited</literal></entry>
                  <entry><literal>0</literal>, <literal>1</literal>, <literal>2</literal>, <literal
                  >3</literal>, …, <literal>255</literal></entry>
                </row>
                <row>
                  <entry valign="top"><literal>exit-code</literal></entry>
                  <entry valign="top"><literal>exited</literal></entry>
                  <entry><literal>1</literal>, <literal>2</literal>, <literal
                  >3</literal>, …, <literal>255</literal></entry>
                </row>
                <row>
                  <entry valign="top"><literal>signal</literal></entry>
                  <entry valign="top"><literal>killed</literal></entry>
                  <entry><literal>HUP</literal>, <literal>INT</literal>, <literal>KILL</literal>, …</entry>
                </row>
                <row>
                  <entry valign="top"><literal>core-dump</literal></entry>
                  <entry valign="top"><literal>dumped</literal></entry>
                  <entry><literal>ABRT</literal>, <literal>SEGV</literal>, <literal>QUIT</literal>, …</entry>
                </row>
                <row>
                  <entry morerows="2" valign="top"><literal>watchdog</literal></entry>
                  <entry><literal>dumped</literal></entry>
                  <entry><literal>ABRT</literal></entry>
                </row>
                <row>
                  <entry><literal>killed</literal></entry>
                  <entry><literal>TERM</literal>, <literal>KILL</literal></entry>
                </row>
                <row>
                  <entry><literal>exited</literal></entry>
                  <entry><literal>0</literal>, <literal>1</literal>, <literal>2</literal>, <literal
                  >3</literal>, …, <literal>255</literal></entry>
                </row>
                <row>
                  <entry valign="top"><literal>exec-condition</literal></entry>
                  <entry><literal>exited</literal></entry>
                  <entry><literal>1</literal>, <literal>2</literal>, <literal>3</literal>, <literal
                  >4</literal>, …, <literal>254</literal></entry>
                </row>
                <row>
                  <entry valign="top"><literal>oom-kill</literal></entry>
                  <entry valign="top"><literal>killed</literal></entry>
                  <entry><literal>TERM</literal>, <literal>KILL</literal></entry>
                </row>
                <row>
                  <entry><literal>start-limit-hit</literal></entry>
                  <entry>not set</entry>
                  <entry>not set</entry>
                </row>
                <row>
                  <entry><literal>resources</literal></entry>
                  <entry>any of the above</entry>
                  <entry>any of the above</entry>
                </row>
                <row>
                  <entry namest="results" nameend="status">Note: the process may be also terminated by a signal not sent by systemd. In particular the process may send an arbitrary signal to itself in a handler for any of the non-maskable signals. Nevertheless, in the <literal>timeout</literal> and <literal>watchdog</literal> rows above only the signals that systemd sends have been included. Moreover, using <varname>SuccessExitStatus=</varname> additional exit statuses may be declared to indicate clean termination, which is not reflected by this table.</entry>
                </row>
              </tbody>
            </tgroup>
          </table>

            <xi:include href="version-info.xml" xpointer="v232"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>$MONITOR_SERVICE_RESULT</varname></term>
          <term><varname>$MONITOR_EXIT_CODE</varname></term>
          <term><varname>$MONITOR_EXIT_STATUS</varname></term>
          <term><varname>$MONITOR_INVOCATION_ID</varname></term>
          <term><varname>$MONITOR_UNIT</varname></term>

          <listitem><para>Only defined for the service unit type. Those environment variables are passed to
          all <varname>ExecStart=</varname> and <varname>ExecStartPre=</varname> processes which run in
          services triggered by <varname>OnFailure=</varname> or <varname>OnSuccess=</varname> dependencies.
          </para>

          <para>Variables <varname>$MONITOR_SERVICE_RESULT</varname>, <varname>$MONITOR_EXIT_CODE</varname>
          and <varname>$MONITOR_EXIT_STATUS</varname> take the same values as for
          <varname>ExecStop=</varname> and <varname>ExecStopPost=</varname> processes. Variables
          <varname>$MONITOR_INVOCATION_ID</varname> and <varname>$MONITOR_UNIT</varname> are set to the
          invocation id and unit name of the service which triggered the dependency.</para>

