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  names-
       pace,  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 names-
              paces.  (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  excep-
       tion: since Linux 3.8, no privilege is required to create a user names-
       pace.

   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):
       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 corre-
       sponding 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 con-
       tent 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 characteristic of these IPC mecha-
       nisms 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  names-
       pace are visible to all other processes that are members of that names-
       pace, but are not visible to processes in other IPC namespaces.

       The following /proc interfaces are distinct in each IPC namespace:
       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 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.

   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 currently 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 having an error condition.

       The  /proc/[pid]/mountstats  file  (present since Linux 2.6.17) exports
       information (statistics, 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.  Cur-
                   rently (as at Linux 2.6.26), only  NFS  filesystems  export
                   information via this field.

       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),    credentials(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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