UNSHARE(1) User Commands UNSHARE(1)
NAME
unshare - run program in new namespaces
SYNOPSIS
unshare [options] [program [arguments]]
DESCRIPTION
The unshare command creates new namespaces (as specified by the
command-line options described below) and then executes the specified
program. If program is not given, then "${SHELL}" is run (default:
/bin/sh).
By default, a new namespace persists only as long as it has member
processes. A new namespace can be made persistent even when it has no
member processes by bind mounting /proc/pid/ns/type files to a
filesystem path. A namespace that has been made persistent in this way
can subsequently be entered with nsenter(1) even after the program
terminates (except PID namespaces where a permanently running init
process is required). Once a persistent namespace is no longer needed,
it can be unpersisted by using umount(8) to remove the bind mount. See
the EXAMPLES section for more details.
unshare since util-linux version 2.36 uses
/proc/[pid]/ns/pid_for_children and /proc/[pid]/ns/time_for_children
files for persistent PID and TIME namespaces. This change requires
Linux kernel 4.17 or newer.
The following types of namespaces can be created with unshare:
mount namespace
Mounting and unmounting filesystems will not affect the rest of the
system, 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). For further details,
see mount_namespaces(7).
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. For further details, see uts_namespaces(7).
IPC namespace
The process will have an independent namespace for POSIX message
queues as well as System V message queues, semaphore sets and
shared memory segments. For further details, see ipc_namespaces(7).
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. For further details, see
network_namespaces(7).
PID namespace
Children will have a distinct set of PID-to-process mappings from
their parent. For further details, see pid_namespaces(7).
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. For
further details, see cgroup_namespaces(7).
user namespace
The process will have a distinct set of UIDs, GIDs and
capabilities. For further details, see user_namespaces(7).
time namespace
The process can have a distinct view of CLOCK_MONOTONIC and/or
CLOCK_BOOTTIME which can be changed using
/proc/self/timens_offsets. For further details, see
time_namespaces(7).
OPTIONS
-i, --ipc[=file]
Create a new IPC namespace. If file is specified, then the
namespace is made persistent by creating a bind mount at file.
-m, --mount[=file]
Create a new mount namespace. If file is specified, then the
namespace is made persistent by creating a bind mount at file. Note
that file must be located on a mount whose propagation type is not
shared (or an error results). Use the command findmnt
-o+PROPAGATION when not sure about the current setting. See also
the examples below.
-n, --net[=file]
Create a new network namespace. If file is specified, then the
namespace is made persistent by creating a bind mount at file.
-p, --pid[=file]
Create a new PID namespace. If file is specified, then the
namespace is made persistent by creating a bind mount at file.
(Creation of a persistent PID namespace will fail if the --fork
option is not also specified.)
See also the --fork and --mount-proc options.
-u, --uts[=file]
Create a new UTS namespace. If file is specified, then the
namespace is made persistent by creating a bind mount at file.
-U, --user[=file]
Create a new user namespace. If file is specified, then the
namespace is made persistent by creating a bind mount at file.
-C, --cgroup[=file]
Create a new cgroup namespace. If file is specified, then the
namespace is made persistent by creating a bind mount at file.
-T, --time[=file]
Create a new time namespace. If file is specified, then the
namespace is made persistent by creating a bind mount at file. The
--monotonic and --boottime options can be used to specify the
corresponding offset in the time namespace.
-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. Note that when unshare is waiting for the child process,
then it ignores SIGINT and SIGTERM and does not forward any signals
to the child. It is necessary to send signals to the child process.
--keep-caps
When the --user option is given, ensure that capabilities granted
in the user namespace are preserved in the child process.
--kill-child[=signame]
When unshare terminates, have signame be sent to the forked child
process. Combined with --pid this allows for an easy and reliable
killing of the entire process tree below unshare. If not given,
signame defaults to SIGKILL. This option implies --fork.
--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).
--map-user=uid|name
Run the program only after the current effective user ID has been
mapped to uid. If this option is specified multiple times, the last
occurrence takes precedence. This option implies --user.
