seccomp: drop execve() from @process list

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

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<refentry id="systemd.exec">
  <refentryinfo>
    <title>systemd.exec</title>
    <productname>systemd</productname>

    <authorgroup>
      <author>
        <contrib>Developer</contrib>
        <firstname>Lennart</firstname>
        <surname>Poettering</surname>
        <email>lennart@poettering.net</email>
      </author>
    </authorgroup>
  </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>Automatic Dependencies</title>

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

    <para>Units with <varname>WorkingDirectory=</varname> or
    <varname>RootDirectory=</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>

    <para>Similar, 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>.</para>

    <para>Units whose standard output or error output is connected to <option>journal</option>, <option>syslog</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>
  </refsect1>

  <refsect1>
    <title>Options</title>

    <variablelist class='unit-directives'>

      <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> 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>PrivateUsers=</varname> setting is particularly useful in conjunction with
        <varname>RootDirectory=</varname>. For details, see below.</para></listitem>
      </varlistentry>

      <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 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></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. 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>,
        <varname>PrivateTmp=</varname> are implied. 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. 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 whitelisted
        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. Defaults to off.</para></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>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></listitem>
      </varlistentry>

      <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). See
        <citerefentry><refentrytitle>setpriority</refentrytitle><manvolnum>2</manvolnum></citerefentry>
        for details.</para></listitem>
      </varlistentry>

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

        <listitem><para>Sets the adjustment level for the
        Out-Of-Memory killer for executed processes. Takes an integer
        between -1000 (to disable OOM killing for this process) and
        1000 (to make killing of this process under memory pressure
        very likely). See <ulink
        url="https://www.kernel.org/doc/Documentation/filesystems/proc.txt">proc.txt</ulink>
        for details.</para></listitem>
      </varlistentry>

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

        <listitem><para>Sets the I/O scheduling class for executed
        processes. Takes an integer between 0 and 3 or one of the
        strings <option>none</option>, <option>realtime</option>,
        <option>best-effort</option> or <option>idle</option>. 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). The available priorities depend on the
        selected I/O scheduling class (see above). See
        <citerefentry><refentrytitle>ioprio_set</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><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. See
        <citerefentry><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 fork, and can hence not leak into child
        processes. See
        <citerefentry><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. 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><refentrytitle>sched_setaffinity</refentrytitle><manvolnum>2</manvolnum></citerefentry>
        for details.</para></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.</para></listitem>
      </varlistentry>

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

        <listitem><para>Sets environment variables for executed
        processes. Takes a space-separated list of variable
        assignments. This option may be specified more than once, in
        which case all listed variables will be set. If the same
        variable is set 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. Variable expansion is not
        performed inside the strings, however, specifier expansion is
        possible. The $ character has no special meaning. If you need
        to assign a value containing spaces to a variable, use double
        quotes (") for the assignment.</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></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 new-line-separated variable assignments.
        Empty lines, lines without an <literal>=</literal> separator,
        or lines starting with ; or # will be ignored,
        which may be used for commenting. A line ending with a
        backslash will be concatenated with the following one,
        allowing multiline variable definitions. The parser strips
        leading and trailing whitespace from the values of
        assignments, unless you use double quotes (").</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).</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 from the systemd system
        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 set. If the empty string is assigned to this
        option, the list of environment variables is reset, all prior
        assignments have no effect. Variables that are not set in the system
        manager will not be passed and will be silently ignored.</para>

        <para>Variables passed from this setting are overridden by those passed
        from <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></listitem>
      </varlistentry>

      <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>socket</option> or
        <option>fd</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>socket</option> option is only valid in
        socket-activated services, and only when the socket
        configuration file (see
        <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>
        for details) specifies 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>
        daemon.</para>