          <para>Note that when multiple services trigger the same unit, those variables will be
          <emphasis>not</emphasis> be passed. Consider using a template handler unit for that case instead:
          <literal>OnFailure=<replaceable>handler</replaceable>@%n.service</literal> for non-templated units,
          or <literal>OnFailure=<replaceable>handler</replaceable>@%p-%i.service</literal> for templated
          units.</para>

          <xi:include href="version-info.xml" xpointer="v251"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>$PIDFILE</varname></term>

          <listitem><para>The path to the configured PID file, in case the process is forked off on behalf of
          a service that uses the <varname>PIDFile=</varname> setting, see
          <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
          for details. Service code may use this environment variable to automatically generate a PID file at
          the location configured in the unit file. This field is set to an absolute path in the file
          system.</para>

          <xi:include href="version-info.xml" xpointer="v242"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>$REMOTE_ADDR</varname></term>
          <term><varname>$REMOTE_PORT</varname></term>

          <listitem><para>If this is a unit started via per-connection socket activation (i.e. via a socket
          unit with <varname>Accept=yes</varname>), these environment variables contain the IP address and
          port number of the remote peer of the socket connection.</para>

          <xi:include href="version-info.xml" xpointer="v254"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>$TRIGGER_UNIT</varname></term>
          <term><varname>$TRIGGER_PATH</varname></term>
          <term><varname>$TRIGGER_TIMER_REALTIME_USEC</varname></term>
          <term><varname>$TRIGGER_TIMER_MONOTONIC_USEC</varname></term>

          <listitem><para>If the unit was activated dynamically (e.g.: a corresponding path unit or timer unit), the
          unit that triggered it and other type-dependent information will be passed via these variables. Note that
          this information is provided in a best-effort way. For example, multiple triggers happening one after
          another will be coalesced and only one will be reported, with no guarantee as to which one it will be.
          Because of this, in most cases this variable will be primarily informational, i.e. useful for debugging
          purposes, is lossy, and should not be relied upon to propagate a comprehensive reason for activation.
          </para>

          <xi:include href="version-info.xml" xpointer="v252"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>$MEMORY_PRESSURE_WATCH</varname></term>
          <term><varname>$MEMORY_PRESSURE_WRITE</varname></term>

          <listitem><para>If memory pressure monitoring is enabled for this service unit, the path to watch
          and the data to write into it. See <ulink url="https://systemd.io/MEMORY_PRESSURE">Memory Pressure
          Handling</ulink> for details about these variables and the service protocol data they
          convey.</para>

          <xi:include href="version-info.xml" xpointer="v254"/></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>$FDSTORE</varname></term>

          <listitem><para>The maximum number of file descriptors that may be stored in the manager for the
          service. This variable is set when the file descriptor store is enabled for the service, i.e.
          <varname>FileDescriptorStoreMax=</varname> is set to a non-zero value (see
          <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
          for details). Applications may check this environment variable before sending file descriptors to
          the service manager via <function>sd_pid_notify_with_fds()</function> (see
          <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry> for
          details).</para>

          <xi:include href="version-info.xml" xpointer="v254"/></listitem>
        </varlistentry>

      </variablelist>

      <para>For system services, when <varname>PAMName=</varname> is enabled and <command>pam_systemd</command> is part
      of the selected PAM stack, additional environment variables defined by systemd may be set for
      services. Specifically, these are <varname>$XDG_SEAT</varname>, <varname>$XDG_VTNR</varname>, see
      <citerefentry><refentrytitle>pam_systemd</refentrytitle><manvolnum>8</manvolnum></citerefentry> for details.</para>
    </refsect2>

