nspawn: use right constant for shifting for uint64_t caps

[thirdparty/systemd.git] / man / systemd-nspawn.xml

<?xml version='1.0'?>
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd" [
<!ENTITY fedora_latest_version "28">
<!ENTITY fedora_cloud_release "1.1">
]>
<!-- SPDX-License-Identifier: LGPL-2.1+ -->

<refentry id="systemd-nspawn"
    xmlns:xi="http://www.w3.org/2001/XInclude">

  <refentryinfo>
    <title>systemd-nspawn</title>
    <productname>systemd</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>systemd-nspawn</refentrytitle>
    <manvolnum>1</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>systemd-nspawn</refname>
    <refpurpose>Spawn a command or OS in a light-weight container</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <cmdsynopsis>
      <command>systemd-nspawn</command>
      <arg choice="opt" rep="repeat">OPTIONS</arg>
      <arg choice="opt"><replaceable>COMMAND</replaceable>
      <arg choice="opt" rep="repeat">ARGS</arg>
      </arg>
    </cmdsynopsis>
    <cmdsynopsis>
      <command>systemd-nspawn</command>
      <arg choice="plain">--boot</arg>
      <arg choice="opt" rep="repeat">OPTIONS</arg>
      <arg choice="opt" rep="repeat">ARGS</arg>
    </cmdsynopsis>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para><command>systemd-nspawn</command> may be used to run a command or OS in a light-weight namespace
    container. In many ways it is similar to <citerefentry
    project='man-pages'><refentrytitle>chroot</refentrytitle><manvolnum>1</manvolnum></citerefentry>, but more powerful
    since it fully virtualizes the file system hierarchy, as well as the process tree, the various IPC subsystems and
    the host and domain name.</para>

    <para><command>systemd-nspawn</command> may be invoked on any directory tree containing an operating system tree,
    using the <option>--directory=</option> command line option. By using the <option>--machine=</option> option an OS
    tree is automatically searched for in a couple of locations, most importantly in
    <filename>/var/lib/machines</filename>, the suggested directory to place OS container images installed on the
    system.</para>

    <para>In contrast to <citerefentry
    project='man-pages'><refentrytitle>chroot</refentrytitle><manvolnum>1</manvolnum></citerefentry> <command>systemd-nspawn</command>
    may be used to boot full Linux-based operating systems in a container.</para>

    <para><command>systemd-nspawn</command> limits access to various kernel interfaces in the container to read-only,
    such as <filename>/sys</filename>, <filename>/proc/sys</filename> or <filename>/sys/fs/selinux</filename>. The
    host's network interfaces and the system clock may not be changed from within the container. Device nodes may not
    be created. The host system cannot be rebooted and kernel modules may not be loaded from within the
    container.</para>

    <para>Use a tool like <citerefentry
    project='mankier'><refentrytitle>dnf</refentrytitle><manvolnum>8</manvolnum></citerefentry>, <citerefentry
    project='die-net'><refentrytitle>debootstrap</refentrytitle><manvolnum>8</manvolnum></citerefentry>, or
    <citerefentry project='archlinux'><refentrytitle>pacman</refentrytitle><manvolnum>8</manvolnum></citerefentry> to
    set up an OS directory tree suitable as file system hierarchy for <command>systemd-nspawn</command> containers. See
    the Examples section below for details on suitable invocation of these commands.</para>

    <para>As a safety check <command>systemd-nspawn</command> will verify the existence of
    <filename>/usr/lib/os-release</filename> or <filename>/etc/os-release</filename> in the container tree before
    starting the container (see
    <citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry>).  It might be
    necessary to add this file to the container tree manually if the OS of the container is too old to contain this
    file out-of-the-box.</para>

    <para><command>systemd-nspawn</command> may be invoked directly from the interactive command line or run as system
    service in the background. In this mode each container instance runs as its own service instance; a default
    template unit file <filename>systemd-nspawn@.service</filename> is provided to make this easy, taking the container
    name as instance identifier. Note that different default options apply when <command>systemd-nspawn</command> is
    invoked by the template unit file than interactively on the command line. Most importantly the template unit file
    makes use of the <option>--boot</option> which is not the default in case <command>systemd-nspawn</command> is
    invoked from the interactive command line. Further differences with the defaults are documented along with the
    various supported options below.</para>

    <para>The <citerefentry><refentrytitle>machinectl</refentrytitle><manvolnum>1</manvolnum></citerefentry> tool may
    be used to execute a number of operations on containers. In particular it provides easy-to-use commands to run
    containers as system services using the <filename>systemd-nspawn@.service</filename> template unit
    file.</para>

    <para>Along with each container a settings file with the <filename>.nspawn</filename> suffix may exist, containing
    additional settings to apply when running the container. See
    <citerefentry><refentrytitle>systemd.nspawn</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
    details. Settings files override the default options used by the <filename>systemd-nspawn@.service</filename>
    template unit file, making it usually unnecessary to alter this template file directly.</para>

    <para>Note that <command>systemd-nspawn</command> will mount file systems private to the container to
    <filename>/dev</filename>, <filename>/run</filename> and similar. These will not be visible outside of the
    container, and their contents will be lost when the container exits.</para>

    <para>Note that running two <command>systemd-nspawn</command> containers from the same directory tree will not make
    processes in them see each other. The PID namespace separation of the two containers is complete and the containers
    will share very few runtime objects except for the underlying file system. Use
    <citerefentry><refentrytitle>machinectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
    <command>login</command> or <command>shell</command> commands to request an additional login session in a running
    container.</para>

    <para><command>systemd-nspawn</command> implements the <ulink
    url="https://www.freedesktop.org/wiki/Software/systemd/ContainerInterface">Container Interface</ulink>
    specification.</para>

    <para>While running, containers invoked with <command>systemd-nspawn</command> are registered with the
    <citerefentry><refentrytitle>systemd-machined</refentrytitle><manvolnum>8</manvolnum></citerefentry> service that
    keeps track of running containers, and provides programming interfaces to interact with them.</para>
  </refsect1>

  <refsect1>
    <title>Options</title>

    <para>If option <option>-b</option> is specified, the arguments
    are used as arguments for the init program. Otherwise,
    <replaceable>COMMAND</replaceable> specifies the program to launch
    in the container, and the remaining arguments are used as
    arguments for this program. If <option>--boot</option> is not used and
    no arguments are specified, a shell is launched in the
    container.</para>

    <para>The following options are understood:</para>

    <variablelist>
      <varlistentry>
        <term><option>-D</option></term>
        <term><option>--directory=</option></term>

        <listitem><para>Directory to use as file system root for the
        container.</para>

        <para>If neither <option>--directory=</option>, nor
        <option>--image=</option> is specified the directory is
        determined by searching for a directory named the same as the
        machine name specified with <option>--machine=</option>. See
        <citerefentry><refentrytitle>machinectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
        section "Files and Directories" for the precise search path.</para>

        <para>If neither <option>--directory=</option>,
        <option>--image=</option>, nor <option>--machine=</option>
        are specified, the current directory will
        be used. May not be specified together with
        <option>--image=</option>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--template=</option></term>

