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man: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  names-
     pace,  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 names-
            paces.  (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.)
     Creation  of new namespaces using clone(2) and unshare(2) in most cases
     requires the CAP_SYS_ADMIN capability.  User namespaces are the  excep-
     tion: since Linux 3.8, no privilege is required to create a user names-
     pace.
 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/$$/ns total 0 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46
         cgroup -> cgroup:[4026531835] lrwxrwxrwx. 1 mtk mtk 0 Apr 28  12:46
         ipc -> ipc:[4026531839] lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 mnt ->
         mnt:[4026531840] lrwxrwxrwx. 1 mtk  mtk  0  Apr  28  12:46  net  ->
         net:[4026531969]  lrwxrwxrwx.  1  mtk  mtk  0  Apr  28 12:46 pid ->
         pid:[4026531836] lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 pid_for_chil-
         dren  -> pid:[4026531834] lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 user
         -> user:[4026531837] lrwxrwxrwx. 1 mtk mtk 0 Apr 28  12:46  uts  ->
         uts:[4026531838]
     Bind  mounting  (see  mount(2))  one  of the files in this directory to
     somewhere else in the filesystem keeps the corresponding  namespace  of
     the  process  specified by pid alive even if all processes currently in
     the namespace terminate.
     Opening one of the files in this directory (or  a  file  that  is  bind
     mounted  to  one  of  these files) returns a file handle for the corre-
     sponding namespace of the process specified by pid.  As  long  as  this
     file  descriptor remains open, the namespace will remain alive, even if
     all processes in the namespace terminate.  The file descriptor  can  be
     passed to setns(2).
     In  Linux  3.7  and  earlier,  these  files were visible as hard links.
     Since Linux 3.8, they appear as symbolic links.  If two  processes  are
     in  the  same namespace, then the device IDs and inode numbers of their
     /proc/[pid]/ns/xxx symbolic links will be the same; an application  can
     check  this  using  the  stat.st_dev and stat.st_ino fields returned by
     stat(2).  The content of this symbolic link is a string containing  the
     namespace type and inode number as in the following example:
         $ readlink /proc/$$/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
            user 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 names-

pace 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  names-
           pace  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  svipc(7))  and (since Linux 2.6.30) POSIX message queues
     (see mq_overview(7)).  The common characteristic of  these  IPC  mecha-
     nisms  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 names-
     pace are visible to all other processes that are members of that names-
     pace, 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 host-
     name and the NIS domain name.  These identifiers are set using sethost-
     name(2)  and  setdomainname(2),  and  can  be retrieved using uname(2),
     gethostname(2), and getdomainname(2).
     Use of UTS namespaces requires a kernel that  is  configured  with  the
     CONFIG_UTS_NS option.

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_names-
     paces(7), lsns(8), switch_root(8)

COLOPHON

     This page is part of release 4.16 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 2018-02-02 NAMESPACES(7)

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