  </refsect1>

  <refsect1>
    <title>Process Exit Codes</title>

    <para>When invoking a unit process the service manager possibly fails to apply the execution parameters configured
    with the settings above. In that case the already created service process will exit with a non-zero exit code
    before the configured command line is executed. (Or in other words, the child process possibly exits with these
    error codes, after having been created by the <citerefentry
    project='man-pages'><refentrytitle>fork</refentrytitle><manvolnum>2</manvolnum></citerefentry> system call, but
    before the matching <citerefentry
    project='man-pages'><refentrytitle>execve</refentrytitle><manvolnum>2</manvolnum></citerefentry> system call is
    called.) Specifically, exit codes defined by the C library, by the LSB specification and by the systemd service
    manager itself are used.</para>

    <para>The following basic service exit codes are defined by the C library.</para>

    <table>
      <title>Basic C library exit codes</title>
      <tgroup cols='3'>
        <thead>
          <row>
            <entry>Exit Code</entry>
            <entry>Symbolic Name</entry>
            <entry>Description</entry>
          </row>
        </thead>
        <tbody>
          <row>
            <entry>0</entry>
            <entry><constant>EXIT_SUCCESS</constant></entry>
            <entry>Generic success code.</entry>
          </row>
          <row>
            <entry>1</entry>
            <entry><constant>EXIT_FAILURE</constant></entry>
            <entry>Generic failure or unspecified error.</entry>
          </row>
        </tbody>
      </tgroup>
    </table>

    <para>The following service exit codes are defined by the <ulink
    url="https://refspecs.linuxbase.org/LSB_5.0.0/LSB-Core-generic/LSB-Core-generic/iniscrptact.html">LSB specification</ulink>.
    </para>

    <table>
      <title>LSB service exit codes</title>
      <tgroup cols='3'>
        <thead>
          <row>
            <entry>Exit Code</entry>
            <entry>Symbolic Name</entry>
            <entry>Description</entry>
          </row>
        </thead>
        <tbody>
          <row>
            <entry>2</entry>
            <entry><constant>EXIT_INVALIDARGUMENT</constant></entry>
            <entry>Invalid or excess arguments.</entry>
          </row>
          <row>
            <entry>3</entry>
            <entry><constant>EXIT_NOTIMPLEMENTED</constant></entry>
            <entry>Unimplemented feature.</entry>
          </row>
          <row>
            <entry>4</entry>
            <entry><constant>EXIT_NOPERMISSION</constant></entry>
            <entry>The user has insufficient privileges.</entry>
          </row>
          <row>
            <entry>5</entry>
            <entry><constant>EXIT_NOTINSTALLED</constant></entry>
            <entry>The program is not installed.</entry>
          </row>
          <row>
            <entry>6</entry>
            <entry><constant>EXIT_NOTCONFIGURED</constant></entry>
            <entry>The program is not configured.</entry>
          </row>
          <row>
            <entry>7</entry>
            <entry><constant>EXIT_NOTRUNNING</constant></entry>
            <entry>The program is not running.</entry>
          </row>
        </tbody>
      </tgroup>
    </table>

