Provided by: util-linux_2.40.2-1ubuntu1.1_amd64 
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).
--mount-binfmt[=mountpoint]
Just before running the program, mount the binfmt_misc filesystem at mountpoint (default is
/proc/sys/fs/binfmt_misc). It also implies creating a new mount namespace since the binfmt_misc mount
would otherwise mess up existing programs on the system. The new binfmt_misc 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=inneruid:outeruid:count|auto|all
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 unshare
was run unprivileged. 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. Use --map-users multiple times to map more than one block of user IDs. 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. The special value all will create a pass-through map for every user ID
available in the parent namespace. This option implies --user.
Before util-linux version 2.39, this option expected a comma-separated argument of the form
outeruid,inneruid,count but that format is now deprecated for consistency with the ordering used in
/proc/[pid]/uid_map and the X-mount.idmap mount option.
--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=innergid:outergid:count|auto|all
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 unshare
was run unprivileged. 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. Use --map-groups multiple times to map more than one block of group IDs.
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. The special value all will create a pass-through map
for every group ID available in the parent namespace. This option implies --user.
Before util-linux version 2.39, this option expected a comma-separated argument of the form
outergid,innergid,count but that format is now deprecated for consistency with the ordering used in
/proc/[pid]/gid_map and the X-mount.idmap mount option.
--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.
-l, --load-interp=string
Load binfmt_misc definition in the namespace (implies --mount-binfmt). The string argument is
:name:type:offset:magic:mask:interpreter:flags. For more details about new binary type registration
see https://www.kernel.org/doc/Documentation/admin-guide/binfmt-misc.rst. To manage the F flag in
flags with --root parameter, binfmt_misc is mounted twice, once before the chroot to load the
interpreter from the caller filesystem and once after to make it available from the chroot userspace.
--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
The following example execute a chroot into the directory /chroot/powerpc/jessie and install the
interpreter /bin/qemu-ppc-static to execute the powerpc binaries.
$ unshare --map-root-user --fork --pid --load-interp=":qemu-ppc:M::\\x7fELF\x01\\x02\\x01\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x02\\x00\\x14:\\xff\\xff\\xff\\xff\\xff\\xff\\xff\\x00\\xff\\xff\\xff\\xff\\xff\\xff\\xff\\xff\\xff\\xfe\\xff\\xff:/bin/qemu-ppc-static:OCF" --root=/chroot/powerpc/jessie /bin/bash -l
The load-interp parameter can be read as following
qemu-ppc
is the name of the new file created below /proc/sys/fs/binfmt_misc to register the interpreter
M
defines the interpreter for a given type of magic number
\\x7fELF\x01\\x02\\x01\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x02\\x00\\x1
is the magic number to recognize the file to interpret (in this case, the ELF header for PPC32)
\\xff\\xff\\xff\\xff\\xff\\xff\\xff\\x00\\xff\\xff\\xff\\xff\\xff\\xff\\xff\\xff\\xff\\xfe\\xff\\xff
the mask to apply to the magic number
/bin/qemu-ppc-static
the interpreter to use with the file
OCF
the file is open by the kernel with credential and security tokens of the file itself and loaded
as soon as we register it.
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/>.