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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.)

       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
           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  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.

   Cgroup namespaces (CLONE_NEWCGROUP)
       See cgroup_namespaces(7).

   IPC namespaces (CLONE_NEWIPC)
       IPC  namespaces  isolate  certain  IPC  resources,  namely,  System  V  IPC  objects  (see
       sysvipc(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).

       When a process creates  a  new  UTS  namespace  using  clone(2)  or  unshare(2)  with  the
       CLONE_NEWUTS  flag,  the  hostname and domain of the new UTS namespace are copied from the
       corresponding values in the caller's UTS namespace.

       Use of UTS namespaces requires a kernel that is configured with the CONFIG_UTS_NS option.

   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 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.

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), pam_namespace(8), switch_root(8)

COLOPHON

       This page is part of release 5.02 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/.