    <para>
      The LSB specification suggests that error codes 200 and above are reserved for implementations. Some of them are
      used by the service manager to indicate problems during process invocation:
    </para>
    <table>
      <title>systemd-specific exit codes</title>
      <tgroup cols='3'>
        <thead>
          <row>
            <entry>Exit Code</entry>
            <entry>Symbolic Name</entry>
            <entry>Description</entry>
          </row>
        </thead>
        <tbody>
          <row>
            <entry>200</entry>
            <entry><constant>EXIT_CHDIR</constant></entry>
            <entry>Changing to the requested working directory failed. See <varname>WorkingDirectory=</varname> above.</entry>
          </row>
          <row>
            <entry>201</entry>
            <entry><constant>EXIT_NICE</constant></entry>
            <entry>Failed to set up process scheduling priority (nice level). See <varname>Nice=</varname> above.</entry>
          </row>
          <row>
            <entry>202</entry>
            <entry><constant>EXIT_FDS</constant></entry>
            <entry>Failed to close unwanted file descriptors, or to adjust passed file descriptors.</entry>
          </row>
          <row>
            <entry>203</entry>
            <entry><constant>EXIT_EXEC</constant></entry>
            <entry>The actual process execution failed (specifically, the <citerefentry project='man-pages'><refentrytitle>execve</refentrytitle><manvolnum>2</manvolnum></citerefentry> system call). Most likely this is caused by a missing or non-accessible executable file.</entry>
          </row>
          <row>
            <entry>204</entry>
            <entry><constant>EXIT_MEMORY</constant></entry>
            <entry>Failed to perform an action due to memory shortage.</entry>
          </row>
          <row>
            <entry>205</entry>
            <entry><constant>EXIT_LIMITS</constant></entry>
            <entry>Failed to adjust resource limits. See <varname>LimitCPU=</varname> and related settings above.</entry>
          </row>
          <row>
            <entry>206</entry>
            <entry><constant>EXIT_OOM_ADJUST</constant></entry>
            <entry>Failed to adjust the OOM setting. See <varname>OOMScoreAdjust=</varname> above.</entry>
          </row>
          <row>
            <entry>207</entry>
            <entry><constant>EXIT_SIGNAL_MASK</constant></entry>
            <entry>Failed to set process signal mask.</entry>
          </row>
          <row>
            <entry>208</entry>
            <entry><constant>EXIT_STDIN</constant></entry>
            <entry>Failed to set up standard input. See <varname>StandardInput=</varname> above.</entry>
          </row>
          <row>
            <entry>209</entry>
            <entry><constant>EXIT_STDOUT</constant></entry>
            <entry>Failed to set up standard output. See <varname>StandardOutput=</varname> above.</entry>
          </row>
          <row>
            <entry>210</entry>
            <entry><constant>EXIT_CHROOT</constant></entry>
            <entry>Failed to change root directory (<citerefentry project='man-pages'><refentrytitle>chroot</refentrytitle><manvolnum>2</manvolnum></citerefentry>). See <varname>RootDirectory=</varname>/<varname>RootImage=</varname> above.</entry>
          </row>
          <row>
            <entry>211</entry>
            <entry><constant>EXIT_IOPRIO</constant></entry>
            <entry>Failed to set up IO scheduling priority. See <varname>IOSchedulingClass=</varname>/<varname>IOSchedulingPriority=</varname> above.</entry>
          </row>
          <row>
            <entry>212</entry>
            <entry><constant>EXIT_TIMERSLACK</constant></entry>
            <entry>Failed to set up timer slack. See <varname>TimerSlackNSec=</varname> above.</entry>
          </row>
          <row>
            <entry>213</entry>
            <entry><constant>EXIT_SECUREBITS</constant></entry>
            <entry>Failed to set process secure bits. See <varname>SecureBits=</varname> above.</entry>
          </row>
          <row>
            <entry>214</entry>
            <entry><constant>EXIT_SETSCHEDULER</constant></entry>
            <entry>Failed to set up CPU scheduling. See <varname>CPUSchedulingPolicy=</varname>/<varname>CPUSchedulingPriority=</varname> above.</entry>
          </row>
          <row>
            <entry>215</entry>
            <entry><constant>EXIT_CPUAFFINITY</constant></entry>
            <entry>Failed to set up CPU affinity. See <varname>CPUAffinity=</varname> above.</entry>
          </row>
          <row>
            <entry>216</entry>
            <entry><constant>EXIT_GROUP</constant></entry>
            <entry>Failed to determine or change group credentials. See <varname>Group=</varname>/<varname>SupplementaryGroups=</varname> above.</entry>
          </row>
          <row>
            <entry>217</entry>
            <entry><constant>EXIT_USER</constant></entry>
            <entry>Failed to determine or change user credentials, or to set up user namespacing. See <varname>User=</varname>/<varname>PrivateUsers=</varname> above.</entry>
          </row>
          <row>
            <entry>218</entry>
            <entry><constant>EXIT_CAPABILITIES</constant></entry>
            <entry>Failed to drop capabilities, or apply ambient capabilities. See <varname>CapabilityBoundingSet=</varname>/<varname>AmbientCapabilities=</varname> above.</entry>
          </row>
          <row>
            <entry>219</entry>
            <entry><constant>EXIT_CGROUP</constant></entry>
            <entry>Setting up the service control group failed.</entry>
          </row>
          <row>
            <entry>220</entry>
            <entry><constant>EXIT_SETSID</constant></entry>
