namespaces(7) Miscellaneous Information Manual namespaces(7)
NAME
namespaces - overview of Linux namespaces
DESCRIPTION
A namespace wraps a global system resource in an abstraction that makes
it appear to the processes within the namespace that they have their own
isolated instance of the global resource. Changes to the global re-
source are visible to other processes that are members of the namespace,
but are invisible to other processes. One use of namespaces is to im-
plement containers.
This page provides pointers to information on the various namespace
types, describes the associated /proc files, and summarizes the APIs for
working with namespaces.
Namespace types
The following table shows the namespace types available on Linux. The
second column of the table shows the flag value that is used to specify
the namespace type in various APIs. The third column identifies the
manual page that provides details on the namespace type. The last col-
umn is a summary of the resources that are isolated by the namespace
type.
Namespace Flag Page Isolates
Cgroup CLONE_NEWCGROUP cgroup_namespaces(7) Cgroup root di-
rectory
IPC CLONE_NEWIPC ipc_namespaces(7) System V IPC,
POSIX message
queues
Network CLONE_NEWNET network_namespaces(7) Network devices,
stacks, ports,
etc.
Mount CLONE_NEWNS mount_namespaces(7) Mount points
PID CLONE_NEWPID pid_namespaces(7) Process IDs
Time CLONE_NEWTIME time_namespaces(7) Boot and monoto-
nic clocks
User CLONE_NEWUSER user_namespaces(7) User and group
IDs
UTS CLONE_NEWUTS uts_namespaces(7) Hostname and NIS
domain name
The namespaces API
As well as various /proc files described below, the namespaces API in-
cludes the following system calls:
clone(2)
The clone(2) system call creates a new process. If the flags ar-
gument of the call specifies one or more of the CLONE_NEW* flags
listed above, then new namespaces are created for each flag, and
the child process is made a member of those namespaces. (This
system call also implements a number of features unrelated to
namespaces.)
setns(2)
The setns(2) system call allows the calling process to join an
existing namespace. The namespace to join is specified via a
file descriptor that refers to one of the /proc/pid/ns files de-
scribed below.
unshare(2)
The unshare(2) system call moves the calling process to a new
namespace. If the flags argument of the call specifies one or
more of the CLONE_NEW* flags listed above, then new namespaces
are created for each flag, and the calling process is made a mem-
ber of those namespaces. (This system call also implements a
number of features unrelated to namespaces.)
ioctl(2)
Various ioctl(2) operations can be used to discover information
about namespaces. These operations are described in
ioctl_nsfs(2).
Creation of new namespaces using clone(2) and unshare(2) in most cases
requires the CAP_SYS_ADMIN capability, since, in the new namespace, the
creator will have the power to change global resources that are visible
to other processes that are subsequently created in, or join the name-
space. User namespaces are the exception: since Linux 3.8, no privilege
is required to create a user namespace.
The /proc/pid/ns/ directory
Each process has a /proc/pid/ns/ subdirectory containing one entry for
each namespace that supports being manipulated by setns(2):
$ ls -l /proc/$$/ns | awk '{print $1, $9, $10, $11}'
total 0
lrwxrwxrwx. cgroup -> cgroup:[4026531835]
lrwxrwxrwx. ipc -> ipc:[4026531839]
lrwxrwxrwx. mnt -> mnt:[4026531840]
lrwxrwxrwx. net -> net:[4026531969]
lrwxrwxrwx. pid -> pid:[4026531836]
lrwxrwxrwx. pid_for_children -> pid:[4026531834]
lrwxrwxrwx. time -> time:[4026531834]
lrwxrwxrwx. time_for_children -> time:[4026531834]
lrwxrwxrwx. user -> user:[4026531837]
lrwxrwxrwx. uts -> uts:[4026531838]
Bind mounting (see mount(2)) one of the files in this directory to some-
where else in the filesystem keeps the corresponding namespace of the
process specified by pid alive even if all processes currently in the
namespace terminate.
Opening one of the files in this directory (or a file that is bind
mounted to one of these files) returns a file handle for the correspond-
ing namespace of the process specified by pid. As long as this file de-
scriptor remains open, the namespace will remain alive, even if all
processes in the namespace terminate. The file descriptor can be passed
to setns(2).
In Linux 3.7 and earlier, these files were visible as hard links. Since
Linux 3.8, they appear as symbolic links. If two processes are in the
same namespace, then the device IDs and inode numbers of their
/proc/pid/ns/xxx symbolic links will be the same; an application can
check this using the stat.st_dev and stat.st_ino fields returned by
stat(2). The content of this symbolic link is a string containing the
namespace type and inode number as in the following example:
$ readlink /proc/$$/ns/uts
uts:[4026531838]
The symbolic links in this subdirectory are as follows:
/proc/pid/ns/cgroup (since Linux 4.6)
This file is a handle for the cgroup namespace of the process.
/proc/pid/ns/ipc (since Linux 3.0)
This file is a handle for the IPC namespace of the process.
/proc/pid/ns/mnt (since Linux 3.8)
This file is a handle for the mount namespace of the process.