        <listitem><para>Directory or <literal>btrfs</literal> subvolume to use as template for the
        container's root directory. If this is specified and the container's root directory (as configured by
        <option>--directory=</option>) does not yet exist it is created as <literal>btrfs</literal> snapshot
        (if supported) or plain directory (otherwise) and populated from this template tree. Ideally, the
        specified template path refers to the root of a <literal>btrfs</literal> subvolume, in which case a
        simple copy-on-write snapshot is taken, and populating the root directory is instant. If the
        specified template path does not refer to the root of a <literal>btrfs</literal> subvolume (or not
        even to a <literal>btrfs</literal> file system at all), the tree is copied (though possibly in a
        'reflink' copy-on-write scheme — if the file system supports that), which can be substantially more
        time-consuming. Note that the snapshot taken is of the specified directory or subvolume, including
        all subdirectories and subvolumes below it, but excluding any sub-mounts. May not be specified
        together with <option>--image=</option> or <option>--ephemeral</option>.</para>

        <para>Note that this switch leaves host name, machine ID and
        all other settings that could identify the instance
        unmodified.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-x</option></term>
        <term><option>--ephemeral</option></term>

        <listitem><para>If specified, the container is run with a temporary snapshot of its file system that is removed
        immediately when the container terminates. May not be specified together with
        <option>--template=</option>.</para>
        <para>Note that this switch leaves host name, machine ID and all other settings that could identify
        the instance unmodified. Please note that — as with <option>--template=</option> — taking the
        temporary snapshot is more efficient on file systems that support subvolume snapshots or 'reflinks'
        natively (<literal>btrfs</literal> or new <literal>xfs</literal>) than on more traditional file
        systems that do not (<literal>ext4</literal>). Note that the snapshot taken is of the specified
        directory or subvolume, including all subdirectories and subvolumes below it, but excluding any
        sub-mounts.</para>

        <para>With this option no modifications of the container image are retained. Use
        <option>--volatile=</option> (described below) for other mechanisms to restrict persistency of
        container images during runtime.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-i</option></term>
        <term><option>--image=</option></term>

        <listitem><para>Disk image to mount the root directory for the
        container from. Takes a path to a regular file or to a block
        device node. The file or block device must contain
        either:</para>

        <itemizedlist>
          <listitem><para>An MBR partition table with a single
          partition of type 0x83 that is marked
          bootable.</para></listitem>

          <listitem><para>A GUID partition table (GPT) with a single
          partition of type
          0fc63daf-8483-4772-8e79-3d69d8477de4.</para></listitem>

          <listitem><para>A GUID partition table (GPT) with a marked
          root partition which is mounted as the root directory of the
          container. Optionally, GPT images may contain a home and/or
          a server data partition which are mounted to the appropriate
          places in the container. All these partitions must be
          identified by the partition types defined by the <ulink
          url="https://www.freedesktop.org/wiki/Specifications/DiscoverablePartitionsSpec/">Discoverable
          Partitions Specification</ulink>.</para></listitem>

          <listitem><para>No partition table, and a single file system spanning the whole image.</para></listitem>
        </itemizedlist>

        <para>On GPT images, if an EFI System Partition (ESP) is discovered, it is automatically mounted to
        <filename>/efi</filename> (or <filename>/boot</filename> as fallback) in case a directory by this name exists
        and is empty.</para>

        <para>Partitions encrypted with LUKS are automatically decrypted. Also, on GPT images dm-verity data integrity
        hash partitions are set up if the root hash for them is specified using the <option>--root-hash=</option>
        option.</para>

        <para>Any other partitions, such as foreign partitions or swap partitions are not mounted. May not be specified
        together with <option>--directory=</option>, <option>--template=</option>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--root-hash=</option></term>

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

      <varlistentry>
        <term><option>-a</option></term>
        <term><option>--as-pid2</option></term>

        <listitem><para>Invoke the shell or specified program as process ID (PID) 2 instead of PID 1 (init). By
        default, if neither this option nor <option>--boot</option> is used, the selected program is run as the process
        with PID 1, a mode only suitable for programs that are aware of the special semantics that the process with
        PID 1 has on UNIX. For example, it needs to reap all processes reparented to it, and should implement
        <command>sysvinit</command> compatible signal handling (specifically: it needs to reboot on SIGINT, reexecute
        on SIGTERM, reload configuration on SIGHUP, and so on). With <option>--as-pid2</option> a minimal stub init
        process is run as PID 1 and the selected program is executed as PID 2 (and hence does not need to implement any
        special semantics). The stub init process will reap processes as necessary and react appropriately to
        signals. It is recommended to use this mode to invoke arbitrary commands in containers, unless they have been
        modified to run correctly as PID 1. Or in other words: this switch should be used for pretty much all commands,
        except when the command refers to an init or shell implementation, as these are generally capable of running
        correctly as PID 1. This option may not be combined with <option>--boot</option>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-b</option></term>
        <term><option>--boot</option></term>

        <listitem><para>Automatically search for an init program and invoke it as PID 1, instead of a shell or a user
        supplied program. If this option is used, arguments specified on the command line are used as arguments for the
        init program. This option may not be combined with <option>--as-pid2</option>.</para>

        <para>The following table explains the different modes of invocation and relationship to
        <option>--as-pid2</option> (see above):</para>

        <table>
          <title>Invocation Mode</title>
          <tgroup cols='2' align='left' colsep='1' rowsep='1'>
            <colspec colname="switch" />
            <colspec colname="explanation" />
            <thead>
              <row>
                <entry>Switch</entry>
                <entry>Explanation</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry>Neither <option>--as-pid2</option> nor <option>--boot</option> specified</entry>
                <entry>The passed parameters are interpreted as the command line, which is executed as PID 1 in the container.</entry>
              </row>

              <row>
                <entry><option>--as-pid2</option> specified</entry>
                <entry>The passed parameters are interpreted as the command line, which is executed as PID 2 in the container. A stub init process is run as PID 1.</entry>
              </row>

              <row>
                <entry><option>--boot</option> specified</entry>
                <entry>An init program is automatically searched for and run as PID 1 in the container. The passed parameters are used as invocation parameters for this process.</entry>
              </row>

            </tbody>
          </tgroup>
        </table>

        <para>Note that <option>--boot</option> is the default mode of operation if the
        <filename>systemd-nspawn@.service</filename> template unit file is used.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--chdir=</option></term>

        <listitem><para>Change to the specified working directory before invoking the process in the container. Expects
        an absolute path in the container's file system namespace.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--pivot-root=</option></term>

        <listitem><para>Pivot the specified directory to <filename>/</filename> inside the container, and either unmount the
        container's old root, or pivot it to another specified directory. Takes one of: a path argument — in which case the
        specified path will be pivoted to <filename>/</filename> and the old root will be unmounted; or a colon-separated pair
        of new root path and pivot destination for the old root. The new root path will be pivoted to <filename>/</filename>,
        and the old <filename>/</filename> will be pivoted to the other directory. Both paths must be absolute, and are resolved
        in the container's file system namespace.</para>

        <para>This is for containers which have several bootable directories in them; for example, several
        <ulink url="https://ostree.readthedocs.io/en/latest/">OSTree</ulink> deployments. It emulates the behavior of
        the boot loader and initial RAM disk which normally select which directory to mount as the root and start the
        container's PID 1 in.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-u</option></term>
        <term><option>--user=</option></term>

        <listitem><para>After transitioning into the container, change
        to the specified user-defined in the container's user
        database. Like all other systemd-nspawn features, this is not
        a security feature and provides protection against accidental
        destructive operations only.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-M</option></term>
        <term><option>--machine=</option></term>

        <listitem><para>Sets the machine name for this container. This
        name may be used to identify this container during its runtime
        (for example in tools like
        <citerefentry><refentrytitle>machinectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
        and similar), and is used to initialize the container's
        hostname (which the container can choose to override,
        however). If not specified, the last component of the root
        directory path of the container is used, possibly suffixed
        with a random identifier in case <option>--ephemeral</option>
        mode is selected. If the root directory selected is the host's
        root directory the host's hostname is used as default
        instead.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--hostname=</option></term>

