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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 above, 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
              above, 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),
       switch_root(8)