unshare man page

unshare — run program with some namespaces unshared from parent

Synopsis

unshare [options] program [arguments]

Description

Unshares the indicated namespaces from the parent process and then executes the specified program.

The namespaces can optionally be made persistent by bind mounting /proc/pid/ns/type files to a filesystem path and entered with nsenter(1) even after the program terminates. Once a persistent namespace is no longer needed, it can be unpersisted with umount(8). See the Examples section for more details.

The namespaces to be unshared are indicated via options. Unshareable namespaces are:

mount namespace
Mounting and unmounting filesystems will not affect the rest of the system (CLONE_NEWNS flag), except for filesystems which are explicitly marked as shared (with mount --make-shared; see /proc/self/mountinfo or findmnt -o+PROPAGATION for the shared flags).

unshare since util-linux version 2.27 automatically sets propagation to private in a new mount namespace to make sure that the new namespace is really unshared. It's possible to disable this feature with option --propagation unchanged. Note that private is the kernel default.
UTS namespace
Setting hostname or domainname will not affect the rest of the system. (CLONE_NEWUTS flag)
IPC namespace
The process will have an independent namespace for System V message queues, semaphore sets and shared memory segments. (CLONE_NEWIPC flag)
network namespace
The process will have independent IPv4 and IPv6 stacks, IP routing tables, firewall rules, the /proc/net and /sys/class/net directory trees, sockets, etc. (CLONE_NEWNET flag)
pid namespace
Children will have a distinct set of PID-to-process mappings from their parent. (CLONE_NEWPID flag)
cgroup namespace
The process will have a virtualized view of /proc/self/cgroup, and new cgroup mounts will be rooted at the namespace cgroup root. (CLONE_NEWCGROUP flag)
user namespace
The process will have a distinct set of UIDs, GIDs and capabilities. (CLONE_NEWUSER flag)

See clone(2) for the exact semantics of the flags.

Options

-i, --ipc[=file]
Unshare the IPC namespace. If file is specified, then a persistent namespace is created by a bind mount.
-m, --mount[=file]
Unshare the mount namespace. If file is specified, then a persistent namespace is created by a bind mount. Note that file has to be located on a filesystem with the propagation flag set to private. Use the command findmnt -o+PROPAGATION when not sure about the current setting. See also the examples below.
-n, --net[=file]
Unshare the network namespace. If file is specified, then a persistent namespace is created by a bind mount.
-p, --pid[=file]
Unshare the PID namespace. If file is specified then persistent namespace is created by a bind mount. See also the --fork and --mount-proc options.
-u, --uts[=file]
Unshare the UTS namespace. If file is specified, then a persistent namespace is created by a bind mount.
-U, --user[=file]
Unshare the user namespace. If file is specified, then a persistent namespace is created by a bind mount.
-C, --cgroup[=file]
Unshare the cgroup namespace. If file is specified then persistent namespace is created by bind mount.
-f, --fork
Fork the specified program as a child process of unshare rather than running it directly. This is useful when creating a new PID namespace.
--mount-proc[=mountpoint]
Just before running the program, mount the proc filesystem at mountpoint (default is /proc). This is useful when creating a new PID namespace. It also implies creating a new mount namespace since the /proc mount would otherwise mess up existing programs on the system. The new proc filesystem is explicitly mounted as private (with MS_PRIVATE|MS_REC).
-r, --map-root-user
Run the program only after the current effective user and group IDs have been mapped to the superuser UID and GID in the newly created user namespace. This makes it possible to conveniently gain capabilities needed to manage various aspects of the newly created namespaces (such as configuring interfaces in the network namespace or mounting filesystems in the mount namespace) even when run unprivileged. As a mere convenience feature, it does not support more sophisticated use cases, such as mapping multiple ranges of UIDs and GIDs. This option implies --setgroups=deny.
--propagation private|shared|slave|unchanged
Recursively set the mount propagation flag in the new mount namespace. The default is to set the propagation to private. It is possible to disable this feature with the argument unchanged. The option is silently ignored when the mount namespace (--mount) is not requested.
--setgroups allow|deny
Allow or deny the setgroups(2) syscall in a user namespace.

To be able to call setgroups(2), the calling process must at least have CAP_SETGID. But since Linux 3.19 a further restriction applies: the kernel gives permission to call setgroups(2) only after the GID map (/proc/pid/gid_map) has been set. The GID map is writable by root when setgroups(2) is enabled (i.e. allow, the default), and the GID map becomes writable by unprivileged processes when setgroups(2) is permanently disabled (with deny).
-V, --version
Display version information and exit.
-h, --help
Display help text and exit.

Examples

# unshare --fork --pid --mount-proc readlink /proc/self
1
Establish a PID namespace, ensure we're PID 1 in it against a newly mounted procfs instance.
$ unshare --map-root-user --user sh -c whoami
root
Establish a user namespace as an unprivileged user with a root user within it.
# touch /root/uts-ns
# unshare --uts=/root/uts-ns hostname FOO
# nsenter --uts=/root/uts-ns hostname
FOO
# umount /root/uts-ns
Establish a persistent UTS namespace, and modify the hostname. The namespace is then entered with nsenter. The namespace is destroyed by unmounting the bind reference.
# mount --bind /root/namespaces /root/namespaces
# mount --make-private /root/namespaces
# touch /root/namespaces/mnt
# unshare --mount=/root/namespaces/mnt
Establish a persistent mount namespace referenced by the bind mount /root/namespaces/mnt. This example shows a portable solution, because it makes sure that the bind mount is created on a shared filesystem.

See Also

unshare(2), clone(2), mount(8)

Authors

Mikhail Gusarov
Karel Zak

Availability

The unshare command is part of the util-linux package and is available from ftp://ftp.kernel.org/pub/linux/utils/ut….

Referenced By

cgroup_namespaces(7), findmnt(8), lsns(8), mount_namespaces(7), namespaces(7), unshare(2).

February 2016 util-linux User Commands