shared: rework switch_root() code

[thirdparty/systemd.git] / 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> 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 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="http://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 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>--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 copy-on-write scheme — if the file system supports that), which
        can be substantially more time-consuming. 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.</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>

          <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 256bits (and hence 64
        hexadecimal characters) long (in case of SHA256 for example). If this option is not specified, 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.</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>.</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>.</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>

        <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>-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>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></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>.</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>
        </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>.</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-zones=</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_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 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>--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. 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></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>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></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: 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>.</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
        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 <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>

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

      <programlisting># machinectl pull-raw --verify=no http://ftp.halifax.rwth-aachen.de/fedora/linux/releases/24/CloudImages/x86_64/images/Fedora-Cloud-Base-24-1.2.x86_64.raw.xz
# systemd-nspawn -M Fedora-Cloud-Base-24-1.2.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=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 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>
  </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>