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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.
          lB lB lB lB l1 lB1 l1 l.  Namespace Flag Page Isolates
          Cgroup    CLONE_NEWCGROUP     cgroup_namespaces(7)     Cgroup
          root directory IPC  CLONE_NEWIPC   ipc_namespaces(7)   T{
          System V IPC,
          POSIX message queues T}
          Network   CLONE_NEWNET   network_namespaces(7)    T{ Network
          devices,
          stacks, ports, etc.  T}
          Mount     CLONE_NEWNS    mount_namespaces(7) Mount points
          PID  CLONE_NEWPID   pid_namespaces(7)   Process IDs
          Time CLONE_NEWTIME  time_namespaces(7)  T{ Boot and mono-
          tonic
          clocks T} User CLONE_NEWUSER  user_namespaces(7)  User and
          group IDs UTS  CLONE_NEWUTS   uts_namespaces(7)   T{ Host-
          name and NIS
          domain name T}

        The namespaces API
          As well as various /proc files described below, the names-
          paces 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

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               implements a number of features unrelated to names-
               paces.)

          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 aq{print $1, $9, $10, $11}aq
              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 direc-
          tory to somewhere else in the filesystem keeps the corre-
          sponding namespace of the process specified by pid alive

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          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:

               This file is a handle for the cgroup namespace of the
               process.

               This file is a handle for the IPC namespace of the pro-
               cess.

               This file is a handle for the mount namespace of the
               process.

               This file is a handle for the network namespace of the
               process.

               This file is a handle for the PID namespace of the pro-
               cess.  This handle is permanent for the lifetime of the
               process (i.e., a process's PID namespace membership
               never changes).

               htmlmanrefend/proc/[pid]/ns/pid_for_children(sinceLinux

                 htmlmanrefstart/proc/[pid]/ns/pid.htmlmanrefend/proc/[pid]/ns/pid.

          htmlmanrefstart/proc/[pid]/ns/time(sinceLinux5.6)

          htmlmanrefstart/proc/[pid]/ns/time_for_children(sinceLinux5.6)    .}f 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 dif-
               fer from /proc/[pid]/ns/time.

               This file is a handle for the user namespace of the
               process.

               This file is a handle for the UTS namespace of the pro-
               cess.

          Permission to dereference or read (readlink(2)) these sym-
          bolic 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 fol-
          lows:

               The value in this file defines a per-user limit on the
               number of cgroup namespaces that may be created in the
               user namespace.

               The value in this file defines a per-user limit on the
               number of ipc namespaces that may be created in the
               user namespace.

               The value in this file defines a per-user limit on the
               number of mount namespaces that may be created in the
               user namespace.

               The value in this file defines a per-user limit on the
               number of network namespaces that may be created in the
               user namespace.

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               The value in this file defines a per-user limit on the
               number of PID namespaces that may be created in the
               user namespace.

               The value in this file defines a per-user limit on the
               number of time namespaces that may be created in the
               user namespace.

               The value in this file defines a per-user limit on the
               number of user namespaces that may be created in the
               user namespace.

               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

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                namespaces, and the kernel ensures that the corre-
                sponding 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 fol-
          lowing:

          *  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 names-
             pace), 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

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          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/.

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