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systemd/man/systemd-nspawn.xml
Zbigniew Jędrzejewski-Szmek 870c4365cf man: document systemd-nspawn behaviour with -b 
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2013-02-27 22:25:40 -05:00

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<?xml version='1.0'?> <!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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Copyright 2010 Lennart Poettering
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<refentry id="systemd-nspawn">
<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">-b</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><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 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. The intended use of
this program is debugging and testing as well as
building of packages, distributions and software
involved with boot and systems management.</para>
<para>In contrast to
<citerefentry><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><refentrytitle>yum</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>debootstrap</refentrytitle><manvolnum>8</manvolnum></citerefentry>
or
<citerefentry><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. It is however possible to
enter an existing container, see
<link linkend='example-nsenter'>Example 4</link> below.
</para>
<para><command>systemd-nspawn</command> implements the
<ulink
url="http://www.freedesktop.org/wiki/Software/systemd/ContainerInterface">Container
Interface</ulink> specification.</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 specifed, a shell is launched in the
container.</para>
<para>The following options are understood:</para>
<variablelist>
<varlistentry>
<term><option>-h</option></term>
<term><option>--help</option></term>
<listitem><para>Prints a short help
text and exits.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>--version</option></term>
<listitem><para>Prints a version string
and exits.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>-D</option></term>
<term><option>--directory=</option></term>
<listitem><para>Directory to use as
file system root for the namespace
container. If omitted the current
directory will be
used.</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
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.
</para></listitem>
</varlistentry>
<varlistentry>
<term><option>-u</option></term>
<term><option>--user=</option></term>
<listitem><para>Run the command
under specified user, create home
directory and cd into it. As rest
of systemd-nspawn, this is not
the security feature and limits
against accidental changes only.
</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.
</para></listitem>
</varlistentry>
<varlistentry>
<term><option>-C</option></term>
<term><option>--controllers=</option></term>
<listitem><para>Makes the container appear in
other hierarchies than the name=systemd:/ one.
Takes a comma-separated list of controllers.
</para></listitem>
</varlistentry>
<varlistentry>
<term><option>--private-network</option></term>
<listitem><para>Turn off networking in
the container. This makes all network
interfaces unavailable in the
container, with the exception of the
loopback device.</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>--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><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.</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>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. 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 doesn't 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=guest</option>.</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. Either takes 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. The
<option>--bind-ro=</option> option
creates read-only bind
mount.</para></listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1>
<title>Example 1</title>
<programlisting># yum -y --releasever=19 --nogpg --installroot=/srv/mycontainer --disablerepo='*' --enablerepo=fedora install systemd passwd yum fedora-release vim-minimal
# systemd-nspawn -bD /srv/mycontainer</programlisting>
<para>This installs a minimal Fedora distribution into
the directory <filename>/srv/mycontainer/</filename> and
then boots an OS in a namespace container in
it.</para>
</refsect1>
<refsect1>
<title>Example 2</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>
</refsect1>
<refsect1>
<title>Example 3</title>
<programlisting># pacstrap -c -d ~/arch-tree/ base
# systemd-nspawn -bD ~/arch-tree/</programlisting>
<para>This installs a mimimal Arch Linux distribution into
the directory <filename>~/arch-tree/</filename> and then
boots an OS in a namespace container in it.</para>
</refsect1>
<refsect1 id='example-nsenter'>
<title>Example 4</title>
<para>To enter the container, PID of one of the
processes sharing the new namespaces must be used.
<command>systemd-nspawn</command> prints the PID
(as viewed from the outside) of the launched process,
and it can be used to enter the container.</para>
<programlisting># nsenter -muinpt $PID</programlisting>
<para><citerefentry><refentrytitle>nsenter</refentrytitle><manvolnum>1</manvolnum></citerefentry>
is part of
<ulink url="https://github.com/karelzak/util-linux">util-linux</ulink>.
Kernel support for entering namespaces was added in
Linux 3.8.</para>
</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>chroot</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>unshare</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>yum</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>debootstrap</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>pacman</refentrytitle><manvolnum>8</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>
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