namespaces — overview of Linux namespaces
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.
Linux provides the following namespaces:
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 |
This page describes the various namespaces and the
associated /proc files, and
summarizes the APIs for working with namespaces.
As well as various /proc
files described below, the namespaces API includes the
following system calls:
The clone(2) system
call creates a new process. If the flags argument of the
call specifies one or more of the 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.)
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.
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 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.)
Creation of new namespaces using clone(2) and unshare(2) in most cases
requires the CAP_SYS_ADMIN
capability. User namespaces are the exception: since Linux
3.8, no privilege is required to create a user
namespace.
Each process has a /proc/[pid]/ns/ subdirectory containing
one entry for each namespace that supports being
manipulated by setns(2):
$ ls -l /proc/$$/ns total 0 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 cgroup -> cgroup:[4026531835] lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 ipc -> ipc:[4026531839] lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 mnt -> mnt:[4026531840] lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 net -> net:[4026531969] lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 pid -> pid:[4026531836] lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 user -> user:[4026531837] lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 uts -> uts:[4026531838]
Bind mounting (see mount(2)) one of the
files in this directory to somewhere else in the filesystem
keeps the corresponding namespace of the process specified
by pid alive even
if all processes currently in the namespace terminate.
Opening one of the files in this directory (or a file
that is bind mounted to one of these files) returns a file
handle for the corresponding namespace of the process
specified by pid.
As long as this file descriptor remains open, the namespace
will remain alive, even if all processes in the namespace
terminate. The file descriptor can be passed to setns(2).
In Linux 3.7 and earlier, these files were visible as
hard links. Since Linux 3.8, they appear as symbolic links.
If two processes are in the same namespace, then the inode
numbers of their /proc/[pid]/ns/xxx symbolic links will be
the same; an application can check this using the
stat.st_ino field
returned by stat(2). The content of
this symbolic link is a string containing the namespace
type and inode number as in the following example:
$ readlink /proc/$$/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.
/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).
IPC namespaces isolate certain IPC resources, namely, System V IPC objects (see svipc(7)) and (since Linux 2.6.30) POSIX message queues (see mq_overview(7)). The common characteristic of these IPC mechanisms is that IPC objects are identified by mechanisms other than filesystem pathnames.
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.
The following /proc
interfaces are distinct in each IPC namespace:
The POSIX message queue interfaces in /proc/sys/fs/mqueue.
The System V IPC interfaces in /proc/sys/kernel, namely:
msgmax,
msgmnb,
msgmni,
sem,
shmall,
shmmax,
shmmni, and
shm_rmid_forced.
The System V IPC interfaces in /proc/sysvipc.
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.
Use of IPC namespaces requires a kernel that is
configured with the CONFIG_IPC_NS option.
Network namespaces provide isolation of the system
resources associated with networking: network devices, IPv4
and IPv6 protocol stacks, IP routing tables, firewalls, the
/proc/net directory, the
/sys/class/net directory,
port numbers (sockets), and so on. A physical network
device can live in exactly one network namespace. A virtual
network device ("veth") pair provides a pipe-like
abstraction that can be used to create tunnels between
network namespaces, and can be used to create a bridge to a
physical network device in another namespace.
When a network namespace is freed (i.e., when the last process in the namespace terminates), its physical network devices are moved back to the initial network namespace (not to the parent of the process).
Use of network namespaces requires a kernel that is
configured with the CONFIG_NET_NS option.
UTS namespaces provide isolation of two system identifiers: the hostname and the NIS domain name. These identifiers are set using sethostname(2) and setdomainname(2), and can be retrieved using uname(2), gethostname(2), and getdomainname(2).
Use of UTS namespaces requires a kernel that is
configured with the CONFIG_UTS_NS option.
lsns(1), nsenter(1), readlink(1), unshare(1), clone(2), setns(2), unshare(2), proc(5), capabilities(7), cgroup_namespaces(7), cgroups(7), credentials(7), pid_namespaces(7), user_namespaces(7), switch_root(8)
This page is part of release 4.07 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/.
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