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.