        <listitem><para>Controls the hostname to set within the container, if different from the machine name. Expects
        a valid hostname as argument. If this option is used, the kernel hostname of the container will be set to this
        value, otherwise it will be initialized to the machine name as controlled by the <option>--machine=</option>
        option described above. The machine name is used for various aspect of identification of the container from the
        outside, the kernel hostname configurable with this option is useful for the container to identify itself from
        the inside. It is usually a good idea to keep both forms of identification synchronized, in order to avoid
        confusion. It is hence recommended to avoid usage of this option, and use <option>--machine=</option>
        exclusively. Note that regardless whether the container's hostname is initialized from the name set with
        <option>--hostname=</option> or the one set with <option>--machine=</option>, the container can later override
        its kernel hostname freely on its own as well.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--uuid=</option></term>

        <listitem><para>Set the specified UUID for the container. The
        init system will initialize
        <filename>/etc/machine-id</filename> from this if this file is
        not set yet. Note that this option takes effect only if
        <filename>/etc/machine-id</filename> in the container is
        unpopulated.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-S</option></term>
        <term><option>--slice=</option></term>

        <listitem><para>Make the container part of the specified slice, instead of the default
        <filename>machine.slice</filename>. This applies only if the machine is run in its own scope unit, i.e. if
        <option>--keep-unit</option> isn't used.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--property=</option></term>

        <listitem><para>Set a unit property on the scope unit to register for the machine. This applies only if the
        machine is run in its own scope unit, i.e. if <option>--keep-unit</option> isn't used. Takes unit property
        assignments in the same format as <command>systemctl set-property</command>. This is useful to set memory
        limits and similar for container.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--private-users=</option></term>

        <listitem><para>Controls user namespacing. If enabled, the container will run with its own private set of UNIX
        user and group ids (UIDs and GIDs). This involves mapping the private UIDs/GIDs used in the container (starting
        with the container's root user 0 and up) to a range of UIDs/GIDs on the host that are not used for other
        purposes (usually in the range beyond the host's UID/GID 65536). The parameter may be specified as follows:</para>

        <orderedlist>
          <listitem><para>If one or two colon-separated numbers are specified, user namespacing is turned on. The first
          parameter specifies the first host UID/GID to assign to the container, the second parameter specifies the
          number of host UIDs/GIDs to assign to the container. If the second parameter is omitted, 65536 UIDs/GIDs are
          assigned.</para></listitem>

          <listitem><para>If the parameter is omitted, or true, user namespacing is turned on. The UID/GID range to
          use is determined automatically from the file ownership of the root directory of the container's directory
          tree. To use this option, make sure to prepare the directory tree in advance, and ensure that all files and
          directories in it are owned by UIDs/GIDs in the range you'd like to use. Also, make sure that used file ACLs
          exclusively reference UIDs/GIDs in the appropriate range. If this mode is used the number of UIDs/GIDs
          assigned to the container for use is 65536, and the UID/GID of the root directory must be a multiple of
          65536.</para></listitem>

          <listitem><para>If the parameter is false, user namespacing is turned off. This is the default.</para>
          </listitem>

          <listitem><para>The special value <literal>pick</literal> turns on user namespacing. In this case the UID/GID
          range is automatically chosen. As first step, the file owner of the root directory of the container's
          directory tree is read, and it is checked that it is currently not used by the system otherwise (in
          particular, that no other container is using it). If this check is successful, the UID/GID range determined
          this way is used, similar to the behavior if "yes" is specified. If the check is not successful (and thus
          the UID/GID range indicated in the root directory's file owner is already used elsewhere) a new – currently
          unused – UID/GID range of 65536 UIDs/GIDs is randomly chosen between the host UID/GIDs of 524288 and
          1878982656, always starting at a multiple of 65536. This setting implies
          <option>--private-users-chown</option> (see below), which has the effect that the files and directories in
          the container's directory tree will be owned by the appropriate users of the range picked. Using this option
          makes user namespace behavior fully automatic. Note that the first invocation of a previously unused
          container image might result in picking a new UID/GID range for it, and thus in the (possibly expensive) file
          ownership adjustment operation. However, subsequent invocations of the container will be cheap (unless of
          course the picked UID/GID range is assigned to a different use by then).</para></listitem>
        </orderedlist>

        <para>It is recommended to assign at least 65536 UIDs/GIDs to each container, so that the usable UID/GID range in the
        container covers 16 bit. For best security, do not assign overlapping UID/GID ranges to multiple containers. It is
        hence a good idea to use the upper 16 bit of the host 32-bit UIDs/GIDs as container identifier, while the lower 16
        bit encode the container UID/GID used. This is in fact the behavior enforced by the
        <option>--private-users=pick</option> option.</para>

        <para>When user namespaces are used, the GID range assigned to each container is always chosen identical to the
        UID range.</para>

        <para>In most cases, using <option>--private-users=pick</option> is the recommended option as it enhances
        container security massively and operates fully automatically in most cases.</para>

        <para>Note that the picked UID/GID range is not written to <filename>/etc/passwd</filename> or
        <filename>/etc/group</filename>. In fact, the allocation of the range is not stored persistently anywhere,
        except in the file ownership of the files and directories of the container.</para>

        <para>Note that when user namespacing is used file ownership on disk reflects this, and all of the container's
        files and directories are owned by the container's effective user and group IDs. This means that copying files
        from and to the container image requires correction of the numeric UID/GID values, according to the UID/GID
        shift applied.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--private-users-chown</option></term>

        <listitem><para>If specified, all files and directories in the container's directory tree will adjusted so that
        they are owned to the appropriate UIDs/GIDs selected for the container (see above). This operation is
        potentially expensive, as it involves descending and iterating through the full directory tree of the
        container. Besides actual file ownership, file ACLs are adjusted as well.</para>

        <para>This option is implied if <option>--private-users=pick</option> is used. This option has no effect if
        user namespacing is not used.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-U</option></term>

        <listitem><para>If the kernel supports the user namespaces feature, equivalent to
        <option>--private-users=pick --private-users-chown</option>, otherwise equivalent to
        <option>--private-users=no</option>.</para>

        <para>Note that <option>-U</option> is the default if the
        <filename>systemd-nspawn@.service</filename> template unit file is used.</para>

        <para>Note: it is possible to undo the effect of <option>--private-users-chown</option> (or
        <option>-U</option>) on the file system by redoing the operation with the first UID of 0:</para>

        <programlisting>systemd-nspawn … --private-users=0 --private-users-chown</programlisting>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--private-network</option></term>

        <listitem><para>Disconnect networking of the container from
        the host. This makes all network interfaces unavailable in the
        container, with the exception of the loopback device and those
        specified with <option>--network-interface=</option> and
        configured with <option>--network-veth</option>. If this
        option is specified, the CAP_NET_ADMIN capability will be
        added to the set of capabilities the container retains. The
        latter may be disabled by using <option>--drop-capability=</option>.
        If this option is not specified (or implied by one of the options
        listed below), the container will have full access to the host network.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--network-namespace-path=</option></term>

        <listitem><para>Takes the path to a file representing a kernel
        network namespace that the container shall run in. The specified path
        should refer to a (possibly bind-mounted) network namespace file, as
        exposed by the kernel below <filename>/proc/$PID/ns/net</filename>.
        This makes the container enter the given network namespace. One of the
        typical use cases is to give a network namespace under
        <filename>/run/netns</filename> created by <citerefentry
        project='man-pages'><refentrytitle>ip-netns</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
        for example, <option>--network-namespace-path=/run/netns/foo</option>.
        Note that this option cannot be used together with other
        network-related options, such as <option>--private-network</option>
        or <option>--network-interface=</option>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--network-interface=</option></term>