        <para>The <option>fd</option> option connects
        the input stream to a single file descriptor provided by a socket unit.
        A custom named file descriptor can be specified as part of this option,
        after a <literal>:</literal> (e.g. <literal>fd:<replaceable>foobar</replaceable></literal>).
        If no name is specified, <literal>stdin</literal> is assumed
        (i.e. <literal>fd</literal> is equivalent to <literal>fd:stdin</literal>).
        At least one socket unit defining such name must be explicitly provided via the
        <varname>Sockets=</varname> option, and 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 ordering.</para>

        <para>This setting defaults to
        <option>null</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>syslog</option>,
        <option>kmsg</option>,
        <option>journal+console</option>,
        <option>syslog+console</option>,
        <option>kmsg+console</option>,
        <option>socket</option> or
        <option>fd</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 syslog or kmsg (see
        below) is implicitly stored in the journal as well, the
        specific two options listed below are hence supersets of this
        one.</para>

        <para><option>syslog</option> connects standard output to the
        <citerefentry project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
        system syslog service, in addition to the journal. Note that
        the journal daemon is usually configured to forward everything
        it receives to syslog anyway, in which case this option is no
        different from <option>journal</option>.</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>,
        <option>syslog+console</option> and
        <option>kmsg+console</option> work in a similar way as the
        three options above but copy the output to the system console
        as well.</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>.</para>

        <para>The <option>fd</option> option connects
        the output stream to a single file descriptor provided by a socket unit.
        A custom named file descriptor can be specified as part of this option,
        after a <literal>:</literal> (e.g. <literal>fd:<replaceable>foobar</replaceable></literal>).
        If no name is specified, <literal>stdout</literal> is assumed
        (i.e. <literal>fd</literal> is equivalent to <literal>fd:stdout</literal>).
        At least one socket unit defining such name must be explicitly provided via the
        <varname>Sockets=</varname> option, and 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 ordering.</para>

        <para>If the standard output (or error output, see below) of a unit is connected to the journal, syslog 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 automatic dependencies section above).</para>

        <para>This setting defaults to the value set with
        <option>DefaultStandardOutput=</option> 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</option> operates on the error
        stream and will look by default for a descriptor named
        <literal>stderr</literal>.</para>

        <para>This setting defaults to the value set with
        <option>DefaultStandardError=</option> 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>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>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>
      <varlistentry>
        <term><varname>SyslogIdentifier=</varname></term>
        <listitem><para>Sets the process name 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>syslog</option>, <option>journal</option> or
        <option>kmsg</option> (or to the same settings in combination
        with <option>+console</option>).</para></listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>SyslogFacility=</varname></term>
        <listitem><para>Sets the syslog facility to use when logging
        to syslog. 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>syslog</option>. Defaults to
        <option>daemon</option>.</para></listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>SyslogLevel=</varname></term>
        <listitem><para>The default syslog level to use when logging to
        syslog 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>syslog</option> or <option>kmsg</option>. Note that
        individual lines output by the daemon might 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>syslog</option>, <option>kmsg</option> or
        <option>journal</option>, log lines written by the executed
        process that are prefixed with a log level will be passed on
        to syslog 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. For details about
        this prefixing see
        <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
        Defaults to true.</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>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 resource limit concept. 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 <varname>infinity</varname>
        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. LimitAS=16G). 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>MemoryLimit=</varname> is a more powerful (and
        working) replacement for <varname>LimitRSS=</varname>.</para>

        <para>For system units these resource limits may be chosen freely. For user units however (i.e. units run by a
        per-user instance of
        <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>), these limits are
        bound by (possibly more restrictive) per-user limits enforced by the OS.</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 above).</para>

