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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 pro‐
       cesses within the namespace that they have their own isolated  instance  of  the  global  re‐
       source.   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.

       This  page provides pointers to information on the various namespace types, describes the as‐
       sociated /proc files, and summarizes the APIs for working with namespaces.

   Namespace types
       The following table shows the namespace types available on Linux.  The second column  of  the
       table  shows  the flag value that is used to specify the namespace type in various APIs.  The
       third column identifies the manual page that provides details on  the  namespace  type.   The
       last column is a summary of the resources that are isolated by the namespace type.

       Namespace Flag            Page                  Isolates
       Cgroup    CLONE_NEWCGROUP cgroup_namespaces(7)  Cgroup root directory
       IPC       CLONE_NEWIPC    ipc_namespaces(7)     System V IPC,
                                                       POSIX message queues
       Network   CLONE_NEWNET    network_namespaces(7) Network devices,
                                                       stacks, ports, etc.
       Mount     CLONE_NEWNS     mount_namespaces(7)   Mount points
       PID       CLONE_NEWPID    pid_namespaces(7)     Process IDs
       Time      CLONE_NEWTIME   time_namespaces(7)    Boot and monotonic
                                                       clocks
       User      CLONE_NEWUSER   user_namespaces(7)    User and group IDs
       UTS       CLONE_NEWUTS    uts_namespaces(7)     Hostname and NIS
                                                       domain name

   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.)

       ioctl(2)
              Various  ioctl(2)  operations  can  be  used to discover information about namespaces.
              These operations are described in ioctl_ns(2).

       Creation of new  namespaces  using  clone(2)  and  unshare(2)  in  most  cases  requires  the
       CAP_SYS_ADMIN  capability,  since,  in  the new namespace, the creator will have the power to
       change global resources that are visible to other processes that are subsequently created in,
       or  join  the namespace.  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 | awk '{print $1, $9, $10, $11}'
           total 0
           lrwxrwxrwx. cgroup -> cgroup:[4026531835]
           lrwxrwxrwx. ipc -> ipc:[4026531839]
           lrwxrwxrwx. mnt -> mnt:[4026531840]
           lrwxrwxrwx. net -> net:[4026531969]
           lrwxrwxrwx. pid -> pid:[4026531836]
           lrwxrwxrwx. pid_for_children -> pid:[4026531834]
           lrwxrwxrwx. time -> time:[4026531834]
           lrwxrwxrwx. time_for_children -> time:[4026531834]
           lrwxrwxrwx. user -> user:[4026531837]
           lrwxrwxrwx. 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  ap‐
       pear  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  fol‐
       lowing 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.   (Be‐
              forehand, readlink(2) of the symbolic link will return an empty buffer.)

       /proc/[pid]/ns/time (since Linux 5.6)
              This file is a handle for the time namespace of the process.

       /proc/[pid]/ns/time_for_children (since Linux 5.6)
              This  file  is  a  handle  for  the  time namespace of child processes created by this
              process.  This can change as a consequence of calls to unshare(2)  and  setns(2)  (see
              time_namespaces(7)), so the file may differ from /proc/[pid]/ns/time.

       /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_time_namespaces (since Linux 5.7)
              The value in this file defines a per-user limit on the number of time 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 uts  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 open‐
          ing 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 de‐
          scendant 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.

   Namespace lifetime
       Absent any other factors, a namespace is automatically torn down when the last process in the
       namespace  terminates  or leaves the namespace.  However, there are a number of other factors
       that may pin a namespace into existence even though it has no member processes.   These  fac‐
       tors include the following:

       *  An  open  file  descriptor  or  a bind mount exists for the corresponding /proc/[pid]/ns/*
          file.

       *  The namespace is hierarchical (i.e., a PID or user namespace), and has a child namespace.

       *  It is a user namespace that owns one or more nonuser namespaces.

       *  It is a PID namespace, and there  is  a  process  that  refers  to  the  namespace  via  a
          /proc/[pid]/ns/pid_for_children symbolic link.

       *  It  is  a  time  namespace,  and  there  is  a  process that refers to the namespace via a
          /proc/[pid]/ns/time_for_children symbolic link.

       *  It is an IPC namespace, and a corresponding mount of an mqueue  filesystem  (see  mq_over‐‐
          view(7)) refers to this namespace.

       *  It  is  a  PID namespace, and a corresponding mount of a proc(5) filesystem refers to this
          namespace.

EXAMPLES
       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), ipc_namespaces(7), net‐‐
       work_namespaces(7),  pid_namespaces(7),   user_namespaces(7),   uts_namespaces(7),   lsns(8),
       pam_namespace(8), switch_root(8)

COLOPHON
       This  page  is  part  of  release  5.10 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/.



Linux                                        2020-11-01                                NAMESPACES(7)