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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.

       This  page  provides pointers to information on the various namespace types, describes the
       associated /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
       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/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
          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.

   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 factors 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_overview(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),
       network_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/.