--map-users=outeruid,inneruid,count|auto
Run the program only after the block of user IDs of size count
beginning at outeruid has been mapped to the block of user IDs
beginning at inneruid. This mapping is created with newuidmap(1).
If the range of user IDs overlaps with the mapping specified by
--map-user, then a "hole" will be removed from the mapping. This
may result in the highest user ID of the mapping not being mapped.
The special value auto will map the first block of user IDs owned
by the effective user from /etc/subuid to a block starting at user
ID 0. If this option is specified multiple times, the last
occurrence takes precedence. This option implies --user.
--map-group=gid|name
Run the program only after the current effective group ID has been
mapped to gid. If this option is specified multiple times, the last
occurrence takes precedence. This option implies --setgroups=deny
and --user.
--map-groups=outergid,innergid,count|auto
Run the program only after the block of group IDs of size count
beginning at outergid has been mapped to the block of group IDs
beginning at innergid. This mapping is created with newgidmap(1).
If the range of group IDs overlaps with the mapping specified by
--map-group, then a "hole" will be removed from the mapping. This
may result in the highest group ID of the mapping not being mapped.
The special value auto will map the first block of user IDs owned
by the effective user from /etc/subgid to a block starting at group
ID 0. If this option is specified multiple times, the last
occurrence takes precedence. This option implies --user.
--map-auto
Map the first block of user IDs owned by the effective user from
/etc/subuid to a block starting at user ID 0. In the same manner,
also map the first block of group IDs owned by the effective group
from /etc/subgid to a block starting at group ID 0. This option is
intended to handle the common case where the first block of
subordinate user and group IDs can map the whole user and group ID
space. This option is equivalent to specifying --map-users=auto and
--map-groups=auto.
-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 and --user.
This option is equivalent to --map-user=0 --map-group=0.
-c, --map-current-user
Run the program only after the current effective user and group IDs
have been mapped to the same UID and GID in the newly created user
namespace. This option implies --setgroups=deny and --user. This
option is equivalent to --map-user=$(id -ru) --map-group=$(id -rg).
--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) system call 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).
-R, --root=dir
run the command with root directory set to dir.
-w, --wd=dir
change working directory to dir.
-S, --setuid uid
Set the user ID which will be used in the entered namespace.
-G, --setgid gid
Set the group ID which will be used in the entered namespace and
drop supplementary groups.
--monotonic offset
Set the offset of CLOCK_MONOTONIC which will be used in the entered
time namespace. This option requires unsharing a time namespace
with --time.
--boottime offset
Set the offset of CLOCK_BOOTTIME which will be used in the entered
time namespace. This option requires unsharing a time namespace
with --time.
-h, --help
Display help text and exit.
-V, --version
Print version and exit.
NOTES
The proc and sysfs filesystems mounting as root in a user namespace
have to be restricted so that a less privileged user cannot get more
access to sensitive files that a more privileged user made unavailable.
In short the rule for proc and sysfs is as close to a bind mount as
possible.
EXAMPLES
The following command creates a PID namespace, using --fork to ensure
that the executed command is performed in a child process that (being
the first process in the namespace) has PID 1. The --mount-proc option
ensures that a new mount namespace is also simultaneously created and
that a new proc(5) filesystem is mounted that contains information
corresponding to the new PID namespace. When the readlink(1) command
terminates, the new namespaces are automatically torn down.
