.\" Copyright (c) 2013 by Michael Kerrisk .\" and Copyright (c) 2012 by Eric W. Biederman .\" .\" %%%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 .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date.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 .\" .\" .TH NAMESPACES 7 2018-02-02 "Linux" "Linux Programmer's Manual" .SH NAME namespaces \- overview of Linux namespaces .SH 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. .PP Linux provides the following namespaces: .TS lB lB lB l lB l. Namespace Constant Isolates Cgroup CLONE_NEWCGROUP Cgroup root directory IPC CLONE_NEWIPC System V IPC, POSIX message queues Network CLONE_NEWNET Network devices, stacks, ports, etc. Mount CLONE_NEWNS Mount points PID CLONE_NEWPID Process IDs User CLONE_NEWUSER User and group IDs UTS CLONE_NEWUTS Hostname and NIS domain name .TE .PP This page describes the various namespaces and the associated .I /proc files, and summarizes the APIs for working with namespaces. .\" .\" ==================== The namespaces API ==================== .\" .SS The namespaces API As well as various .I /proc files described below, the namespaces API includes the following system calls: .TP .BR clone (2) The .BR clone (2) system call creates a new process. If the .I flags argument of the call specifies one or more of the .B CLONE_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.) .TP .BR setns (2) The .BR 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 .IR /proc/[pid]/ns files described below. .TP .BR unshare (2) The .BR unshare (2) system call moves the calling process to a new namespace. If the .I flags argument of the call specifies one or more of the .B CLONE_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.) .PP Creation of new namespaces using .BR clone (2) and .BR unshare (2) in most cases requires the .BR CAP_SYS_ADMIN capability. User namespaces are the exception: since Linux 3.8, no privilege is required to create a user namespace. .\" .\" ==================== The /proc/[pid]/ns/ directory ==================== .\" .SS The /proc/[pid]/ns/ directory Each process has a .IR /proc/[pid]/ns/ .\" See commit 6b4e306aa3dc94a0545eb9279475b1ab6209a31f subdirectory containing one entry for each namespace that supports being manipulated by .BR setns (2): .PP .in +4n .EX $ \fBls \-l /proc/$$/ns\fP total 0 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 cgroup \-> cgroup:[4026531835] lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 ipc \-> ipc:[4026531839] lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 mnt \-> mnt:[4026531840] lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 net \-> net:[4026531969] lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 pid \-> pid:[4026531836] lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 pid_for_children \-> pid:[4026531834] lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 user \-> user:[4026531837] lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 uts \-> uts:[4026531838] .EE .in .PP Bind mounting (see .BR mount (2)) one of the files in this directory to somewhere else in the filesystem keeps the corresponding namespace of the process specified by .I pid alive even if all processes currently in the namespace terminate. .PP Opening one of the files in this directory (or a file that is bind mounted to one of these files) returns a file handle for the corresponding namespace of the process specified by .IR pid . As long as this file descriptor remains open, the namespace will remain alive, even if all processes in the namespace terminate. The file descriptor can be passed to .BR setns (2). .PP In Linux 3.7 and earlier, these files were visible as hard links. Since Linux 3.8, .\" commit bf056bfa80596a5d14b26b17276a56a0dcb080e5 they appear as symbolic links. If two processes are in the same namespace, then the device IDs and inode numbers of their .IR /proc/[pid]/ns/xxx symbolic links will be the same; an application can check this using the .I stat.st_dev and .I stat.st_ino fields returned by .BR stat (2). The content of this symbolic link is a string containing the namespace type and inode number as in the following example: .PP .in +4n .EX $ \fBreadlink /proc/$$/ns/uts\fP uts:[4026531838] .EE .in .PP The symbolic links in this subdirectory are as follows: .TP .IR /proc/[pid]/ns/cgroup " (since Linux 4.6)" This file is a handle for the cgroup namespace of the process. .TP .IR /proc/[pid]/ns/ipc " (since Linux 3.0)" This file is a handle for the IPC namespace of the process. .TP .IR /proc/[pid]/ns/mnt " (since Linux 3.8)" .\" commit 8823c079ba7136dc1948d6f6dcb5f8022bde438e This file is a handle for the mount namespace of the process. .TP .IR /proc/[pid]/ns/net " (since Linux 3.0)" This file is a handle for the network namespace of the process. .TP .IR /proc/[pid]/ns/pid " (since Linux 3.8)" .\" commit 57e8391d327609cbf12d843259c968b9e5c1838f 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). .TP .IR /proc/[pid]/ns/pid_for_children " (since Linux 4.12)" .\" commit eaa0d190bfe1ed891b814a52712dcd852554cb08 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 .BR unshare (2) and .BR setns (2) (see .BR pid_namespaces (7)), so the file may differ from .IR /proc/[pid]/ns/pid . The symbolic link gains a value only after the first child process is created in the namespace. (Beforehand, .BR readlink (2) of the symbolic link will return an empty buffer.) .TP .IR /proc/[pid]/ns/user " (since Linux 3.8)" .