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NAMESPACES(7)                       Linux Programmer's 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 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
       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.

   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 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.)

       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 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 privi‐
       lege 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 Jan 14 01:20 ipc -> ipc:[4026531839]
           lrwxrwxrwx. 1 mtk mtk 0 Jan 14 01:20 mnt -> mnt:[4026531840]
           lrwxrwxrwx. 1 mtk mtk 0 Jan 14 01:20 net -> net:[4026531956]
           lrwxrwxrwx. 1 mtk mtk 0 Jan 14 01:20 pid -> pid:[4026531836]
           lrwxrwxrwx. 1 mtk mtk 0 Jan 14 01:20 user -> user:[4026531837]
           lrwxrwxrwx. 1 mtk mtk 0 Jan 14 01:20 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 num‐
       bers 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 files in this subdirectory are as follows:

       /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.

   IPC namespaces (CLONE_NEWIPC)
       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 character‐
       istic  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  names‐
       paces.

       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 (CLONE_NEWNET)
       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 termi‐
       nates), 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.

   Mount namespaces (CLONE_NEWNS)
       Mount namespaces isolate the set of filesystem mount points,  meaning  that  processes  in
       different  mount namespaces can have different views of the filesystem hierarchy.  The set
       of mounts in a mount namespace is modified using mount(2) and umount(2).

       The /proc/[pid]/mounts file (present since Linux 2.4.19) lists all  the  filesystems  cur‐
       rently mounted in the process's mount namespace.  The format of this file is documented in
       fstab(5).  Since kernel version 2.6.15, this file is pollable: after opening the file  for
       reading,  a  change in this file (i.e., a filesystem mount or unmount) causes select(2) to
       mark the file descriptor as readable, and poll(2) and epoll_wait(2) mark the file as  hav‐
       ing an error condition.

       The  /proc/[pid]/mountstats file (present since Linux 2.6.17) exports information (statis‐
       tics, configuration information) about the mount points in the process's mount  namespace.
       This file is readable only by the owner of the process.  Lines in this file have the form:

            device /dev/sda7 mounted on /home with fstype ext3 [statistics]
            (       1      )            ( 2 )             (3 ) (4)

              The fields in each line are:

              (1)  The  name  of  the  mounted device (or "nodevice" if there is no corresponding
                   device).

              (2)  The mount point within the filesystem tree.

              (3)  The filesystem type.

              (4)  Optional statistics and configuration information.   Currently  (as  at  Linux
                   2.6.26), only NFS filesystems export information via this field.

   PID namespaces (CLONE_NEWPID)
       See pid_namespaces(7).

   User namespaces (CLONE_NEWUSER)
       See user_namespaces(7).

   UTS namespaces (CLONE_NEWUTS)
       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.

CONFORMING TO
       Namespaces are a Linux-specific feature.

EXAMPLE
       See user_namespaces(7).

SEE ALSO
       nsenter(1),  readlink(1),  unshare(1),  clone(2),  setns(2),  unshare(2), proc(5), creden‐
       tials(7), capabilities(7), pid_namespaces(7), user_namespaces(7), switch_root(8)

COLOPHON
       This page is part of release 4.04 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 http://www.kernel.org/doc/man-pages/.

Linux                                       2014-09-21                              NAMESPACES(7)

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