        <listitem><para>Assign the specified network interface to the
        container. This will remove the specified interface from the
        calling namespace and place it in the container. When the
        container terminates, it is moved back to the host namespace.
        Note that <option>--network-interface=</option> implies
        <option>--private-network</option>. This option may be used
        more than once to add multiple network interfaces to the
        container.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--network-macvlan=</option></term>

        <listitem><para>Create a <literal>macvlan</literal> interface
        of the specified Ethernet network interface and add it to the
        container. A <literal>macvlan</literal> interface is a virtual
        interface that adds a second MAC address to an existing
        physical Ethernet link. The interface in the container will be
        named after the interface on the host, prefixed with
        <literal>mv-</literal>. Note that
        <option>--network-macvlan=</option> implies
        <option>--private-network</option>. This option may be used
        more than once to add multiple network interfaces to the
        container.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--network-ipvlan=</option></term>

        <listitem><para>Create an <literal>ipvlan</literal> interface
        of the specified Ethernet network interface and add it to the
        container. An <literal>ipvlan</literal> interface is a virtual
        interface, similar to a <literal>macvlan</literal> interface,
        which uses the same MAC address as the underlying interface.
        The interface in the container will be named after the
        interface on the host, prefixed with <literal>iv-</literal>.
        Note that <option>--network-ipvlan=</option> implies
        <option>--private-network</option>. This option may be used
        more than once to add multiple network interfaces to the
        container.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-n</option></term>
        <term><option>--network-veth</option></term>

        <listitem><para>Create a virtual Ethernet link (<literal>veth</literal>) between host and container. The host
        side of the Ethernet link will be available as a network interface named after the container's name (as
        specified with <option>--machine=</option>), prefixed with <literal>ve-</literal>. The container side of the
        Ethernet link will be named <literal>host0</literal>. The <option>--network-veth</option> option implies
        <option>--private-network</option>.</para>

        <para>Note that
        <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
        includes by default a network file <filename>/usr/lib/systemd/network/80-container-ve.network</filename>
        matching the host-side interfaces created this way, which contains settings to enable automatic address
        provisioning on the created virtual link via DHCP, as well as automatic IP routing onto the host's external
        network interfaces. It also contains <filename>/usr/lib/systemd/network/80-container-host0.network</filename>
        matching the container-side interface created this way, containing settings to enable client side address
        assignment via DHCP. In case <filename>systemd-networkd</filename> is running on both the host and inside the
        container, automatic IP communication from the container to the host is thus available, with further
        connectivity to the external network.</para>

        <para>Note that <option>--network-veth</option> is the default if the
        <filename>systemd-nspawn@.service</filename> template unit file is used.</para>

        <para>Note that on Linux network interface names may have a length of 15 characters at maximum, while
        container names may have a length up to 64 characters. As this option derives the host-side interface
        name from the container name the name is possibly truncated. Thus, care needs to be taken to ensure
        that interface names remain unique in this case, or even better container names are generally not
        chosen longer than 12 characters, to avoid the truncation. Alternatively, the
        <option>--network-veth-extra=</option> option may be used, which allows free configuration of the
        host-side interface name independently of the container name — but might require a bit more
        additional configuration in case bridging in a fashion similar to <option>--network-bridge=</option>
        is desired.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--network-veth-extra=</option></term>

        <listitem><para>Adds an additional virtual Ethernet link
        between host and container. Takes a colon-separated pair of
        host interface name and container interface name. The latter
        may be omitted in which case the container and host sides will
        be assigned the same name. This switch is independent of
        <option>--network-veth</option>, and — in contrast — may be
        used multiple times, and allows configuration of the network
        interface names. Note that <option>--network-bridge=</option>
        has no effect on interfaces created with
        <option>--network-veth-extra=</option>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--network-bridge=</option></term>

        <listitem><para>Adds the host side of the Ethernet link created with <option>--network-veth</option>
        to the specified Ethernet bridge interface. Expects a valid network interface name of a bridge device
        as argument. Note that <option>--network-bridge=</option> implies <option>--network-veth</option>. If
        this option is used, the host side of the Ethernet link will use the <literal>vb-</literal> prefix
        instead of <literal>ve-</literal>. Regardless of the used naming prefix the same network interface
        name length limits imposed by Linux apply, along with the complications this creates (for details see
        above).</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--network-zone=</option></term>

        <listitem><para>Creates a virtual Ethernet link (<literal>veth</literal>) to the container and adds it to an
        automatically managed Ethernet bridge interface. The bridge interface is named after the passed argument,
        prefixed with <literal>vz-</literal>. The bridge interface is automatically created when the first container
        configured for its name is started, and is automatically removed when the last container configured for its
        name exits. Hence, each bridge interface configured this way exists only as long as there's at least one
        container referencing it running. This option is very similar to <option>--network-bridge=</option>, besides
        this automatic creation/removal of the bridge device.</para>

        <para>This setting makes it easy to place multiple related containers on a common, virtual Ethernet-based
        broadcast domain, here called a "zone". Each container may only be part of one zone, but each zone may contain
        any number of containers. Each zone is referenced by its name. Names may be chosen freely (as long as they form
        valid network interface names when prefixed with <literal>vz-</literal>), and it is sufficient to pass the same
        name to the <option>--network-zone=</option> switch of the various concurrently running containers to join
        them in one zone.</para>

        <para>Note that
        <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
        includes by default a network file <filename>/usr/lib/systemd/network/80-container-vz.network</filename>
        matching the bridge interfaces created this way, which contains settings to enable automatic address
        provisioning on the created virtual network via DHCP, as well as automatic IP routing onto the host's external
        network interfaces. Using <option>--network-zone=</option> is hence in most cases fully automatic and
        sufficient to connect multiple local containers in a joined broadcast domain to the host, with further
        connectivity to the external network.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-p</option></term>
        <term><option>--port=</option></term>

        <listitem><para>If private networking is enabled, maps an IP
        port on the host onto an IP port on the container. Takes a
        protocol specifier (either <literal>tcp</literal> or
        <literal>udp</literal>), separated by a colon from a host port
        number in the range 1 to 65535, separated by a colon from a
        container port number in the range from 1 to 65535. The
        protocol specifier and its separating colon may be omitted, in
        which case <literal>tcp</literal> is assumed. The container
        port number and its colon may be omitted, in which case the
        same port as the host port is implied. This option is only
        supported if private networking is used, such as with
        <option>--network-veth</option>, <option>--network-zone=</option>
        <option>--network-bridge=</option>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-Z</option></term>
        <term><option>--selinux-context=</option></term>

        <listitem><para>Sets the SELinux security context to be used
        to label processes in the container.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-L</option></term>
        <term><option>--selinux-apifs-context=</option></term>

        <listitem><para>Sets the SELinux security context to be used
        to label files in the virtual API file systems in the
        container.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--capability=</option></term>

