namespaces

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



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
       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
           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.

   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), 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 5.04 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                             2019-08-02                     NAMESPACES(7)