namespaces(7) - Linux manual page (original) (raw)
namespaces(7) Miscellaneous Information Manual namespaces(7)
NAME top
namespaces - overview of Linux namespacesDESCRIPTION top
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 resource are visible to other processes that are
members of the namespace, but are invisible to other processes.
One use of namespaces is to implement 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 column 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
directory
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
monotonic
clocks
User **CLONE_NEWUSER user_namespaces**(7) User and
group IDs
UTS **CLONE_NEWUTS uts_namespaces**(7) Hostname and
NIS domain
nameThe namespaces API As well as various /proc files described below, the namespaces API includes the following system calls:
[clone(2)](../man2/clone.2.html)
The [clone(2)](../man2/clone.2.html) system call creates a new process. 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 child process is made a
member of those namespaces. (This system call also
implements a number of features unrelated to namespaces.)
[setns(2)](../man2/setns.2.html)
The [setns(2)](../man2/setns.2.html) 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 described below.
[unshare(2)](../man2/unshare.2.html)
The [unshare(2)](../man2/unshare.2.html) 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 member of those namespaces. (This system
call also implements a number of features unrelated to
namespaces.)
[ioctl(2)](../man2/ioctl.2.html)
Various [ioctl(2)](../man2/ioctl.2.html) operations can be used to discover
information about namespaces. These operations are
described in [ioctl_nsfs(2)](../man2/ioctl%5Fnsfs.2.html).
Creation of new namespaces using [clone(2)](../man2/clone.2.html) and [unshare(2)](../man2/unshare.2.html) 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 namespace. 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 <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mn>1</mn><mo separator="true">,</mo></mrow><annotation encoding="application/x-tex">1, </annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.8389em;vertical-align:-0.1944em;"></span><span class="mord">1</span><span class="mpunct">,</span></span></span></span>9, <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mn>10</mn><mo separator="true">,</mo></mrow><annotation encoding="application/x-tex">10, </annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.8389em;vertical-align:-0.1944em;"></span><span class="mord">10</span><span class="mpunct">,</span></span></span></span>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)](../man2/mount.2.html)) one of the files in this directory to
somewhere 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
corresponding namespace of the process specified by _pid_. As long
as this file descriptor remains open, the namespace will remain
alive, even if all processes in the namespace terminate. The file
descriptor can be passed to [setns(2)](../man2/setns.2.html).
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.stdev_ and _stat.stino_
fields returned by [stat(2)](../man2/stat.2.html). 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/pidforchildren_ (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)](../man2/unshare.2.html) and [setns(2)](../man2/setns.2.html) (see
[pid_namespaces(7)](../man7/pid%5Fnamespaces.7.html)), 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 namespace.
(Beforehand, [readlink(2)](../man2/readlink.2.html) 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/timeforchildren_ (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)](../man2/unshare.2.html) and [setns(2)](../man2/setns.2.html) (see
[time_namespaces(7)](../man7/time%5Fnamespaces.7.html)), 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)](../man2/readlink.2.html)) these symbolic
links is governed by a ptrace access mode **PTRACE_MODE_READ_FSCREDS**
check; see [ptrace(2)](../man2/ptrace.2.html).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:
_maxcgroupnamespaces_
The value in this file defines a per-user limit on the
number of cgroup namespaces that may be created in the user
namespace.
_maxipcnamespaces_
The value in this file defines a per-user limit on the
number of ipc namespaces that may be created in the user
namespace.
_maxmntnamespaces_
The value in this file defines a per-user limit on the
number of mount namespaces that may be created in the user
namespace.
_maxnetnamespaces_
The value in this file defines a per-user limit on the
number of network namespaces that may be created in the
user namespace.
_maxpidnamespaces_
The value in this file defines a per-user limit on the
number of PID namespaces that may be created in the user
namespace.
_maxtimenamespaces_ (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.
_maxusernamespaces_
The value in this file defines a per-user limit on the
number of user namespaces that may be created in the user
namespace.
_maxutsnamespaces_
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
namespace 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)](../man2/clone.2.html) and [unshare(2)](../man2/unshare.2.html) 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 ancestor 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 ensures 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
namespace 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/pidforchildren_ symbolic
link.
• It is a time namespace, and there is a process that refers to
the namespace via a _/proc/_pid_/ns/timeforchildren_ symbolic
link.
• It is an IPC namespace, and a corresponding mount of an _mqueue_
filesystem (see [mq_overview(7)](../man7/mq%5Foverview.7.html)) refers to this namespace.
• It is a PID namespace, and a corresponding mount of a [proc(5)](../man5/proc.5.html)
filesystem refers to this namespace.EXAMPLES top
See [clone(2)](../man2/clone.2.html) and [user_namespaces(7)](../man7/user%5Fnamespaces.7.html).SEE ALSO top
[nsenter(1)](../man1/nsenter.1.html), [readlink(1)](../man1/readlink.1.html), [unshare(1)](../man1/unshare.1.html), [clone(2)](../man2/clone.2.html), [ioctl_nsfs(2)](../man2/ioctl%5Fnsfs.2.html),
[setns(2)](../man2/setns.2.html), [unshare(2)](../man2/unshare.2.html), [proc(5)](../man5/proc.5.html), [capabilities(7)](../man7/capabilities.7.html),
[cgroup_namespaces(7)](../man7/cgroup%5Fnamespaces.7.html), [cgroups(7)](../man7/cgroups.7.html), [credentials(7)](../man7/credentials.7.html),
[ipc_namespaces(7)](../man7/ipc%5Fnamespaces.7.html), [network_namespaces(7)](../man7/network%5Fnamespaces.7.html), [pid_namespaces(7)](../man7/pid%5Fnamespaces.7.html),
[user_namespaces(7)](../man7/user%5Fnamespaces.7.html), [uts_namespaces(7)](../man7/uts%5Fnamespaces.7.html), [lsns(8)](../man8/lsns.8.html), [switch_root(8)](../man8/switch%5Froot.8.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-09-01 namespaces(7)
Pages that refer to this page:nsenter(1), ps(1), systemd-detect-virt(1), unshare(1), clone(2), getdomainname(2), gethostname(2), ioctl_nsfs(2), setns(2), unshare(2), cap_get_proc(3), lttng-ust(3), proc(5), proc_pid_ns(5), proc_sys_user(5), systemd.exec(5), cgroup_namespaces(7), cgroups(7), credentials(7), ipc_namespaces(7), mount_namespaces(7), mq_overview(7), network_namespaces(7), pid_namespaces(7), time_namespaces(7), user_namespaces(7), uts_namespaces(7), lsns(8), rdma-system(8)