        <listitem><para>List one or more additional capabilities to grant the container.
        Takes a comma-separated list of capability names, see
        <citerefentry project='man-pages'><refentrytitle>capabilities</refentrytitle><manvolnum>7</manvolnum></citerefentry>
        for more information. Note that the following capabilities will be granted in any way:
        CAP_AUDIT_CONTROL, CAP_AUDIT_WRITE, CAP_CHOWN, CAP_DAC_OVERRIDE, CAP_DAC_READ_SEARCH,
        CAP_FOWNER, CAP_FSETID, CAP_IPC_OWNER, CAP_KILL, CAP_LEASE, CAP_LINUX_IMMUTABLE,
        CAP_MKNOD, CAP_NET_BIND_SERVICE, CAP_NET_BROADCAST, CAP_NET_RAW, CAP_SETFCAP,
        CAP_SETGID, CAP_SETPCAP, CAP_SETUID, CAP_SYS_ADMIN, CAP_SYS_BOOT, CAP_SYS_CHROOT,
        CAP_SYS_NICE, CAP_SYS_PTRACE, CAP_SYS_RESOURCE, CAP_SYS_TTY_CONFIG. Also CAP_NET_ADMIN
        is retained if <option>--private-network</option> is specified.  If the special value
        <literal>all</literal> is passed, all capabilities are retained.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--drop-capability=</option></term>

        <listitem><para>Specify one or more additional capabilities to
        drop for the container. This allows running the container with
        fewer capabilities than the default (see
        above).</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--no-new-privileges=</option></term>

        <listitem><para>Takes a boolean argument. Specifies the value of the <constant>PR_SET_NO_NEW_PRIVS</constant>
        flag for the container payload. Defaults to off. When turned on the payload code of the container cannot
        acquire new privileges, i.e. the "setuid" file bit as well as file system capabilities will not have an effect
        anymore. See <citerefentry
        project='man-pages'><refentrytitle>prctl</refentrytitle><manvolnum>2</manvolnum></citerefentry> for details
        about this flag. </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--system-call-filter=</option></term>

        <listitem><para>Alter the system call filter applied to containers. Takes a space-separated list of system call
        names or group names (the latter prefixed with <literal>@</literal>, as listed by the
        <command>syscall-filter</command> command of
        <citerefentry><refentrytitle>systemd-analyze</refentrytitle><manvolnum>1</manvolnum></citerefentry>). Passed
        system calls will be permitted. The list may optionally be prefixed by <literal>~</literal>, in which case all
        listed system calls are prohibited. If this command line option is used multiple times the configured lists are
        combined. If both a positive and a negative list (that is one system call list without and one with the
        <literal>~</literal> prefix) are configured, the negative list takes precedence over the positive list. Note
        that <command>systemd-nspawn</command> always implements a system call whitelist (as opposed to a blacklist),
        and this command line option hence adds or removes entries from the default whitelist, depending on the
        <literal>~</literal> prefix. Note that the applied system call filter is also altered implicitly if additional
        capabilities are passed using the <command>--capabilities=</command>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--rlimit=</option></term>

        <listitem><para>Sets the specified POSIX resource limit for the container payload. Expects an assignment of the
        form
        <literal><replaceable>LIMIT</replaceable>=<replaceable>SOFT</replaceable>:<replaceable>HARD</replaceable></literal>
        or <literal><replaceable>LIMIT</replaceable>=<replaceable>VALUE</replaceable></literal>, where
        <replaceable>LIMIT</replaceable> should refer to a resource limit type, such as
        <constant>RLIMIT_NOFILE</constant> or <constant>RLIMIT_NICE</constant>. The <replaceable>SOFT</replaceable> and
        <replaceable>HARD</replaceable> fields should refer to the numeric soft and hard resource limit values. If the
        second form is used, <replaceable>VALUE</replaceable> may specify a value that is used both as soft and hard
        limit. In place of a numeric value the special string <literal>infinity</literal> may be used to turn off
        resource limiting for the specific type of resource. This command line option may be used multiple times to
        control limits on multiple limit types. If used multiple times for the same limit type, the last use
        wins. For details about resource limits see <citerefentry
        project='man-pages'><refentrytitle>setrlimit</refentrytitle><manvolnum>2</manvolnum></citerefentry>. By default
        resource limits for the container's init process (PID 1) are set to the same values the Linux kernel originally
        passed to the host init system. Note that some resource limits are enforced on resources counted per user, in
        particular <constant>RLIMIT_NPROC</constant>. This means that unless user namespacing is deployed
        (i.e. <option>--private-users=</option> is used, see above), any limits set will be applied to the resource
        usage of the same user on all local containers as well as the host. This means particular care needs to be
        taken with these limits as they might be triggered by possibly less trusted code. Example:
        <literal>--rlimit=RLIMIT_NOFILE=8192:16384</literal>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--oom-score-adjust=</option></term>

        <listitem><para>Changes the OOM ("Out Of Memory") score adjustment value for the container payload. This controls
        <filename>/proc/self/oom_score_adj</filename> which influences the preference with which this container is
        terminated when memory becomes scarce. For details see <citerefentry
        project='man-pages'><refentrytitle>proc</refentrytitle><manvolnum>5</manvolnum></citerefentry>. Takes an
        integer in the range -1000…1000.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--cpu-affinity=</option></term>

        <listitem><para>Controls the CPU affinity of the container payload. Takes a comma separated list of CPU numbers
        or number ranges (the latter's start and end value separated by dashes). See <citerefentry
        project='man-pages'><refentrytitle>sched_setaffinity</refentrytitle><manvolnum>2</manvolnum></citerefentry> for
        details.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--kill-signal=</option></term>

        <listitem><para>Specify the process signal to send to the container's PID 1 when nspawn itself receives
        <constant>SIGTERM</constant>, in order to trigger an orderly shutdown of the container. Defaults to
        <constant>SIGRTMIN+3</constant> if <option>--boot</option> is used (on systemd-compatible init systems
        <constant>SIGRTMIN+3</constant> triggers an orderly shutdown). If <option>--boot</option> is not used and this
        option is not specified the container's processes are terminated abruptly via <constant>SIGKILL</constant>. For
        a list of valid signals, see <citerefentry
        project='man-pages'><refentrytitle>signal</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--link-journal=</option></term>

        <listitem><para>Control whether the container's journal shall
        be made visible to the host system. If enabled, allows viewing
        the container's journal files from the host (but not vice
        versa). Takes one of <literal>no</literal>,
        <literal>host</literal>, <literal>try-host</literal>,
        <literal>guest</literal>, <literal>try-guest</literal>,
        <literal>auto</literal>. If <literal>no</literal>, the journal
        is not linked. If <literal>host</literal>, the journal files
        are stored on the host file system (beneath
        <filename>/var/log/journal/<replaceable>machine-id</replaceable></filename>)
        and the subdirectory is bind-mounted into the container at the
        same location. If <literal>guest</literal>, the journal files
        are stored on the guest file system (beneath
        <filename>/var/log/journal/<replaceable>machine-id</replaceable></filename>)
        and the subdirectory is symlinked into the host at the same
        location. <literal>try-host</literal> and
        <literal>try-guest</literal> do the same but do not fail if
        the host does not have persistent journaling enabled. If
        <literal>auto</literal> (the default), and the right
        subdirectory of <filename>/var/log/journal</filename> exists,
        it will be bind mounted into the container. If the
        subdirectory does not exist, no linking is performed.
        Effectively, booting a container once with
        <literal>guest</literal> or <literal>host</literal> will link
        the journal persistently if further on the default of
        <literal>auto</literal> is used.</para>

        <para>Note that <option>--link-journal=try-guest</option> is the default if the
        <filename>systemd-nspawn@.service</filename> template unit file is used.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-j</option></term>

        <listitem><para>Equivalent to
        <option>--link-journal=try-guest</option>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--resolv-conf=</option></term>