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

          <tgroup cols='3'>
            <colspec colname='directive' />
            <colspec colname='equivalent' />
            <colspec colname='unit' />
            <thead>
              <row>
                <entry>Directive</entry>
                <entry><command>ulimit</command> equivalent</entry>
                <entry>Unit</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry>LimitCPU=</entry>
                <entry>ulimit -t</entry>
                <entry>Seconds</entry>
              </row>
              <row>
                <entry>LimitFSIZE=</entry>
                <entry>ulimit -f</entry>
                <entry>Bytes</entry>
              </row>
              <row>
                <entry>LimitDATA=</entry>
                <entry>ulimit -d</entry>
                <entry>Bytes</entry>
              </row>
              <row>
                <entry>LimitSTACK=</entry>
                <entry>ulimit -s</entry>
                <entry>Bytes</entry>
              </row>
              <row>
                <entry>LimitCORE=</entry>
                <entry>ulimit -c</entry>
                <entry>Bytes</entry>
              </row>
              <row>
                <entry>LimitRSS=</entry>
                <entry>ulimit -m</entry>
                <entry>Bytes</entry>
              </row>
              <row>
                <entry>LimitNOFILE=</entry>
                <entry>ulimit -n</entry>
                <entry>Number of File Descriptors</entry>
              </row>
              <row>
                <entry>LimitAS=</entry>
                <entry>ulimit -v</entry>
                <entry>Bytes</entry>
              </row>
              <row>
                <entry>LimitNPROC=</entry>
                <entry>ulimit -u</entry>
                <entry>Number of Processes</entry>
              </row>
              <row>
                <entry>LimitMEMLOCK=</entry>
                <entry>ulimit -l</entry>
                <entry>Bytes</entry>
              </row>
              <row>
                <entry>LimitLOCKS=</entry>
                <entry>ulimit -x</entry>
                <entry>Number of Locks</entry>
              </row>
              <row>
                <entry>LimitSIGPENDING=</entry>
                <entry>ulimit -i</entry>
                <entry>Number of Queued Signals</entry>
              </row>
              <row>
                <entry>LimitMSGQUEUE=</entry>
                <entry>ulimit -q</entry>
                <entry>Bytes</entry>
              </row>
              <row>
                <entry>LimitNICE=</entry>
                <entry>ulimit -e</entry>
                <entry>Nice Level</entry>
              </row>
              <row>
                <entry>LimitRTPRIO=</entry>
                <entry>ulimit -r</entry>
                <entry>Realtime Priority</entry>
              </row>
              <row>
                <entry>LimitRTTIME=</entry>
                <entry>No equivalent</entry>
                <entry>Microseconds</entry>
              </row>
            </tbody>
          </tgroup>
        </table></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. 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></listitem>
      </varlistentry>

      <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. 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></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.  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 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></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>

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

        <listitem><para>Sets up a new file system namespace for executed processes. These options may be used to limit
        access a process might have to the file system hierarchy. 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>.</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 whitelist
        specific paths for write access if <varname>ProtectSystem=strict</varname> is used. 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).</para>

        <para>Note that restricting access with these options does not extend to submounts of a directory that are
        created later on.  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> and
        <varname>InaccessiblePaths=</varname> may be prefixed with <literal>-</literal>, in which case they will be ignored
        when they do not exist. 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. 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></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 is 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 this is enabled, 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.</para></listitem>

      </varlistentry>

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

        <listitem><para>Takes a boolean argument. If true, sets up a new /dev namespace 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 securely turn off physical device access by the
        executed process. Defaults to false. 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, will also remove
        <constant>CAP_MKNOD</constant> and <constant>CAP_SYS_RAWIO</constant> from the capability bounding set for
        the unit (see above), 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 /dev namespace 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>. 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.</para></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
        securely 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, this includes AF_NETLINK and AF_UNIX.
        The latter has the effect that AF_UNIX sockets in the abstract
        socket namespace will become unavailable to the processes
        (however, those located in the file system will continue to be
        accessible).</para></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>This setting is particularly useful in conjunction with <varname>RootDirectory=</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></listitem>
      </varlistentry>

      <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 <filename>/boot</filename>
        directories 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. 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.</para></listitem>
      </varlistentry>