# unshare --fork --pid --mount-proc readlink /proc/self
1
As an unprivileged user, create a new user namespace where the user’s
credentials are mapped to the root IDs inside the namespace:
$ id -u; id -g
1000
1000
$ unshare --user --map-root-user \
sh -c ''whoami; cat /proc/self/uid_map /proc/self/gid_map''
root
0 1000 1
0 1000 1
As an unprivileged user, create a user namespace where the first 65536
IDs are all mapped, and the user’s credentials are mapped to the root
IDs inside the namespace. The map is determined by the subordinate IDs
assigned in subuid(5) and subgid(5). Demonstrate this mapping by
creating a file with user ID 1 and group ID 1. For brevity, only the
user ID mappings are shown:
$ id -u
1000
$ cat /etc/subuid
1000:100000:65536
$ unshare --user --map-auto --map-root-user
# id -u
0
# cat /proc/self/uid_map
0 1000 1
1 100000 65535
# touch file; chown 1:1 file
# ls -ln --time-style=+ file
-rw-r--r-- 1 1 1 0 file
# exit
$ ls -ln --time-style=+ file
-rw-r--r-- 1 100000 100000 0 file
The first of the following commands creates a new persistent UTS
namespace and modifies the hostname as seen in that namespace. The
namespace is then entered with nsenter(1) in order to display the
modified hostname; this step demonstrates that the UTS namespace
continues to exist even though the namespace had no member processes
after the unshare command terminated. The namespace is then destroyed
by removing the bind mount.
# touch /root/uts-ns
# unshare --uts=/root/uts-ns hostname FOO
# nsenter --uts=/root/uts-ns hostname
FOO
# umount /root/uts-ns
The following commands establish a persistent mount namespace
referenced by the bind mount /root/namespaces/mnt. In order to ensure
that the creation of that bind mount succeeds, the parent directory
(/root/namespaces) is made a bind mount whose propagation type is not
shared.
# mount --bind /root/namespaces /root/namespaces
# mount --make-private /root/namespaces
# touch /root/namespaces/mnt
# unshare --mount=/root/namespaces/mnt
The following commands demonstrate the use of the --kill-child option
when creating a PID namespace, in order to ensure that when unshare is
killed, all of the processes within the PID namespace are killed.
# set +m # Don't print job status messages
# unshare --pid --fork --mount-proc --kill-child -- \
bash --norc -c ''(sleep 555 &) && (ps a &) && sleep 999'' &
[1] 53456
# PID TTY STAT TIME COMMAND
1 pts/3 S+ 0:00 sleep 999
3 pts/3 S+ 0:00 sleep 555
5 pts/3 R+ 0:00 ps a
# ps h -o 'comm' $! # Show that background job is unshare(1)
unshare
# kill $! # Kill unshare(1)
# pidof sleep
The pidof(1) command prints no output, because the sleep processes have
been killed. More precisely, when the sleep process that has PID 1 in
the namespace (i.e., the namespace’s init process) was killed, this
caused all other processes in the namespace to be killed. By contrast,
a similar series of commands where the --kill-child option is not used
shows that when unshare terminates, the processes in the PID namespace
are not killed:
# unshare --pid --fork --mount-proc -- \
bash --norc -c ''(sleep 555 &) && (ps a &) && sleep 999'' &
[1] 53479
# PID TTY STAT TIME COMMAND
1 pts/3 S+ 0:00 sleep 999
3 pts/3 S+ 0:00 sleep 555
5 pts/3 R+ 0:00 ps a
# kill $!
# pidof sleep
53482 53480
The following example demonstrates the creation of a time namespace
where the boottime clock is set to a point several years in the past:
# uptime -p # Show uptime in initial time namespace
up 21 hours, 30 minutes
# unshare --time --fork --boottime 300000000 uptime -p
up 9 years, 28 weeks, 1 day, 2 hours, 50 minutes
AUTHORS
Mikhail Gusarov <dottedmag@dottedmag.net>, Karel Zak <kzak@redhat.com>
SEE ALSO
newuidmap(1) newgidmap(1) clone(2), unshare(2), namespaces(7), mount(8)
REPORTING BUGS
For bug reports, use the issue tracker at
https://github.com/util-linux/util-linux/issues.
AVAILABILITY
The unshare command is part of the util-linux package which can be
downloaded from Linux Kernel Archive
<https://www.kernel.org/pub/linux/utils/util-linux/>.
util-linux 2.38.1 2022-05-11 UNSHARE(1)
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