namespaces(7) — Linux manual page

NAME | DESCRIPTION | EXAMPLES | SEE ALSO | COLOPHON

NAMESPACES(7)           Linux Programmer's Manual          NAMESPACES(7)

NAME         top

       namespaces - overview of Linux namespaces

DESCRIPTION         top

       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)    T{User and group IDs
       T}
       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         top

       See clone(2) and user_namespaces(7).

SEE ALSO         top

       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         top

       This page is part of release 5.11 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                          2021-03-22                  NAMESPACES(7)

Pages that refer to this page: nsenter(1)procps(1)ps(1)systemd-detect-virt(1)unshare(1)clone(2)getdomainname(2)gethostname(2)ioctl_ns(2)setns(2)unshare(2)proc(5)systemd.exec(5)cgroup_namespaces(7)cgroups(7)credentials(7)ipc_namespaces(7)mount_namespaces(7)mq_overview(7)network_namespaces(7)pid_namespaces(7)time_namespaces(7)user_namespaces(7)uts_namespaces(7)lsns(8)rdma-system(8)