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

        <listitem><para>Takes a boolean argument or <literal>read-only</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. 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. For this setting the same restrictions regarding mount propagation and privileges apply as for
        <varname>ReadOnlyPaths=</varname> and related calls, see above.</para></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 only be written at
        boot-time, with the <citerefentry><refentrytitle>sysctl.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>
        mechanism. Almost no 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.
        Note that this option does not prevent kernel tuning through IPC interfaces and external programs. However
        <varname>InaccessiblePaths=</varname> can be used to make some IPC file system objects
        inaccessible.</para></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.</para></listitem>
      </varlistentry>

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

        <listitem><para>Takes a mount propagation flag: <option>shared</option>, <option>slave</option> or
        <option>private</option>, which control whether mounts in the file system namespace set up for this unit's
        processes will receive or propagate mounts or unmounts. See <citerefentry
        project='man-pages'><refentrytitle>mount</refentrytitle><manvolnum>2</manvolnum></citerefentry> for
        details. Defaults to <option>shared</option>. Use <option>shared</option> to ensure that mounts and unmounts
        are propagated from the host to the container and vice versa. Use <option>slave</option> to run processes so
        that none of their mounts and unmounts will propagate to the host. Use <option>private</option> to also ensure
        that no mounts and unmounts from the host will propagate into the unit processes' namespace. Note that
        <option>slave</option> means that file systems mounted on the host might stay mounted continuously in the
        unit's namespace, and thus keep the device busy. Note that the file system namespace related options
        (<varname>PrivateTmp=</varname>, <varname>PrivateDevices=</varname>, <varname>ProtectSystem=</varname>,
        <varname>ProtectHome=</varname>, <varname>ProtectKernelTunables=</varname>,
        <varname>ProtectControlGroups=</varname>, <varname>ReadOnlyPaths=</varname>,
        <varname>InaccessiblePaths=</varname>, <varname>ReadWritePaths=</varname>) require that mount and unmount
        propagation from the unit's file system namespace is disabled, and hence downgrade <option>shared</option> to
        <option>slave</option>. </para></listitem>
      </varlistentry>

      <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></listitem>
      </varlistentry>

      <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>, all errors
        will be ignored. This does not affect commands prefixed with <literal>+</literal>.
        See <citerefentry project='die-net'><refentrytitle>setexeccon</refentrytitle><manvolnum>3</manvolnum></citerefentry>
        for details.</para></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. This result in a non operation if AppArmor is not
        enabled. If prefixed by <literal>-</literal>, all errors will
        be ignored. This does not affect commands prefixed with <literal>+</literal>.</para></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>
        </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>

      <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. This option is more
        powerful than the respective secure bits flags (see above), as it also prohibits
        UID changes of any kind. 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 in the user manager instance certain settings force
        <varname>NoNewPrivileges=yes</varname>, ignoring the value of this setting.
        Those is the case when <varname>SystemCallFilter=</varname>,
        <varname>SystemCallArchitectures=</varname>,
        <varname>RestrictAddressFamilies=</varname>,
        <varname>PrivateDevices=</varname>,
        <varname>ProtectKernelTunables=</varname>,
        <varname>ProtectKernelModules=</varname>,
        <varname>MemoryDenyWriteExecute=</varname>, or
        <varname>RestrictRealtime=</varname> are specified.
        </para></listitem>
      </varlistentry>

      <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 (whitelisting). 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
        (blacklisting). If running in user mode, or in system mode, but without the <constant>CAP_SYS_ADMIN</constant>
        capability (e.g. setting <varname>User=nobody</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 whitelisted 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>If you specify both types of this option (i.e.
        whitelisting and blacklisting), 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 a whitelisting of
        <function>read</function> and <function>write</function>, and
        right after it add a blacklisting of
        <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>@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>@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>SysV IPC, POSIX Message Queues or 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>@module</entry>
                <entry>Kernel module control (<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>File system mounting and unmounting (<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>@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, namespaces (<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>
            </tbody>
          </tgroup>
        </table>

