bionic (1) systemd-nspawn.1.gz
NAME
systemd-nspawn - Spawn a namespace container for debugging, testing and building
SYNOPSIS
systemd-nspawn [OPTIONS...] [COMMAND [ARGS...]]
systemd-nspawn --boot [OPTIONS...] [ARGS...]
DESCRIPTION
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 may be invoked on any directory tree containing an operating system tree, using the
--directory= command line option. By using the --machine= option an OS tree is automatically searched for
in a couple of locations, most importantly in /var/lib/machines, the suggested directory to place
container images installed on the system.
In contrast to chroot(1) systemd-nspawn may be used to boot full Linux-based operating systems in a
container.
systemd-nspawn limits access to various kernel interfaces in the container to read-only, such as /sys,
/proc/sys or /sys/fs/selinux. 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.
Use a tool like dnf(8), debootstrap(8), or pacman(8) to set up an OS directory tree suitable as file
system hierarchy for systemd-nspawn containers. See the Examples section below for details on suitable
invocation of these commands.
As a safety check systemd-nspawn will verify the existence of /usr/lib/os-release or /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.
systemd-nspawn 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 systemd-nspawn@.service is provided to make this easy, taking the container name as instance
identifier. Note that different default options apply when systemd-nspawn is invoked by the template unit
file than interactively on the command line. Most importantly the template unit file makes use of the
--boot which is not the default in case systemd-nspawn is invoked from the interactive command line.
Further differences with the defaults are documented along with the various supported options below.
The machinectl(1) 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 systemd-nspawn@.service
template unit file.
Along with each container a settings file with the .nspawn suffix may exist, containing additional
settings to apply when running the container. See systemd.nspawn(5) for details. Settings files override
the default options used by the systemd-nspawn@.service template unit file, making it usually unnecessary
to alter this template file directly.
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. Use machinectl(1)'s login or
shell commands to request an additional login session in a running container.
systemd-nspawn implements the Container Interface[1] specification.
While running, containers invoked with systemd-nspawn are registered with the systemd-machined(8) service
that keeps track of running containers, and provides programming interfaces to interact with them.
OPTIONS
If option -b is specified, the arguments are used as arguments for the init program. Otherwise, COMMAND
specifies the program to launch in the container, and the remaining arguments are used as arguments for
this program. If --boot is not used and no arguments are specified, a shell is launched in the container.
The following options are understood:
-D, --directory=
Directory to use as file system root for the container.
If neither --directory=, nor --image= is specified the directory is determined by searching for a
directory named the same as the machine name specified with --machine=. See machinectl(1) section
"Files and Directories" for the precise search path.
If neither --directory=, --image=, nor --machine= are specified, the current directory will be used.
May not be specified together with --image=.
--template=
Directory or "btrfs" subvolume to use as template for the container's root directory. If this is
specified and the container's root directory (as configured by --directory=) does not yet exist it is
created as "btrfs" snapshot (if supported) or plain directory (otherwise) and populated from this
template tree. Ideally, the specified template path refers to the root of a "btrfs" 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 "btrfs" subvolume (or not even to a
"btrfs" 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 --image= or --ephemeral.
Note that this switch leaves host name, machine ID and all other settings that could identify the
instance unmodified.
-x, --ephemeral
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 --template=.
Note that this switch leaves host name, machine ID and all other settings that could identify the
instance unmodified.
-i, --image=
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:
• An MBR partition table with a single partition of type 0x83 that is marked bootable.
• A GUID partition table (GPT) with a single partition of type
0fc63daf-8483-4772-8e79-3d69d8477de4.
• 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 Discoverable Partitions Specification[2].
• No partition table, and a single file system spanning the whole image.
On GPT images, if an EFI System Partition (ESP) is discovered, it is automatically mounted to /efi
(or /boot as fallback) in case a directory by this name exists and is empty.
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 --root-hash=
option.
Any other partitions, such as foreign partitions or swap partitions are not mounted. May not be
specified together with --directory=, --template=.
--root-hash=
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 256
bits (and hence 64 formatted hexadecimal characters) long (in case of SHA256 for example). If this
option is not specified, but the image file carries the "user.verity.roothash" extended file
attribute (see xattr(7)), then the root hash is read from it, also as formatted hexadecimal
characters. If the extended file attribute is not found (or is not supported by the underlying file
system), but a file with the .roothash suffix is found next to the image file, bearing otherwise the
same name, the root hash is read from it and automatically used, also as formatted hexadecimal
characters.