\" commit cde1975bc242f3e1072bde623ef378e547b73f91 This file is a handle for the user namespace of the process. .TP .IR /proc/[pid]/ns/uts " (since Linux 3.0)" This file is a handle for the UTS namespace of the process. .PP Permission to dereference or read .RB ( readlink (2)) these symbolic links is governed by a ptrace access mode .B PTRACE_MODE_READ_FSCREDS check; see .BR ptrace (2). .\" .\" ==================== The /proc/sys/user directory ==================== .\" .SS The /proc/sys/user directory The files in the .I /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: .TP .IR 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. .TP .IR 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. .TP .IR 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. .TP .IR 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. .TP .IR 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. .TP .IR 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. .TP .IR max_uts_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. .PP Note the following details about these files: .IP * 3 The values in these files are modifiable by privileged processes. .IP * The values exposed by these files are the limits for the user namespace in which the opening process resides. .IP * The limits are per-user. Each user in the same user namespace can create namespaces up to the defined limit. .IP * The limits apply to all users, including UID 0. .IP * These limits apply in addition to any other per-namespace limits (such as those for PID and user namespaces) that may be enforced. .IP * Upon encountering these limits, .BR clone (2) and .BR unshare (2) fail with the error .BR ENOSPC . .IP * 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 .RI ( /proc/sys/kernel/threads-max ). In all descendant user namespaces, the default value in each file is .BR MAXINT . .IP * When a namespace is created, the object is also accounted against ancestor namespaces. More precisely: .RS .IP + 3 Each user namespace has a creator UID. .IP + 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. .IP + 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. .RE .PP .\" .\" ==================== Cgroup namespaces ==================== .\" .SS Cgroup namespaces (CLONE_NEWCGROUP) See .BR cgroup_namespaces (7). .\" .\" ==================== IPC namespaces ==================== .\" .SS IPC namespaces (CLONE_NEWIPC) IPC namespaces isolate certain IPC resources, namely, System V IPC objects (see .BR svipc (7)) and (since Linux 2.6.30) .\" commit 7eafd7c74c3f2e67c27621b987b28397110d643f .\" https://lwn.net/Articles/312232/ POSIX message queues (see .BR mq_overview (7)). The common characteristic of these IPC mechanisms is that IPC objects are identified by mechanisms other than filesystem pathnames. .PP Each IPC namespace has its own set of System V IPC identifiers and its own POSIX message queue filesystem. Objects created in an IPC namespace are visible to all other processes that are members of that namespace, but are not visible to processes in other IPC namespaces. .PP The following .I /proc interfaces are distinct in each IPC namespace: .IP * 3 The POSIX message queue interfaces in .IR /proc/sys/fs/mqueue . .IP * The System V IPC interfaces in .IR /proc/sys/kernel , namely: .IR msgmax , .IR msgmnb , .IR msgmni , .IR sem , .IR shmall , .IR shmmax , .IR shmmni , and .IR shm_rmid_forced . .IP * The System V IPC interfaces in .IR /proc/sysvipc . .PP When an IPC namespace is destroyed (i.e., when the last process that is a member of the namespace terminates), all IPC objects in the namespace are automatically destroyed. .PP Use of IPC namespaces requires a kernel that is configured with the .B CONFIG_IPC_NS option. .\" .\" ==================== Network namespaces ==================== .\" .SS Network namespaces (CLONE_NEWNET) See .BR network_namespaces (7). .\" .\" ==================== Mount namespaces ==================== .\" .SS Mount namespaces (CLONE_NEWNS) See .BR mount_namespaces (7). .\" .\" ==================== PID namespaces ==================== .\" .SS PID namespaces (CLONE_NEWPID) See .BR pid_namespaces (7). .\" .\" ==================== User namespaces ==================== .\" .SS User namespaces (CLONE_NEWUSER) See .BR user_namespaces (7). .\" .\" ==================== UTS namespaces ==================== .\" .SS UTS namespaces (CLONE_NEWUTS) UTS namespaces provide isolation of two system identifiers: the hostname and the NIS domain name. These identifiers are set using .BR sethostname (2) and .BR setdomainname (2), and can be retrieved using .BR uname (2), .BR gethostname (2), and .BR getdomainname (2). .PP Use of UTS namespaces requires a kernel that is configured with the .B CONFIG_UTS_NS option. .SH EXAMPLE See .BR clone (2) and .BR user_namespaces (7). .SH SEE ALSO .BR nsenter (1), .BR readlink (1), .BR unshare (1), .BR clone (2), .BR ioctl_ns (2), .BR setns (2), .BR unshare (2), .BR proc (5), .BR capabilities (7), .BR cgroup_namespaces (7), .BR cgroups (7), .BR credentials (7), .BR network_namespaces (7), .BR pid_namespaces (7), .BR user_namespaces (7), .BR lsns (8), .BR switch_root (8) .SH COLOPHON This page is part of release 4.16 of the Linux .I 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/.