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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
       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.

   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 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]

       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 device IDs
       and  inode  numbers  of  their  /proc/[pid]/ns/xxx  symbolic  links  will  be the same; an
       application can check this using  the  stat.st_dev  and  stat.st_ino  fields  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.   This  handle  is
              permanent  for  the  lifetime  of  the  process  (i.e.,  a  process's PID namespace
              membership never changes).

       /proc/[pid]/ns/pid_for_children (since Linux 4.12)
              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 unshare(2) and setns(2) (see
              pid_namespaces(7)), so the file may differ from /proc/[pid]/ns/pid.   The  symbolic
              link  gains a value only after the first child process is created in the namespace.
              (Beforehand, readlink(2) of the symbolic link will return an empty buffer.)

       /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).

   The /proc/sys/user directory
       The files in the /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:

       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.

       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.

       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.

       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.

       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.

       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.

       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.

       Note the following details about these files:

       *  The values in these files are modifiable by privileged processes.

       *  The values exposed by these files are the limits for the user namespace  in  which  the
          opening process resides.

       *  The limits are per-user.  Each user in the same user namespace can create namespaces up
          to the defined limit.

       *  The limits apply to all users, including UID 0.

       *  These limits apply in addition to any other per-namespace limits (such as those for PID
          and user namespaces) that may be enforced.

       *  Upon encountering these limits, clone(2) and unshare(2) fail with the error ENOSPC.

       *  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 (/proc/sys/kernel/threads-max).   In
          all descendant user namespaces, the default value in each file is MAXINT.

       *  When  a namespace is created, the object is also accounted against ancestor namespaces.
          More precisely:

          +  Each user namespace has a creator UID.

          +  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.

          +  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.

   Cgroup namespaces (CLONE_NEWCGROUP)
       See cgroup_namespaces(7).

   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)
       See network_namespaces(7).

   Mount namespaces (CLONE_NEWNS)
       See mount_namespaces(7).

   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.

EXAMPLE

       See clone(2) and user_namespaces(7).

SEE ALSO

       nsenter(1), readlink(1), unshare(1), clone(2), ioctl_ns(2), setns(2), unshare(2), proc(5),
       capabilities(7), cgroup_namespaces(7), cgroups(7), credentials(7),  network_namespaces(7),
       pid_namespaces(7), user_namespaces(7), lsns(8), switch_root(8)

COLOPHON

       This  page  is  part of release 4.16 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/.