        <listitem><para>Configures how <filename>/etc/resolv.conf</filename> inside of the container (i.e. DNS
        configuration synchronization from host to container) shall be handled. Takes one of <literal>off</literal>,
        <literal>copy-host</literal>, <literal>copy-static</literal>, <literal>bind-host</literal>,
        <literal>bind-static</literal>, <literal>delete</literal> or <literal>auto</literal>. If set to
        <literal>off</literal> the <filename>/etc/resolv.conf</filename> file in the container is left as it is
        included in the image, and neither modified nor bind mounted over. If set to <literal>copy-host</literal>, the
        <filename>/etc/resolv.conf</filename> file from the host is copied into the container. Similar, if
        <literal>bind-host</literal> is used, the file is bind mounted from the host into the container. If set to
        <literal>copy-static</literal> the static <filename>resolv.conf</filename> file supplied with
        <citerefentry><refentrytitle>systemd-resolved.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> is
        copied into the container, and correspondingly <literal>bind-static</literal> bind mounts it there. If set to
        <literal>delete</literal> the <filename>/etc/resolv.conf</filename> file in the container is deleted if it
        exists. Finally, if set to <literal>auto</literal> the file is left as it is if private networking is turned on
        (see <option>--private-network</option>). Otherwise, if <filename>systemd-resolved.service</filename> is
        connectible its static <filename>resolv.conf</filename> file is used, and if not the host's
        <filename>/etc/resolv.conf</filename> file is used. In the latter cases the file is copied if the image is
        writable, and bind mounted otherwise. It's recommended to use <literal>copy</literal> if the container shall be
        able to make changes to the DNS configuration on its own, deviating from the host's settings. Otherwise
        <literal>bind</literal> is preferable, as it means direct changes to <filename>/etc/resolv.conf</filename> in
        the container are not allowed, as it is a read-only bind mount (but note that if the container has enough
        privileges, it might simply go ahead and unmount the bind mount anyway). Note that both if the file is bind
        mounted and if it is copied no further propagation of configuration is generally done after the one-time early
        initialization (this is because the file is usually updated through copying and renaming). Defaults to
        <literal>auto</literal>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--timezone=</option></term>

        <listitem><para>Configures how <filename>/etc/localtime</filename> inside of the container (i.e. local timezone
        synchronization from host to container) shall be handled. Takes one of <literal>off</literal>,
        <literal>copy</literal>, <literal>bind</literal>, <literal>symlink</literal>, <literal>delete</literal> or
        <literal>auto</literal>. If set to <literal>off</literal> the <filename>/etc/localtime</filename> file in the
        container is left as it is included in the image, and neither modified nor bind mounted over. If set to
        <literal>copy</literal> the <filename>/etc/localtime</filename> file of the host is copied into the
        container. Similar, if <literal>bind</literal> is used, it is bind mounted from the host into the container. If
        set to <literal>symlink</literal> a symlink from <filename>/etc/localtime</filename> in the container is
        created pointing to the matching the timezone file of the container that matches the timezone setting on the
        host. If set to <literal>delete</literal> the file in the container is deleted, should it exist. If set to
        <literal>auto</literal> and the <filename>/etc/localtime</filename> file of the host is a symlink, then
        <literal>symlink</literal> mode is used, and <literal>copy</literal> otherwise, except if the image is
        read-only in which case <literal>bind</literal> is used instead. Defaults to
        <literal>auto</literal>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--read-only</option></term>

        <listitem><para>Mount the container's root file system (and any other file systems container in the container
        image) read-only. This has no effect on additional mounts made with <option>--bind=</option>,
        <option>--tmpfs=</option> and similar options. This mode is implied if the container image file or directory is
        marked read-only itself. It is also implied if <option>--volatile=</option> is used. In this case the container
        image on disk is strictly read-only, while changes are permitted but kept non-persistently in memory only. For
        further details, see below.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--bind=</option></term>
        <term><option>--bind-ro=</option></term>

        <listitem><para>Bind mount a file or directory from the host into the container. Takes one of: a path
        argument — in which case the specified path will be mounted from the host to the same path in the container, or
        a colon-separated pair of paths — in which case the first specified path is the source in the host, and the
        second path is the destination in the container, or a colon-separated triple of source path, destination path
        and mount options. The source path may optionally be prefixed with a <literal>+</literal> character. If so, the
        source path is taken relative to the image's root directory. This permits setting up bind mounts within the
        container image. The source path may be specified as empty string, in which case a temporary directory below
        the host's <filename>/var/tmp</filename> directory is used. It is automatically removed when the container is
        shut down. Mount options are comma-separated and currently, only <option>rbind</option> and
        <option>norbind</option> are allowed, controlling whether to create a recursive or a regular bind
        mount. Defaults to "rbind". Backslash escapes are interpreted, so <literal>\:</literal> may be used to embed
        colons in either path.  This option may be specified multiple times for creating multiple independent bind
        mount points. The <option>--bind-ro=</option> option creates read-only bind mounts.</para>

        <para>Note that when this option is used in combination with <option>--private-users</option>, the resulting
        mount points will be owned by the <constant>nobody</constant> user. That's because the mount and its files and
        directories continue to be owned by the relevant host users and groups, which do not exist in the container,
        and thus show up under the wildcard UID 65534 (nobody). If such bind mounts are created, it is recommended to
        make them read-only, using <option>--bind-ro=</option>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--tmpfs=</option></term>

        <listitem><para>Mount a tmpfs file system into the container.  Takes a single absolute path argument that
        specifies where to mount the tmpfs instance to (in which case the directory access mode will be chosen as 0755,
        owned by root/root), or optionally a colon-separated pair of path and mount option string that is used for
        mounting (in which case the kernel default for access mode and owner will be chosen, unless otherwise
        specified). Backslash escapes are interpreted in the path, so <literal>\:</literal> may be used to embed colons
        in the path.</para>

        <para>Note that this option cannot be used to replace the root file system of the container with a temporary
        file system. However, the <option>--volatile=</option> option described below provides similar
        functionality, with a focus on implementing stateless operating system images.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--overlay=</option></term>
        <term><option>--overlay-ro=</option></term>

        <listitem><para>Combine multiple directory trees into one
        overlay file system and mount it into the container. Takes a
        list of colon-separated paths to the directory trees to
        combine and the destination mount point.</para>

        <para>Backslash escapes are interpreted in the paths, so
        <literal>\:</literal> may be used to embed colons in the paths.
        </para>

        <para>If three or more paths are specified, then the last
        specified path is the destination mount point in the
        container, all paths specified before refer to directory trees
        on the host and are combined in the specified order into one
        overlay file system. The left-most path is hence the lowest
        directory tree, the second-to-last path the highest directory
        tree in the stacking order. If <option>--overlay-ro=</option>
        is used instead of <option>--overlay=</option>, a read-only
        overlay file system is created. If a writable overlay file
        system is created, all changes made to it are written to the
        highest directory tree in the stacking order, i.e. the
        second-to-last specified.</para>

        <para>If only two paths are specified, then the second
        specified path is used both as the top-level directory tree in
        the stacking order as seen from the host, as well as the mount
        point for the overlay file system in the container. At least
        two paths have to be specified.</para>

        <para>The source paths may optionally be prefixed with <literal>+</literal> character. If so they are taken
        relative to the image's root directory. The uppermost source path may also be specified as empty string, in
        which case a temporary directory below the host's <filename>/var/tmp</filename> is used. The directory is
        removed automatically when the container is shut down. This behaviour is useful in order to make read-only
        container directories writable while the container is running. For example, use the
        <literal>--overlay=+/var::/var</literal> option in order to automatically overlay a writable temporary
        directory on a read-only <filename>/var</filename> directory.</para>