-a, --as-pid2
Invoke the shell or specified program as process ID (PID) 2 instead of PID 1 (init). By default, if
neither this option nor --boot is used, the selected program is run as the 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 sysvinit
compatible signal handling (specifically: it needs to reboot on SIGINT, reexecute on SIGTERM, reload
configuration on SIGHUP, and so on). With --as-pid2 a minimal stub init process is run as PID 1 and
the selected program 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 --boot.
-b, --boot
Automatically search for an init program 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 program. This option may not be combined with --as-pid2.
The following table explains the different modes of invocation and relationship to --as-pid2 (see
above):
Table 1. Invocation Mode
┌─────────────────────────────┬───────────────────────────────────────┐
│Switch │ Explanation │
├─────────────────────────────┼───────────────────────────────────────┤
│Neither --as-pid2 nor --boot │ The passed parameters are interpreted │
│specified │ as the command line, which is │
│ │ executed as PID 1 in the container. │
├─────────────────────────────┼───────────────────────────────────────┤
│--as-pid2 specified │ 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. │
├─────────────────────────────┼───────────────────────────────────────┤
│--boot specified │ An init program is automatically │
│ │ searched for and run as PID 1 in the │
│ │ container. The passed parameters are │
│ │ used as invocation parameters for │
│ │ this process. │
└─────────────────────────────┴───────────────────────────────────────┘
Note that --boot is the default mode of operation if the systemd-nspawn@.service template unit file
is used.
--chdir=
Change to the specified working directory before invoking the process in the container. Expects an
absolute path in the container's file system namespace.
--pivot-root=
Pivot the specified directory to / inside the container, and either unmount the container's old root,
or pivot it to another specified directory. Takes one of: a path argument — in which case the
specified path will be pivoted to / and the old root will be unmounted; or a colon-separated pair of
new root path and pivot destination for the old root. The new root path will be pivoted to /, and the
old / will be pivoted to the other directory. Both paths must be absolute, and are resolved in the
container's file system namespace.
This is for containers which have several bootable directories in them; for example, several
OSTree[3] deployments. It emulates the behavior of the boot loader and initial RAM disk which
normally select which directory to mount as the root and start the container's PID 1 in.
-u, --user=
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.
-M, --machine=
Sets the machine name for this container. This name may be used to identify this container during its
runtime (for example in tools like machinectl(1) 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 --ephemeral mode is selected. If the root directory selected is the host's root
directory the host's hostname is used as default instead.
--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. Note that this option takes effect only if /etc/machine-id in the
container is unpopulated.
-S, --slice=
Make the container part of the specified slice, instead of the default machine.slice. This applies
only if the machine is run in its own scope unit, i.e. if --keep-unit isn't used.
--property=
Set a unit property on the scope unit to register for the machine. This applies only if the machine
is run in its own scope unit, i.e. if --keep-unit isn't used. Takes unit property assignments in the
same format as systemctl set-property. This is useful to set memory limits and similar for container.
--private-users=
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:
1. 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.
2. 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.
3. If the parameter is false, user namespacing is turned off. This is the default.
4. The special value "pick" 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 --private-users-chown (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).
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 --private-users=pick option.
When user namespaces are used, the GID range assigned to each container is always chosen identical to
the UID range.
In most cases, using --private-users=pick is the recommended option as it enhances container security
massively and operates fully automatically in most cases.
Note that the picked UID/GID range is not written to /etc/passwd or /etc/group. 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.
Note that when user namespacing is used file ownership on disk reflects this, and all of the
container's files and directories are owned by the container's effective user and group IDs. This
means that copying files from and to the container image requires correction of the numeric UID/GID
values, according to the UID/GID shift applied.
--private-users-chown
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.
This option is implied if --private-users=pick is used. This option has no effect if user namespacing
is not used.
-U
If the kernel supports the user namespaces feature, equivalent to --private-users=pick
--private-users-chown, otherwise equivalent to --private-users=no.
Note that -U is the default if the systemd-nspawn@.service template unit file is used.
Note: it is possible to undo the effect of --private-users-chown (or -U) on the file system by
redoing the operation with the first UID of 0:
systemd-nspawn ... --private-users=0 --private-users-chown
--private-network
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
--network-interface= and configured with --network-veth. 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 --drop-capability=.
--network-namespace-path=
Takes the path to a file representing a kernel network namespace that the container shall run in. The
specified path should refer to a (possibly bind-mounted) network namespace file, as exposed by the
kernel below /proc/$PID/ns/net. This makes the container enter the given network namespace. One of
the typical use cases is to give a network namespace under /run/netns created by ip-netns(8), for
example, --network-namespace-path=/run/netns/foo. Note that this option cannot be used together with
other network-related options, such as --private-network or --network-interface=.
