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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 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 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   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 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 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 (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 only readable 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),    credentials(7),     capabilities(7),     pid_namespaces(7),
       user_namespaces(7), switch_root(8)

COLOPHON

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       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
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