        <para>For details about overlay file systems, see <ulink
        url="https://www.kernel.org/doc/Documentation/filesystems/overlayfs.txt">overlayfs.txt</ulink>. Note
        that the semantics of overlay file systems are substantially
        different from normal file systems, in particular regarding
        reported device and inode information. Device and inode
        information may change for a file while it is being written
        to, and processes might see out-of-date versions of files at
        times. Note that this switch automatically derives the
        <literal>workdir=</literal> mount option for the overlay file
        system from the top-level directory tree, making it a sibling
        of it. It is hence essential that the top-level directory tree
        is not a mount point itself (since the working directory must
        be on the same file system as the top-most directory
        tree). Also note that the <literal>lowerdir=</literal> mount
        option receives the paths to stack in the opposite order of
        this switch.</para>

        <para>Note that this option cannot be used to replace the root file system of the container with an overlay
        file system. However, the <option>--volatile=</option> option described below provides similar functionality,
        with a focus on implementing stateless operating system images.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-E <replaceable>NAME</replaceable>=<replaceable>VALUE</replaceable></option></term>
        <term><option>--setenv=<replaceable>NAME</replaceable>=<replaceable>VALUE</replaceable></option></term>

        <listitem><para>Specifies an environment variable assignment
        to pass to the init process in the container, in the format
        <literal>NAME=VALUE</literal>. This may be used to override
        the default variables or to set additional variables. This
        parameter may be used more than once.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--register=</option></term>

        <listitem><para>Controls whether the container is registered with
        <citerefentry><refentrytitle>systemd-machined</refentrytitle><manvolnum>8</manvolnum></citerefentry>.  Takes a
        boolean argument, which defaults to <literal>yes</literal>.  This option should be enabled when the container
        runs a full Operating System (more specifically: a system and service manager as PID 1), and is useful to
        ensure that the container is accessible via
        <citerefentry><refentrytitle>machinectl</refentrytitle><manvolnum>1</manvolnum></citerefentry> and shown by
        tools such as <citerefentry
        project='man-pages'><refentrytitle>ps</refentrytitle><manvolnum>1</manvolnum></citerefentry>.  If the container
        does not run a service manager, it is recommended to set this option to
        <literal>no</literal>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--keep-unit</option></term>

        <listitem><para>Instead of creating a transient scope unit to run the container in, simply use the service or
        scope unit <command>systemd-nspawn</command> has been invoked in. If <option>--register=yes</option> is set
        this unit is registered with
        <citerefentry><refentrytitle>systemd-machined</refentrytitle><manvolnum>8</manvolnum></citerefentry>. This
        switch should be used if <command>systemd-nspawn</command> is invoked from within a service unit, and the
        service unit's sole purpose is to run a single <command>systemd-nspawn</command> container. This option is not
        available if run from a user session.</para>
        <para>Note that passing <option>--keep-unit</option> disables the effect of <option>--slice=</option> and
        <option>--property=</option>. Use <option>--keep-unit</option> and <option>--register=no</option> in
        combination to disable any kind of unit allocation or registration with
        <command>systemd-machined</command>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--personality=</option></term>

        <listitem><para>Control the architecture ("personality")
        reported by
        <citerefentry project='man-pages'><refentrytitle>uname</refentrytitle><manvolnum>2</manvolnum></citerefentry>
        in the container. Currently, only <literal>x86</literal> and
        <literal>x86-64</literal> are supported. This is useful when
        running a 32-bit container on a 64-bit host. If this setting
        is not used, the personality reported in the container is the
        same as the one reported on the host.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-q</option></term>
        <term><option>--quiet</option></term>

        <listitem><para>Turns off any status output by the tool
        itself. When this switch is used, the only output from nspawn
        will be the console output of the container OS
        itself.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--volatile</option></term>
        <term><option>--volatile=</option><replaceable>MODE</replaceable></term>

        <listitem><para>Boots the container in volatile mode. When no mode parameter is passed or when mode is
        specified as <option>yes</option>, full volatile mode is enabled. This means the root directory is mounted as a
        mostly unpopulated <literal>tmpfs</literal> instance, and <filename>/usr/</filename> from the OS tree is
        mounted into it in read-only mode (the system thus starts up with read-only OS image, but pristine state and
        configuration, any changes are lost on shutdown). When the mode parameter is specified as
        <option>state</option>, the OS tree is mounted read-only, but <filename>/var/</filename> is mounted as a
        writable <literal>tmpfs</literal> instance into it (the system thus starts up with read-only OS resources and
        configuration, but pristine state, and any changes to the latter are lost on shutdown). When the mode parameter
        is specified as <option>overlay</option> the read-only root file system is combined with a writable
        <filename>tmpfs</filename> instance through <literal>overlayfs</literal>, so that it appears at it normally
        would, but any changes are applied to the temporary file system only and lost when the container is
        terminated. When the mode parameter is specified as <option>no</option> (the default), the whole OS tree is
        made available writable (unless <option>--read-only</option> is specified, see above).</para>

        <para>Note that if one of the volatile modes is chosen, its effect is limited to the root file system (or
        <filename>/var/</filename> in case of <option>state</option>), and any other mounts placed in the hierarchy are
        unaffected — regardless if they are established automatically (e.g. the EFI system partition that might be
        mounted to <filename>/efi/</filename> or <filename>/boot/</filename>) or explicitly (e.g. through an additional
        command line option such as <option>--bind=</option>, see above). This means, even if
        <option>--volatile=overlay</option> is used changes to <filename>/efi/</filename> or
        <filename>/boot/</filename> are prohibited in case such a partition exists in the container image operated on,
        and even if <option>--volatile=state</option> is used the hypothetical file <filename>/etc/foobar</filename> is
        potentially writable if <option>--bind=/etc/foobar</option> if used to mount it from outside the read-only
        container <filename>/etc</filename> directory.</para>

        <para>The <option>--ephemeral</option> option is closely related to this setting, and provides similar
        behaviour by making a temporary, ephemeral copy of the whole OS image and executing that. For further details,
        see above.</para>

        <para>The <option>--tmpfs=</option> and <option>--overlay=</option> options provide similar functionality, but
        for specific sub-directories of the OS image only. For details, see above.</para>

        <para>This option provides similar functionality for containers as the <literal>systemd.volatile=</literal>
        kernel command line switch provides for host systems. See
        <citerefentry><refentrytitle>kernel-command-line</refentrytitle><manvolnum>7</manvolnum></citerefentry> for
        details.</para>

        <para>Note that setting this option to <option>yes</option> or <option>state</option> will only work correctly
        with operating systems in the container that can boot up with only <filename>/usr</filename> mounted, and are
        able to automatically populate <filename>/var</filename>, and also <filename>/etc</filename> in case of
        <literal>--volatile=yes</literal>. The <option>overlay</option> option does not require any particular
        preparations in the OS, but do note that <literal>overlayfs</literal> behaviour differs from regular file
        systems in a number of ways, and hence compatibility is limited.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--settings=</option><replaceable>MODE</replaceable></term>

        <listitem><para>Controls whether
        <command>systemd-nspawn</command> shall search for and use
        additional per-container settings from
        <filename>.nspawn</filename> files. Takes a boolean or the
        special values <option>override</option> or
        <option>trusted</option>.</para>