--network-interface=
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 --network-interface= implies --private-network. This option may
be used more than once to add multiple network interfaces to the container.
--network-macvlan=
Create a "macvlan" interface of the specified Ethernet network interface and add it to the container.
A "macvlan" 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 "mv-". Note that --network-macvlan= implies --private-network. This option may be used more than
once to add multiple network interfaces to the container.
--network-ipvlan=
Create an "ipvlan" interface of the specified Ethernet network interface and add it to the container.
An "ipvlan" interface is a virtual interface, similar to a "macvlan" 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 "iv-". Note that --network-ipvlan= implies --private-network.
This option may be used more than once to add multiple network interfaces to the container.
-n, --network-veth
Create a virtual Ethernet link ("veth") 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
--machine=), prefixed with "ve-". The container side of the Ethernet link will be named "host0". The
--network-veth option implies --private-network.
Note that systemd-networkd.service(8) includes by default a network file
/lib/systemd/network/80-container-ve.network 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
/lib/systemd/network/80-container-host0.network matching the container-side interface created this
way, containing settings to enable client side address assignment via DHCP. In case systemd-networkd
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.
Note that --network-veth is the default if the systemd-nspawn@.service template unit file is used.
--network-veth-extra=
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
--network-veth, and — in contrast — may be used multiple times, and allows configuration of the
network interface names. Note that --network-bridge= has no effect on interfaces created with
--network-veth-extra=.
--network-bridge=
Adds the host side of the Ethernet link created with --network-veth to the specified Ethernet bridge
interface. Expects a valid network interface name of a bridge device as argument. Note that
--network-bridge= implies --network-veth. If this option is used, the host side of the Ethernet link
will use the "vb-" prefix instead of "ve-".
--network-zone=
Creates a virtual Ethernet link ("veth") to the container and adds it to an automatically managed
Ethernet bridge interface. The bridge interface is named after the passed argument, prefixed with
"vz-". 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 --network-bridge=, besides this
automatic creation/removal of the bridge device.
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 "vz-"), and it is sufficient
to pass the same name to the --network-zone= switch of the various concurrently running containers to
join them in one zone.
Note that systemd-networkd.service(8) includes by default a network file
/lib/systemd/network/80-container-vz.network 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 --network-zone= 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.
-p, --port=
If private networking is enabled, maps an IP port on the host onto an IP port on the container. Takes
a protocol specifier (either "tcp" or "udp"), 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 "tcp" 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 --network-veth,
--network-zone= --network-bridge=.
-Z, --selinux-context=
Sets the SELinux security context to be used to label processes in the container.
-L, --selinux-apifs-context=
Sets the SELinux security context to be used to label files in the virtual API file systems in 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_AUDIT_CONTROL, CAP_AUDIT_WRITE, CAP_CHOWN, CAP_DAC_OVERRIDE,
CAP_DAC_READ_SEARCH, CAP_FOWNER, CAP_FSETID, CAP_IPC_OWNER, CAP_KILL, CAP_LEASE, CAP_LINUX_IMMUTABLE,
CAP_MKNOD, CAP_NET_BIND_SERVICE, CAP_NET_BROADCAST, CAP_NET_RAW, CAP_SETFCAP, CAP_SETGID,
CAP_SETPCAP, CAP_SETUID, CAP_SYS_ADMIN, CAP_SYS_BOOT, CAP_SYS_CHROOT, CAP_SYS_NICE, CAP_SYS_PTRACE,
CAP_SYS_RESOURCE, CAP_SYS_TTY_CONFIG. Also CAP_NET_ADMIN is retained if --private-network is
specified. If the special value "all" is passed, all capabilities are retained.
--drop-capability=
Specify one or more additional capabilities to drop for the container. This allows running the
container with fewer capabilities than the default (see above).
--system-call-filter=
Alter the system call filter applied to containers. Takes a space-separated list of system call names
or group names (the latter prefixed with "@", as listed by the syscall-filter command of systemd-
analyze(1)). Passed system calls will be permitted. The list may optionally be prefixed by "~", in
which case all listed system calls are prohibited. If this command line option is used multiple times
the configured lists are combined. If both a positive and a negative list (that is one system call
list without and one with the "~" prefix) are configured, the negative list takes precedence over the
positive list. Note that systemd-nspawn always implements a system call whitelist (as opposed to a
blacklist), and this command line option hence adds or removes entries from the default whitelist,
depending on the "~" prefix. Note that the applied system call filter is also altered implicitly if
additional capabilities are passed using the --capabilities=.
