systemd/man/systemd-nspawn.xml

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

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

  <refnamediv>
    <refname>systemd-nspawn</refname>
    <refpurpose>Spawn a namespace container for debugging, testing and building</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> 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>. 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>Note that even though these security precautions are taken
    <command>systemd-nspawn</command> is not suitable for fully secure
    container setups. Many of the security features may be
    circumvented and are hence primarily useful to avoid accidental
    changes to the host system from the container.</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>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.</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> command to request an additional login
    prompt in a running container.</para>

    <para><command>systemd-nspawn</command> implements the
    <ulink
    url="http://www.freedesktop.org/wiki/Software/systemd/ContainerInterface">Container
    Interface</ulink> specification.</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>
  </refsect1>

  <refsect1>
    <title>Options</title>

    <para>If option <option>-b</option> is specified, the arguments
    are used as arguments for the init binary. 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>-b</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>
        subvolume 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, which can be substantially more
        time-consuming. Note that if this option is used the
        container's root directory (in contrast to the template
        directory!) must be located on a <literal>btrfs</literal> file
        system, so that the <literal>btrfs</literal> subvolume may be
        created. 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 <literal>btrfs</literal> snapshot of its root
        directory (as configured with <option>--directory=</option>),
        that is removed immediately when the container terminates.
        This option is only supported if the root file system is
        <literal>btrfs</literal>. May not be specified together with
        <option>--image=</option> or
        <option>--template=</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>-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="http://www.freedesktop.org/wiki/Specifications/DiscoverablePartitionsSpec/">Discoverable
          Partitions Specification</ulink>.</para></listitem>
        </itemizedlist>

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

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

        <listitem><para>Automatically search for an init binary 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 binary. This option may not be combined with <option>--as-pid2</option> or
        <option>--share-system</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 binary as automatically searched and run as PID 1 in the container. The passed parameters are used as invocation parameters for this process.</entry>
              </row>

            </tbody>
          </tgroup>
        </table>
        </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>-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>--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>--slice=</option></term>

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

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

        <listitem><para>Enables user namespacing. If enabled, the
        container will run with its own private set of Unix user and
        group ids (UIDs and GIDs). Takes none, one or two
        colon-separated parameters: the first parameter specifies the
        first host UID to assign to the container, the second
        parameter specifies the number of host UIDs to assign to the
        container. If the second parameter is omitted, 65536 UIDs are
        assigned. If the first parameter is also omitted (and hence
        no parameter passed at all), the first UID assigned to the
        container is read from the owner of the root directory of the
        container's directory tree. By default, no user namespacing is
        applied.</para>

        <para>Note that user namespacing currently requires OS trees
        that are prepared for the UID shift that is being applied:
        UIDs and GIDs used for file ownership or in file ACL entries
        must be shifted to the container UID base that is
        used during container runtime.</para>

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

        <para>When user namespaces are used, the GID range assigned to
        each container is always chosen identical to the UID
        range.</para></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>.</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>. Note that
        <option>--network-veth</option> implies
        <option>--private-network</option>.</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
        bridge. 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>.</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> or
        <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_CHOWN, CAP_DAC_OVERRIDE,
        CAP_DAC_READ_SEARCH, CAP_FOWNER, CAP_FSETID, CAP_IPC_OWNER,
        CAP_KILL, CAP_LEASE, CAP_LINUX_IMMUTABLE,
        CAP_NET_BIND_SERVICE, CAP_NET_BROADCAST, CAP_NET_RAW,
        CAP_SETGID, CAP_SETFCAP, CAP_SETPCAP, CAP_SETUID,
        CAP_SYS_ADMIN, CAP_SYS_CHROOT, CAP_SYS_NICE, CAP_SYS_PTRACE,
        CAP_SYS_TTY_CONFIG, CAP_SYS_RESOURCE, CAP_SYS_BOOT,
        CAP_AUDIT_WRITE, CAP_AUDIT_CONTROL. 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>--kill-signal=</option></term>

        <listitem><para>Specify the process signal to send to the
        container's PID 1 when nspawn itself receives SIGTERM, in
        order to trigger an orderly shutdown of the
        container. Defaults to SIGRTMIN+3 if <option>--boot</option>
        is used (on systemd-compatible init systems SIGRTMIN+3
        triggers an orderly shutdown). 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 journalling 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></listitem>
      </varlistentry>

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

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

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

        <listitem><para>Mount the root file system read-only for the
        container.</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. Mount options are
        comma-separated and currently, only "rbind" and "norbind"
        are allowed. 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></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). This option is particularly useful for
        mounting directories such as <filename>/var</filename> as
        tmpfs, to allow state-less systems, in particular when
        combined with <option>--read-only</option>.
        Backslash escapes are interpreted in the path, so
        <literal>\:</literal> may be used to embed colons in the path.
        </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>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></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>--share-system</option></term>

        <listitem><para>Allows the container to share certain system
        facilities with the host. More specifically, this turns off
        PID namespacing, UTS namespacing and IPC namespacing, and thus
        allows the guest to see and interact more easily with
        processes outside of the container. Note that using this
        option makes it impossible to start up a full Operating System
        in the container, as an init system cannot operate in this
        mode. It is only useful to run specific programs or
        applications this way, without involving an init system in the
        container. This option implies <option>--register=no</option>.
        This option may not be combined with
        <option>--boot</option>.</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: an init system), 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 an init system, it is
        recommended to set this option to <literal>no</literal>. Note
        that <option>--share-system</option> implies
        <option>--register=no</option>. </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 register the service or scope
        unit <command>systemd-nspawn</command> has been invoked in
        with
        <citerefentry><refentrytitle>systemd-machined</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
        This has no effect if <option>--register=no</option> is used.
        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></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
        resources, but pristine state and configuration, any changes
        to the either 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 <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>no</option> (the default), the whole OS tree is made
        available writable.</para>

        <para>Note that setting this 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 populate
        <filename>/var</filename> automatically, as
        needed.</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>

      <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 Fedora image and start a shell in it</title>

      <programlisting># machinectl pull-raw --verify=no http://ftp.halifax.rwth-aachen.de/fedora/linux/releases/21/Cloud/Images/x86_64/Fedora-Cloud-Base-20141203-21.x86_64.raw.xz
# systemd-nspawn -M Fedora-Cloud-Base-20141203-21</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=23 --installroot=/srv/mycontainer --disablerepo='*' --enablerepo=fedora --enablerepo=updates install systemd passwd dnf fedora-release vim-minimal
# systemd-nspawn -bD /srv/mycontainer</programlisting>

      <para>This installs a minimal Fedora distribution into the
      directory <filename noindex='true'>/srv/mycontainer/</filename>
      and then boots an OS in a namespace container in it.</para>
    </example>

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

      <programlisting># debootstrap --arch=amd64 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>
    </example>

    <example>
      <title>Boot a minimal Arch Linux 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>Boot into an ephemeral <literal>btrfs</literal> snapshot of the host system</title>

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

      <para>This runs a copy of the host system in a
      <literal>btrfs</literal> 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>
  </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><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>