        Note, that as new system calls are added to the kernel, additional system calls might be added to the groups
        above, so the contents of the sets may change between systemd versions.</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>ReadOnlyPaths=</varname>, <varname>InaccessiblePaths=</varname> and
        <varname>ReadWritePaths=</varname>.</para></listitem>
      </varlistentry>

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

        <listitem><para>Takes an <literal>errno</literal> error number
        name to return when the system call filter configured with
        <varname>SystemCallFilter=</varname> is triggered, instead of
        terminating the process immediately. Takes an error name such
        as <constant>EPERM</constant>, <constant>EACCES</constant> or
        <constant>EUCLEAN</constant>. When this setting is not used,
        or when the empty string is assigned, the process will be
        terminated immediately when the filter is
        triggered.</para></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>. Only system calls of the specified architectures will
        be permitted to processes of this unit. This is an effective way to disable
        compatibility with non-native architectures for processes, for example to prohibit
        execution of 32-bit x86 binaries on 64-bit x86-64 systems. The special
        <constant>native</constant> identifier implicitly maps to the native architecture
        of the system (or more strictly: 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=nobody</varname>), <varname>NoNewPrivileges=yes</varname> is
        implied. Note that setting this option to a non-empty list implies that
        <constant>native</constant> is included too. By default, this option is set to the
        empty list, i.e. no architecture system call filtering is applied.
        </para></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 a
        space-separated list of address family names to whitelist,
        such as
        <constant>AF_UNIX</constant>,
        <constant>AF_INET</constant> or
        <constant>AF_INET6</constant>. When
        prefixed with <constant>~</constant> the listed address
        families will be applied as blacklist, otherwise as whitelist.
        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 and is ignored (but works
        correctly on x86-64). If running in user mode, or in system
        mode, but without the <constant>CAP_SYS_ADMIN</constant>
        capability (e.g. setting <varname>User=nobody</varname>),
        <varname>NoNewPrivileges=yes</varname> is implied. By
        default, no restriction applies, all address families are
        accessible to processes. If assigned the empty string, any
        previous list changes are undone.</para>

        <para>Use this option to limit exposure of processes to remote
        systems, in particular via exotic network protocols. Note that
        in most cases, the local <constant>AF_UNIX</constant> address
        family should be included in the configured whitelist as it is
        frequently used for local communication, including for
        <citerefentry><refentrytitle>syslog</refentrytitle><manvolnum>2</manvolnum></citerefentry>
        logging. This does not affect commands prefixed with <literal>+</literal>.</para></listitem>
      </varlistentry>

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

        <listitem><para>Takes a boolean argument. If true, explicit module loading will
        be denied. This allows to turn off module load and unload operations 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. Default 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.</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>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 system architecture. Usually the 64bit versions of the various
        system architectures support their immediate 32bit 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.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>RuntimeDirectory=</varname></term>
        <term><varname>RuntimeDirectoryMode=</varname></term>

        <listitem><para>Takes a list of directory names. If set, one
        or more directories by the specified names will be created
        below <filename>/run</filename> (for system services) or below
        <varname>$XDG_RUNTIME_DIR</varname> (for user services) when
        the unit is started, and removed when the unit is stopped. The
        directories will have the access mode specified in
        <varname>RuntimeDirectoryMode=</varname>, and will be owned by
        the user and group specified in <varname>User=</varname> and
        <varname>Group=</varname>. Use this to manage one or more
        runtime directories of the unit and bind their lifetime to the
        daemon runtime. The specified directory names must be
        relative, and may not include a <literal>/</literal>, i.e.
        must refer to simple directories to create or remove. 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></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 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>
        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
        that generate program code dynamically at runtime, such as JIT execution engines, or programs compiled making
        use of the code "trampoline" feature of various C compilers. This option improves service security, as it makes
        harder for software exploits to change running code dynamically.
        </para></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. 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></listitem>
      </varlistentry>