--kill-signal=
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 --boot is used (on
systemd-compatible init systems SIGRTMIN+3 triggers an orderly shutdown). For a list of valid
signals, see signal(7).
--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",
"try-host", "guest", "try-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/machine-id) and the subdirectory
is bind-mounted into the container at the same location. If "guest", the journal files are stored on
the guest file system (beneath /var/log/journal/machine-id) and the subdirectory is symlinked into
the host at the same location. "try-host" and "try-guest" do the same but do not fail if the host
does not have persistent journaling enabled. If "auto" (the default), and the right subdirectory of
/var/log/journal 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 "guest" or "host" will
link the journal persistently if further on the default of "auto" is used.
Note that --link-journal=try-guest is the default if the systemd-nspawn@.service template unit file
is used.
-j
Equivalent to --link-journal=try-guest.
--read-only
Mount the root file system read-only for the container.
--bind=, --bind-ro=
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 "+" 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 /var/tmp directory is used. It is automatically removed when the
container is shut down. Mount options are comma-separated and currently, only rbind and norbind are
allowed, controlling whether to create a recursive or a regular bind mount. Defaults to "rbind".
Backslash escapes are interpreted, so "\:" may be used to embed colons in either path. This option
may be specified multiple times for creating multiple independent bind mount points. The --bind-ro=
option creates read-only bind mounts.
Note that when this option is used in combination with --private-users, the resulting mount points
will be owned by the nobody user. That's because the mount and its files and directories continue to
be owned by the relevant host users and groups, which do not exist in the container, and thus show up
under the wildcard UID 65534 (nobody). If such bind mounts are created, it is recommended to make
them read-only, using --bind-ro=.
--tmpfs=
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 /var as
tmpfs, to allow state-less systems, in particular when combined with --read-only. Backslash escapes
are interpreted in the path, so "\:" may be used to embed colons in the path.
--overlay=, --overlay-ro=
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.
Backslash escapes are interpreted in the paths, so "\:" may be used to embed colons in the paths.
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 --overlay-ro= is
used instead of --overlay=, 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.
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.
The source paths may optionally be prefixed with "+" 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 /var/tmp 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 "--overlay=+/var::/var"
option in order to automatically overlay a writable temporary directory on a read-only /var
directory.
For details about overlay file systems, see overlayfs.txt[4]. 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 "workdir=" 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 "lowerdir=" mount option receives the paths to stack in the
opposite order of this switch.
-E NAME=VALUE, --setenv=NAME=VALUE
Specifies an environment variable assignment to pass to the init process in the container, in the
format "NAME=VALUE". This may be used to override the default variables or to set additional
variables. This parameter may be used more than once.
--register=
Controls whether the container is registered with systemd-machined(8). Takes a boolean argument,
which defaults to "yes". This option should be enabled when the container runs a full Operating
System (more specifically: a system and service manager as PID 1), and is useful to ensure that the
container is accessible via machinectl(1) and shown by tools such as ps(1). If the container does not
run a service manager, it is recommended to set this option to "no".
--keep-unit
Instead of creating a transient scope unit to run the container in, simply use the service or scope
unit systemd-nspawn has been invoked in. If --register=yes is set this unit is registered with
systemd-machined(8). This switch should be used if systemd-nspawn is invoked from within a service
unit, and the service unit's sole purpose is to run a single systemd-nspawn container. This option is
not available if run from a user session.
Note that passing --keep-unit disables the effect of --slice= and --property=. Use --keep-unit and
--register=no in combination to disable any kind of unit allocation or registration with
systemd-machined.
--personality=
Control the architecture ("personality") reported by uname(2) in the container. Currently, only "x86"
and "x86-64" 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.
-q, --quiet
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.
--volatile, --volatile=MODE
Boots the container in volatile mode. When no mode parameter is passed or when mode is specified as
yes, full volatile mode is enabled. This means the root directory is mounted as a mostly unpopulated
"tmpfs" instance, and /usr 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 state, the OS tree is mounted read-only, but /var
is mounted as a "tmpfs" 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 no (the default), the whole OS tree is made available writable.
This option provides similar functionality for containers as the "systemd.volatile=" kernel command
line switch provides for host systems. See kernel-command-line(7) for details.
Note that enabling this setting will only work correctly with operating systems in the container that
can boot up with only /usr mounted, and are able to automatically populate /var, and also /etc in
case of "--volatile=yes".
--settings=MODE
Controls whether systemd-nspawn shall search for and use additional per-container settings from
.nspawn files. Takes a boolean or the special values override or trusted.
