systemd-nspawn — Spawn a namespace container for debugging, testing and building
systemd-nspawn [OPTIONS...] [COMMAND
[ARGS...]
]
systemd-nspawn -b [OPTIONS...] [ARGS...]
systemd-nspawn may be used to run a command or OS in a light-weight namespace container. In many ways it is similar to chroot(1), 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.
systemd-nspawn limits access
to various kernel interfaces in the container to
read-only, such as /sys,
/proc/sys or
/sys/fs/selinux. 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.
Note that even though these security precautions are taken systemd-nspawn 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.
In contrast to chroot(1) systemd-nspawn may be used to boot full Linux-based operating systems in a container.
Use a tool like yum(8), debootstrap(8) or pacman(8) to set up an OS directory tree suitable as file system hierarchy for systemd-nspawn containers.
Note that systemd-nspawn will
mount file systems private to the container to
/dev,
/run and similar. These will
not be visible outside of the container, and their
contents will be lost when the container exits.
Note that running two systemd-nspawn 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 Example 4 below.
systemd-nspawn implements the Container Interface specification.
As a safety check
systemd-nspawn will verify the
existance of /etc/os-release in
the container tree before starting the container (see
os-release(5)). 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.
Note that the kernel auditing subsystem is
currently broken when used together with
containers. We hence recommend turning it off entirely
when using systemd-nspawn by
booting with audit=0 on the kernel
command line, or by turning it off at kernel build
time. If auditing is enabled in the kernel operating
systems booted in an nspawn container might refuse
log-in attempts.
If option -b is specified, the
arguments are used as arguments for the init
binary. Otherwise, COMMAND
specifies the program to launch in the container, and
the remaining arguments are used as arguments for this
program. If -b is not used and no
arguments are specifed, a shell is launched in the
container.
The following options are understood:
-h, --help¶Prints a short help text and exits.
--version¶Prints a version string and exits.
-D, --directory=¶Directory to use as file system root for the namespace container. If omitted the current directory will be used.
-b, --boot¶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.
-u, --user=¶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.
-M, --machine=¶Sets the machine name for this container. This name may be used to identify this container on the host, 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 of the container is used.
--uuid=¶Set the specified uuid
for the container. The init system
will initialize
/etc/machine-id
from this if this file is not set yet.
-C, --controllers=¶Makes the container appear in other hierarchies than the name=systemd:/ one. Takes a comma-separated list of controllers.
--private-network¶Turn off networking in the container. This makes all network interfaces unavailable in the container, with the exception of the loopback device.
--read-only¶Mount the root file system read only for the container.
--capability=¶List one or more additional capabilities to grant the container. Takes a comma separated list of capability names, see capabilities(7) 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.
--link-journal=¶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
no,
host,
guest,
auto. If
no, the journal is
not linked. If host,
the journal files are stored on the
host file system (beneath
/var/log/journal/)
and the subdirectory is bind-mounted
into the container at the same
location. If machine-idguest,
the journal files are stored on the
guest file system (beneath
/var/log/journal/)
and the subdirectory is symlinked into the host
at the same location. If
machine-idauto (the default),
and the right subdirectory of
/var/log/journal
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
guest or
host will link the
journal persistently if further on
the default of auto
is used.
-j¶Equivalent to
--link-journal=guest.
--bind=, --bind-ro=¶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
--bind-ro= option
creates read-only bind
mount.
# yum -y --releasever=19 --nogpg --installroot=/srv/mycontainer --disablerepo='*' --enablerepo=fedora install systemd passwd yum fedora-release vim-minimal # systemd-nspawn -bD /srv/mycontainer
This installs a minimal Fedora distribution into
the directory /srv/mycontainer/ and
then boots an OS in a namespace container in
it.
# debootstrap --arch=amd64 unstable ~/debian-tree/ # systemd-nspawn -D ~/debian-tree/
This installs a minimal Debian unstable
distribution into the directory
~/debian-tree/ and then spawns a
shell in a namespace container in it.
# pacstrap -c -d ~/arch-tree/ base # systemd-nspawn -bD ~/arch-tree/
This installs a mimimal Arch Linux distribution into
the directory ~/arch-tree/ and then
boots an OS in a namespace container in it.
To enter the container, PID of one of the processes sharing the new namespaces must be used. systemd-nspawn prints the PID (as viewed from the outside) of the launched process, and it can be used to enter the container.
# nsenter -m -u -i -n -p -t $PID
nsenter(1) is part of util-linux. Kernel support for entering namespaces was added in Linux 3.8.