        <para>If enabled (the default), a settings file named after the
        machine (as specified with the <option>--machine=</option>
        setting, or derived from the directory or image file name)
        with the suffix <filename>.nspawn</filename> is searched in
        <filename>/etc/systemd/nspawn/</filename> and
        <filename>/run/systemd/nspawn/</filename>. If it is found
        there, its settings are read and used. If it is not found
        there, it is subsequently searched in the same directory as the
        image file or in the immediate parent of the root directory of
        the container. In this case, if the file is found, its settings
        will be also read and used, but potentially unsafe settings
        are ignored. Note that in both these cases, settings on the
        command line take precedence over the corresponding settings
        from loaded <filename>.nspawn</filename> files, if both are
        specified. Unsafe settings are considered all settings that
        elevate the container's privileges or grant access to
        additional resources such as files or directories of the
        host. For details about the format and contents of
        <filename>.nspawn</filename> files, consult
        <citerefentry><refentrytitle>systemd.nspawn</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>

        <para>If this option is set to <option>override</option>, the
        file is searched, read and used the same way, however, the order of
        precedence is reversed: settings read from the
        <filename>.nspawn</filename> file will take precedence over
        the corresponding command line options, if both are
        specified.</para>

        <para>If this option is set to <option>trusted</option>, the
        file is searched, read and used the same way, but regardless
        of being found in <filename>/etc/systemd/nspawn/</filename>,
        <filename>/run/systemd/nspawn/</filename> or next to the image
        file or container root directory, all settings will take
        effect, however, command line arguments still take precedence
        over corresponding settings.</para>

        <para>If disabled, no <filename>.nspawn</filename> file is read
        and no settings except the ones on the command line are in
        effect.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--notify-ready=</option></term>

        <listitem><para>Configures support for notifications from the container's init process.
        <option>--notify-ready=</option> takes a boolean (<option>no</option> and  <option>yes</option>).
        With option <option>no</option> systemd-nspawn notifies systemd
        with a <literal>READY=1</literal> message when the init process is created.
        With option <option>yes</option> systemd-nspawn waits for the
        <literal>READY=1</literal> message from the init process in the container
        before sending its own to systemd. For more details about notifications
        see <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>).</para></listitem>
      </varlistentry>

      <xi:include href="standard-options.xml" xpointer="help" />
      <xi:include href="standard-options.xml" xpointer="version" />
    </variablelist>

  </refsect1>

  <refsect1>
    <title>Examples</title>

    <example>
      <title>Download a
      <ulink url="https://getfedora.org">Fedora</ulink> image and start a shell in it</title>

      <programlisting># machinectl pull-raw --verify=no \
      https://download.fedoraproject.org/pub/fedora/linux/releases/&fedora_latest_version;/Cloud/x86_64/images/Fedora-Cloud-Base-&fedora_latest_version;-&fedora_cloud_release;.x86_64.raw.xz
# systemd-nspawn -M Fedora-Cloud-Base-&fedora_latest_version;-&fedora_cloud_release;.x86_64.raw</programlisting>

      <para>This downloads an image using
      <citerefentry><refentrytitle>machinectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
      and opens a shell in it.</para>
    </example>

    <example>
      <title>Build and boot a minimal Fedora distribution in a container</title>

      <programlisting># dnf -y --releasever=&fedora_latest_version; --installroot=/var/lib/machines/f&fedora_latest_version; \
      --disablerepo='*' --enablerepo=fedora --enablerepo=updates install \
      systemd passwd dnf fedora-release vim-minimal
# systemd-nspawn -bD /var/lib/machines/f&fedora_latest_version;</programlisting>

      <para>This installs a minimal Fedora distribution into the
      directory <filename noindex='true'>/var/lib/machines/f&fedora_latest_version;</filename>
      and then boots an OS in a namespace container in it. Because the installation
      is located underneath the standard <filename>/var/lib/machines/</filename>
      directory, it is also possible to start the machine using
      <command>systemd-nspawn -M f&fedora_latest_version;</command>.</para>
    </example>

    <example>
      <title>Spawn a shell in a container of a minimal Debian unstable distribution</title>

      <programlisting># debootstrap unstable ~/debian-tree/
# systemd-nspawn -D ~/debian-tree/</programlisting>

      <para>This installs a minimal Debian unstable distribution into
      the directory <filename>~/debian-tree/</filename> and then
      spawns a shell in a namespace container in it.</para>

      <para><command>debootstrap</command> supports
      <ulink url="https://www.debian.org">Debian</ulink>,
      <ulink url="https://www.ubuntu.com">Ubuntu</ulink>,
      and <ulink url="https://www.tanglu.org">Tanglu</ulink>
      out of the box, so the same command can be used to install any of those. For other
      distributions from the Debian family, a mirror has to be specified, see
      <citerefentry project='die-net'><refentrytitle>debootstrap</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
      </para>
    </example>

    <example>
      <title>Boot a minimal
      <ulink url="https://www.archlinux.org">Arch Linux</ulink> distribution in a container</title>

      <programlisting># pacstrap -c -d ~/arch-tree/ base
# systemd-nspawn -bD ~/arch-tree/</programlisting>

      <para>This installs a minimal Arch Linux distribution into the
      directory <filename>~/arch-tree/</filename> and then boots an OS
      in a namespace container in it.</para>
    </example>

    <example>
      <title>Install the
      <ulink url="https://software.opensuse.org/distributions/tumbleweed">OpenSUSE Tumbleweed</ulink>
      rolling distribution</title>

      <programlisting># zypper --root=/var/lib/machines/tumbleweed ar -c \
      https://download.opensuse.org/tumbleweed/repo/oss tumbleweed
# zypper --root=/var/lib/machines/tumbleweed refresh
# zypper --root=/var/lib/machines/tumbleweed install --no-recommends \
      systemd shadow zypper openSUSE-release vim
# systemd-nspawn -M tumbleweed passwd root
# systemd-nspawn -M tumbleweed -b</programlisting>
    </example>

    <example>
      <title>Boot into an ephemeral snapshot of the host system</title>

      <programlisting># systemd-nspawn -D / -xb</programlisting>

      <para>This runs a copy of the host system in a snapshot which is removed immediately when the container
      exits. All file system changes made during runtime will be lost on shutdown, hence.</para>
    </example>

    <example>
      <title>Run a container with SELinux sandbox security contexts</title>

      <programlisting># chcon system_u:object_r:svirt_sandbox_file_t:s0:c0,c1 -R /srv/container
# systemd-nspawn -L system_u:object_r:svirt_sandbox_file_t:s0:c0,c1 \
      -Z system_u:system_r:svirt_lxc_net_t:s0:c0,c1 -D /srv/container /bin/sh</programlisting>
    </example>

    <example>
      <title>Run a container with an OSTree deployment</title>

      <programlisting># systemd-nspawn -b -i ~/image.raw \
      --pivot-root=/ostree/deploy/$OS/deploy/$CHECKSUM:/sysroot \
      --bind=+/sysroot/ostree/deploy/$OS/var:/var</programlisting>
    </example>
  </refsect1>

  <refsect1>
    <title>Exit status</title>

    <para>The exit code of the program executed in the container is
    returned.</para>
  </refsect1>

  <refsect1>
    <title>See Also</title>
    <para>
      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd.nspawn</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
      <citerefentry project='man-pages'><refentrytitle>chroot</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry project='mankier'><refentrytitle>dnf</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry project='die-net'><refentrytitle>debootstrap</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry project='archlinux'><refentrytitle>pacman</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry project='mankier'><refentrytitle>zypper</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd.slice</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>machinectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry project='man-pages'><refentrytitle>btrfs</refentrytitle><manvolnum>8</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>