            <entry>Failed to create new process session.</entry>
          </row>
          <row>
            <entry>221</entry>
            <entry><constant>EXIT_CONFIRM</constant></entry>
            <entry>Execution has been cancelled by the user. See the <varname>systemd.confirm_spawn=</varname> kernel command line setting on <citerefentry><refentrytitle>kernel-command-line</refentrytitle><manvolnum>7</manvolnum></citerefentry> for details.</entry>
          </row>
          <row>
            <entry>222</entry>
            <entry><constant>EXIT_STDERR</constant></entry>
            <entry>Failed to set up standard error output. See <varname>StandardError=</varname> above.</entry>
          </row>
          <row>
            <entry>224</entry>
            <entry><constant>EXIT_PAM</constant></entry>
            <entry>Failed to set up PAM session. See <varname>PAMName=</varname> above.</entry>
          </row>
          <row>
            <entry>225</entry>
            <entry><constant>EXIT_NETWORK</constant></entry>
            <entry>Failed to set up network namespacing. See <varname>PrivateNetwork=</varname> above.</entry>
          </row>
          <row>
            <entry>226</entry>
            <entry><constant>EXIT_NAMESPACE</constant></entry>
            <entry>Failed to set up mount, UTS, or IPC namespacing. See <varname>ReadOnlyPaths=</varname>, <varname>ProtectHostname=</varname>, <varname>PrivateIPC=</varname>, and related settings above.</entry>
          </row>
          <row>
            <entry>227</entry>
            <entry><constant>EXIT_NO_NEW_PRIVILEGES</constant></entry>
            <entry>Failed to disable new privileges. See <varname>NoNewPrivileges=yes</varname> above.</entry>
          </row>
          <row>
            <entry>228</entry>
            <entry><constant>EXIT_SECCOMP</constant></entry>
            <entry>Failed to apply system call filters. See <varname>SystemCallFilter=</varname> and related settings above.</entry>
          </row>
          <row>
            <entry>229</entry>
            <entry><constant>EXIT_SELINUX_CONTEXT</constant></entry>
            <entry>Determining or changing SELinux context failed. See <varname>SELinuxContext=</varname> above.</entry>
          </row>
          <row>
            <entry>230</entry>
            <entry><constant>EXIT_PERSONALITY</constant></entry>
            <entry>Failed to set up an execution domain (personality). See <varname>Personality=</varname> above.</entry>
          </row>
          <row>
            <entry>231</entry>
            <entry><constant>EXIT_APPARMOR_PROFILE</constant></entry>
            <entry>Failed to prepare changing AppArmor profile. See <varname>AppArmorProfile=</varname> above.</entry>
          </row>
          <row>
            <entry>232</entry>
            <entry><constant>EXIT_ADDRESS_FAMILIES</constant></entry>
            <entry>Failed to restrict address families. See <varname>RestrictAddressFamilies=</varname> above.</entry>
          </row>
          <row>
            <entry>233</entry>
            <entry><constant>EXIT_RUNTIME_DIRECTORY</constant></entry>
            <entry>Setting up runtime directory failed. See <varname>RuntimeDirectory=</varname> and related settings above.</entry>
          </row>
          <row>
            <entry>235</entry>
            <entry><constant>EXIT_CHOWN</constant></entry>
            <entry>Failed to adjust socket ownership. Used for socket units only.</entry>
          </row>
          <row>
            <entry>236</entry>
            <entry><constant>EXIT_SMACK_PROCESS_LABEL</constant></entry>
            <entry>Failed to set SMACK label. See <varname>SmackProcessLabel=</varname> above.</entry>
          </row>
          <row>
            <entry>237</entry>
            <entry><constant>EXIT_KEYRING</constant></entry>
            <entry>Failed to set up kernel keyring.</entry>
          </row>
          <row>
            <entry>238</entry>
            <entry><constant>EXIT_STATE_DIRECTORY</constant></entry>
            <entry>Failed to set up unit's state directory. See <varname>StateDirectory=</varname> above.</entry>
          </row>
          <row>
            <entry>239</entry>
            <entry><constant>EXIT_CACHE_DIRECTORY</constant></entry>
            <entry>Failed to set up unit's cache directory. See <varname>CacheDirectory=</varname> above.</entry>
          </row>
          <row>
            <entry>240</entry>
            <entry><constant>EXIT_LOGS_DIRECTORY</constant></entry>
            <entry>Failed to set up unit's logging directory. See <varname>LogsDirectory=</varname> above.</entry>
          </row>
          <row>
            <entry>241</entry>
            <entry><constant>EXIT_CONFIGURATION_DIRECTORY</constant></entry>
            <entry>Failed to set up unit's configuration directory. See <varname>ConfigurationDirectory=</varname> above.</entry>
          </row>
          <row>
            <entry>242</entry>
            <entry><constant>EXIT_NUMA_POLICY</constant></entry>
            <entry>Failed to set up unit's NUMA memory policy. See <varname>NUMAPolicy=</varname> and <varname>NUMAMask=</varname> above.</entry>
          </row>
          <row>
            <entry>243</entry>
            <entry><constant>EXIT_CREDENTIALS</constant></entry>
            <entry>Failed to set up unit's credentials. See <varname>ImportCredential=</varname>, <varname>LoadCredential=</varname> and <varname>SetCredential=</varname> above.</entry>
          </row>
          <row>
            <entry>245</entry>
            <entry><constant>EXIT_BPF</constant></entry>
            <entry>Failed to apply BPF restrictions. See <varname>RestrictFileSystems=</varname> above.</entry>
          </row>
        </tbody>
      </tgroup>
    </table>