    </variablelist>
  </refsect1>

  <refsect1>
    <title>Environment variables in spawned processes</title>

    <para>Processes started by the system are executed in a clean
    environment in which select variables listed below are set. System
    processes started by systemd do not inherit variables from PID 1,
    but processes started by user systemd instances inherit all
    environment variables from the user systemd instance.
    </para>

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

        <listitem><para>Colon-separated list of directories to use
        when launching executables. Systemd uses a fixed value of
        <filename>/usr/local/sbin</filename>:<filename>/usr/local/bin</filename>:<filename>/usr/sbin</filename>:<filename>/usr/bin</filename>:<filename>/sbin</filename>:<filename>/bin</filename>.
        </para></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></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></listitem>
      </varlistentry>

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

        <listitem><para>Contains a randomized, unique 128bit 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></listitem>
      </varlistentry>

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

        <listitem><para>The directory for volatile state. Set for the
        user <command>systemd</command> instance, and also in user
        sessions. See
        <citerefentry><refentrytitle>pam_systemd</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>$XDG_SESSION_ID</varname></term>
        <term><varname>$XDG_SEAT</varname></term>
        <term><varname>$XDG_VTNR</varname></term>

        <listitem><para>The identifier of the session, the seat name,
        and virtual terminal of the session. Set by
        <citerefentry><refentrytitle>pam_systemd</refentrytitle><manvolnum>8</manvolnum></citerefentry>
        for login sessions. <varname>$XDG_SEAT</varname> and
        <varname>$XDG_VTNR</varname> will only be set when attached to
        a seat and a tty.</para></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></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></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></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></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></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></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>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></listitem>
      </varlistentry>

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

        <listitem><para>Only defined 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: <literal>timeout</literal> (in case of an operation
        timeout), <literal>exit-code</literal> (if a service process exited with a non-zero exit code; see
        <varname>$EXIT_CODE</varname> below for the actual exit code returned), <literal>signal</literal> (if a
        service process was terminated abnormally by a signal; see <varname>$EXIT_CODE</varname> below for the actual
        signal used for the termination), <literal>core-dump</literal> (if a service process terminated abnormally and
        dumped core), <literal>watchdog</literal> (if the watchdog keep-alive ping was enabled for the service but it
        missed the deadline), or <literal>resources</literal> (a catch-all condition in case a system operation
        failed).</para>

        <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></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='status' />
            <colspec colname='code' />
            <thead>
              <row>
                <entry><varname>$SERVICE_RESULT</varname></entry>
                <entry><varname>$EXIT_STATUS</varname></entry>
                <entry><varname>$EXIT_CODE</varname></entry>
              </row>
            </thead>

            <tbody>
              <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>0</literal>, <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><literal>resources</literal></entry>
                <entry>any of the above</entry>
                <entry>any of the above</entry>
              </row>

              <row>
                <entry namest="results" nameend="code">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.</entry>
              </row>
            </tbody>
          </tgroup>
        </table>

        </listitem>
      </varlistentry>
    </variablelist>

    <para>Additional variables may be configured by the following
    means: for processes spawned in specific units, use the
    <varname>Environment=</varname>, <varname>EnvironmentFile=</varname>
    and <varname>PassEnvironment=</varname> options above; to specify
    variables globally, use <varname>DefaultEnvironment=</varname>
    (see
    <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>)
    or the kernel option <varname>systemd.setenv=</varname> (see
    <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>).
    Additional variables may also be set through PAM,
    cf. <citerefentry project='man-pages'><refentrytitle>pam_env</refentrytitle><manvolnum>8</manvolnum></citerefentry>.</para>
  </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>journalctl</refentrytitle><manvolnum>8</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>
      </para>
  </refsect1>


</refentry>