/proc/pid/ns/net (since Linux 3.0)
This file is a handle for the network namespace of the process.
/proc/pid/ns/pid (since Linux 3.8)
This file is a handle for the PID namespace of the process. This
handle is permanent for the lifetime of the process (i.e., a
process's PID namespace membership never changes).
/proc/pid/ns/pid_for_children (since Linux 4.12)
This file is a handle for the PID namespace of child processes
created by this process. This can change as a consequence of
calls to unshare(2) and setns(2) (see pid_namespaces(7)), so the
file may differ from /proc/pid/ns/pid. The symbolic link gains a
value only after the first child process is created in the name-
space. (Beforehand, readlink(2) of the symbolic link will return
an empty buffer.)
/proc/pid/ns/time (since Linux 5.6)
This file is a handle for the time namespace of the process.
/proc/pid/ns/time_for_children (since Linux 5.6)
This file is a handle for the time namespace of child processes
created by this process. This can change as a consequence of
calls to unshare(2) and setns(2) (see time_namespaces(7)), so the
file may differ from /proc/pid/ns/time.
/proc/pid/ns/user (since Linux 3.8)
This file is a handle for the user namespace of the process.
/proc/pid/ns/uts (since Linux 3.0)
This file is a handle for the UTS namespace of the process.
Permission to dereference or read (readlink(2)) these symbolic links is
governed by a ptrace access mode PTRACE_MODE_READ_FSCREDS check; see
ptrace(2).
The /proc/sys/user directory
The files in the /proc/sys/user directory (which is present since Linux
4.9) expose limits on the number of namespaces of various types that can
be created. The files are as follows:
max_cgroup_namespaces
The value in this file defines a per-user limit on the number of
cgroup namespaces that may be created in the user namespace.
max_ipc_namespaces
The value in this file defines a per-user limit on the number of
ipc namespaces that may be created in the user namespace.
max_mnt_namespaces
The value in this file defines a per-user limit on the number of
mount namespaces that may be created in the user namespace.
max_net_namespaces
The value in this file defines a per-user limit on the number of
network namespaces that may be created in the user namespace.
max_pid_namespaces
The value in this file defines a per-user limit on the number of
PID namespaces that may be created in the user namespace.
max_time_namespaces (since Linux 5.7)
The value in this file defines a per-user limit on the number of
time namespaces that may be created in the user namespace.
max_user_namespaces
The value in this file defines a per-user limit on the number of
user namespaces that may be created in the user namespace.
max_uts_namespaces
The value in this file defines a per-user limit on the number of
uts namespaces that may be created in the user namespace.
Note the following details about these files:
• The values in these files are modifiable by privileged processes.
• The values exposed by these files are the limits for the user name-
space in which the opening process resides.
• The limits are per-user. Each user in the same user namespace can
create namespaces up to the defined limit.
• The limits apply to all users, including UID 0.
• These limits apply in addition to any other per-namespace limits
(such as those for PID and user namespaces) that may be enforced.
• Upon encountering these limits, clone(2) and unshare(2) fail with the
error ENOSPC.
• For the initial user namespace, the default value in each of these
files is half the limit on the number of threads that may be created
(/proc/sys/kernel/threads-max). In all descendant user namespaces,
the default value in each file is MAXINT.
• When a namespace is created, the object is also accounted against an-
cestor namespaces. More precisely:
• Each user namespace has a creator UID.
• When a namespace is created, it is accounted against the creator
UIDs in each of the ancestor user namespaces, and the kernel en-
sures that the corresponding namespace limit for the creator UID
in the ancestor namespace is not exceeded.
• The aforementioned point ensures that creating a new user name-
space cannot be used as a means to escape the limits in force in
the current user namespace.
Namespace lifetime
Absent any other factors, a namespace is automatically torn down when
the last process in the namespace terminates or leaves the namespace.
However, there are a number of other factors that may pin a namespace
into existence even though it has no member processes. These factors
include the following:
• An open file descriptor or a bind mount exists for the corresponding
/proc/pid/ns/* file.
• The namespace is hierarchical (i.e., a PID or user namespace), and
has a child namespace.
• It is a user namespace that owns one or more nonuser namespaces.
• It is a PID namespace, and there is a process that refers to the
namespace via a /proc/pid/ns/pid_for_children symbolic link.
• It is a time namespace, and there is a process that refers to the
namespace via a /proc/pid/ns/time_for_children symbolic link.
• It is an IPC namespace, and a corresponding mount of an mqueue
filesystem (see mq_overview(7)) refers to this namespace.
• It is a PID namespace, and a corresponding mount of a proc(5)
filesystem refers to this namespace.
EXAMPLES
See clone(2) and user_namespaces(7).
SEE ALSO
nsenter(1), readlink(1), unshare(1), clone(2), ioctl_nsfs(2), setns(2),
unshare(2), proc(5), capabilities(7), cgroup_namespaces(7), cgroups(7),
credentials(7), ipc_namespaces(7), network_namespaces(7), pid_name-
spaces(7), user_namespaces(7), uts_namespaces(7), lsns(8),
switch_root(8)
Linux man-pages 6.9.1 2024-06-13 namespaces(7)
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