    <para>Finally, the BSD operating systems define a set of exit codes, typically defined on Linux systems too:</para>

    <table>
      <title>BSD exit codes</title>
      <tgroup cols='3'>
        <thead>
          <row>
            <entry>Exit Code</entry>
            <entry>Symbolic Name</entry>
            <entry>Description</entry>
          </row>
        </thead>
        <tbody>
          <row>
            <entry>64</entry>
            <entry><constant>EX_USAGE</constant></entry>
            <entry>Command line usage error</entry>
          </row>
          <row>
            <entry>65</entry>
            <entry><constant>EX_DATAERR</constant></entry>
            <entry>Data format error</entry>
          </row>
          <row>
            <entry>66</entry>
            <entry><constant>EX_NOINPUT</constant></entry>
            <entry>Cannot open input</entry>
          </row>
          <row>
            <entry>67</entry>
            <entry><constant>EX_NOUSER</constant></entry>
            <entry>Addressee unknown</entry>
          </row>
          <row>
            <entry>68</entry>
            <entry><constant>EX_NOHOST</constant></entry>
            <entry>Host name unknown</entry>
          </row>
          <row>
            <entry>69</entry>
            <entry><constant>EX_UNAVAILABLE</constant></entry>
            <entry>Service unavailable</entry>
          </row>
          <row>
            <entry>70</entry>
            <entry><constant>EX_SOFTWARE</constant></entry>
            <entry>internal software error</entry>
          </row>
          <row>
            <entry>71</entry>
            <entry><constant>EX_OSERR</constant></entry>
            <entry>System error (e.g., can't fork)</entry>
          </row>
          <row>
            <entry>72</entry>
            <entry><constant>EX_OSFILE</constant></entry>
            <entry>Critical OS file missing</entry>
          </row>
          <row>
            <entry>73</entry>
            <entry><constant>EX_CANTCREAT</constant></entry>
            <entry>Can't create (user) output file</entry>
          </row>
          <row>
            <entry>74</entry>
            <entry><constant>EX_IOERR</constant></entry>
            <entry>Input/output error</entry>
          </row>
          <row>
            <entry>75</entry>
            <entry><constant>EX_TEMPFAIL</constant></entry>
            <entry>Temporary failure; user is invited to retry</entry>
          </row>
          <row>
            <entry>76</entry>
            <entry><constant>EX_PROTOCOL</constant></entry>
            <entry>Remote error in protocol</entry>
          </row>
          <row>
            <entry>77</entry>
            <entry><constant>EX_NOPERM</constant></entry>
            <entry>Permission denied</entry>
          </row>
          <row>
            <entry>78</entry>
            <entry><constant>EX_CONFIG</constant></entry>
            <entry>Configuration error</entry>
          </row>
        </tbody>
      </tgroup>
    </table>
  </refsect1>