If enabled (the default), a settings file named after the machine (as specified with the --machine=
setting, or derived from the directory or image file name) with the suffix .nspawn is searched in
/etc/systemd/nspawn/ and /run/systemd/nspawn/. 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
.nspawn 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 .nspawn files, consult systemd.nspawn(5).
If this option is set to override, the file is searched, read and used the same way, however, the
order of precedence is reversed: settings read from the .nspawn file will take precedence over the
corresponding command line options, if both are specified.
If this option is set to trusted, the file is searched, read and used the same way, but regardless of
being found in /etc/systemd/nspawn/, /run/systemd/nspawn/ 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.
If disabled, no .nspawn file is read and no settings except the ones on the command line are in
effect.
--notify-ready=
Configures support for notifications from the container's init process. --notify-ready= takes a
boolean (no and yes). With option no systemd-nspawn notifies systemd with a "READY=1" message when
the init process is created. With option yes systemd-nspawn waits for the "READY=1" message from the
init process in the container before sending its own to systemd. For more details about notifications
see sd_notify(3)).
-h, --help
Print a short help text and exit.
--version
Print a short version string and exit.
EXAMPLES
Example 1. Download a Fedora image and start a shell in it
# machinectl pull-raw --verify=no \
https://download.fedoraproject.org/pub/fedora/linux/releases/25/CloudImages/x86_64/images/Fedora-Cloud-Base-25-1.3.x86_64.raw.xz
# systemd-nspawn -M Fedora-Cloud-Base-25-1.3.x86_64.raw
This downloads an image using machinectl(1) and opens a shell in it.
Example 2. Build and boot a minimal Fedora distribution in a container
# dnf -y --releasever=27 --installroot=/var/lib/machines/f27container \
--disablerepo='*' --enablerepo=fedora --enablerepo=updates install \
systemd passwd dnf fedora-release vim-minimal
# systemd-nspawn -bD /var/lib/machines/f27container
This installs a minimal Fedora distribution into the directory /var/lib/machines/f27container and then
boots an OS in a namespace container in it. Because the installation is located underneath the standard
/var/lib/machines/ directory, it is also possible to start the machine using systemd-nspawn -M
f27container.
Example 3. Spawn a shell in a container of a minimal Debian unstable distribution
# debootstrap 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.
debootstrap supports Debian[6], Ubuntu[7], and Tanglu[8] out of the box, so the same command can be used
to install any of those. For other distributions from the Debian family, a mirror has to be specified,
see debootstrap(8).
Example 4. Boot a minimal Arch Linux distribution in a container
# pacstrap -c -d ~/arch-tree/ base
# systemd-nspawn -bD ~/arch-tree/
This installs a minimal Arch Linux distribution into the directory ~/arch-tree/ and then boots an OS in a
namespace container in it.
Example 5. Install the OpenSUSE Tumbleweed rolling distribution
# zypper --root=/var/lib/machines/tumbleweed ar -c \
https://download.opensuse.org/tumbleweed/repo/oss tumbleweed
# zypper --root=/var/lib/machines/tumbleweed refresh
# zypper --root=/var/lib/machines/tumbleweed install --no-recommends \
systemd shadow zypper openSUSE-release vim
# systemd-nspawn -M tumbleweed passwd root
# systemd-nspawn -M tumbleweed -b
Example 6. Boot into an ephemeral snapshot of the host system
# systemd-nspawn -D / -xb
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.
Example 7. Run a container with SELinux sandbox security contexts
# 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
Example 8. Run a container with an OSTree deployment
# systemd-nspawn -b -i ~/image.raw \
--pivot-root=/ostree/deploy/$OS/deploy/$CHECKSUM:/sysroot \
--bind=+/sysroot/ostree/deploy/$OS/var:/var
EXIT STATUS
The exit code of the program executed in the container is returned.
SEE ALSO
systemd(1), systemd.nspawn(5), chroot(1), dnf(8), debootstrap(8), pacman(8), zypper(8), systemd.slice(5), machinectl(1), btrfs(8)
NOTES
1. Container Interface
https://www.freedesktop.org/wiki/Software/systemd/ContainerInterface
2. Discoverable Partitions Specification
https://www.freedesktop.org/wiki/Specifications/DiscoverablePartitionsSpec/
3. OSTree
https://ostree.readthedocs.io/en/latest/
4. overlayfs.txt
https://www.kernel.org/doc/Documentation/filesystems/overlayfs.txt
5. Fedora
https://getfedora.org
6. Debian
https://www.debian.org
7. Ubuntu
https://www.ubuntu.com
8. Tanglu
https://www.tanglu.org
9. Arch Linux
https://www.archlinux.org
10. OpenSUSE Tumbleweed
https://software.opensuse.org/distributions/tumbleweed