Section (7) namespaces
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 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 |
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 includes the
following system calls:
- clone(2)
-
The clone(2) system call creates a new process. If the
flags
argument of the call specifies one or more of theCLONE_NEW*
flags listed below, 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 described 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 theCLONE_NEW*
flags listed below, 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)
-
Various ioctl(2) operations can be used to discover information about namespaces. These operations are described in ioctl_ns(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 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/$$/ns total 0 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 cgroup −> cgroup:[4026531835] lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 ipc −> ipc:[4026531839] lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 mnt −> mnt:[4026531840] lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 net −> net:[4026531969] lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 pid −> pid:[4026531836] lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 pid_for_children −> pid:[4026531834] lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 user −> user:[4026531837] lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 uts −> uts:[4026531838]
Bind mounting (see mount(2)) one of the
files in this directory to somewhere else in the filesystem
keeps the corresponding namespace of the process specified
by pid
alive even
if all processes currently in the namespace terminate.
Opening one of the files in this directory (or a file
that is bind mounted to one of these files) returns a file
handle for the corresponding namespace of the process
specified by pid
.
As long as this file descriptor remains open, the namespace
will remain alive, even if all processes in the namespace
terminate. The file descriptor can be passed to setns(2).
In Linux 3.7 and earlier, these files were visible as
hard links. Since Linux 3.8, they appear as symbolic links.
If two processes are in the same namespace, then the device
IDs and inode numbers of their /proc/[pid]/ns/xxx
symbolic links will be
the same; an application can check this using the
stat.st_dev
and
stat.st_ino
fields returned by stat(2). The content of
this symbolic link is a string containing the namespace
type and inode number as in the following example:
$ readlink /proc/$$/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_zsingle_quotesz_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 namespace. (Beforehand, readlink(2) of the symbolic link will return an empty buffer.) /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_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 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) 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 isMAXINT
. -
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/pid_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.
SEE ALSO
nsenter(1), readlink(1), unshare(1), clone(2), ioctl_ns(2), setns(2), unshare(2), proc(5), capabilities(7), cgroup_namespaces(7), cgroups(7), credentials(7), ipc_namespaces(7), network_namespaces(7), pid_namespaces(7), user_namespaces(7), uts_namespaces(7), lsns(8), pam_namespace(8), switch_root(8)
COLOPHON
This page is part of release 5.04 of the Linux man-pages
project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.
Copyright (c) 2013, 2016, 2017 by Michael Kerrisk <mtk.manpagesgmail.com> and Copyright (c) 2012 by Eric W. Biederman <ebiedermxmission.com> %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END |