  <refsect1>
    <title>Examples</title>

      <example>
        <title><varname>$MONITOR_<replaceable>*</replaceable></varname> usage</title>

        <para>A service <filename index="false">myfailer.service</filename> which can trigger an
        <varname>OnFailure=</varname> dependency.</para>

        <programlisting>
[Unit]
Description=Service which can trigger an OnFailure= dependency
OnFailure=myhandler.service

[Service]
ExecStart=/bin/myprogram
        </programlisting>

        <para>A service <filename index="false">mysuccess.service</filename> which can trigger an
        <varname>OnSuccess=</varname> dependency.</para>

        <programlisting>
[Unit]
Description=Service which can trigger an OnSuccess= dependency
OnSuccess=myhandler.service

[Service]
ExecStart=/bin/mysecondprogram
        </programlisting>

        <para>A service <filename index="false">myhandler.service</filename> which can be triggered
        by any of the above services.</para>

        <programlisting>
[Unit]
Description=Acts on service failing or succeeding

[Service]
ExecStart=/bin/bash -c "echo $MONITOR_SERVICE_RESULT $MONITOR_EXIT_CODE $MONITOR_EXIT_STATUS $MONITOR_INVOCATION_ID $MONITOR_UNIT"
        </programlisting>

        <para>If <filename index="false">myfailer.service</filename> were to run and exit in failure,
        then <filename index="false">myhandler.service</filename> would be triggered and the
        monitor variables would be set as follows:</para>

        <programlisting>
MONITOR_SERVICE_RESULT=exit-code
MONITOR_EXIT_CODE=exited
MONITOR_EXIT_STATUS=1
MONITOR_INVOCATION_ID=cc8fdc149b2b4ca698d4f259f4054236
MONITOR_UNIT=myfailer.service
        </programlisting>

        <para>If <filename index="false">mysuccess.service</filename> were to run and exit in success,
        then <filename index="false">myhandler.service</filename> would be triggered and the
        monitor variables would be set as follows:</para>

        <programlisting>
MONITOR_SERVICE_RESULT=success
MONITOR_EXIT_CODE=exited
MONITOR_EXIT_STATUS=0
MONITOR_INVOCATION_ID=6ab9af147b8c4a3ebe36e7a5f8611697
MONITOR_UNIT=mysuccess.service
        </programlisting>

    </example>

  </refsect1>

  <refsect1>
      <title>See Also</title>
      <para>
        <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>systemd-analyze</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>systemd.swap</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>systemd.resource-control</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>systemd.time</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>tmpfiles.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
        <citerefentry project='man-pages'><refentrytitle>exec</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
        <citerefentry project='man-pages'><refentrytitle>fork</refentrytitle><manvolnum>2</manvolnum></citerefentry>
      </para>
  </refsect1>

</refentry>