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.

   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.)

   Creation  of new namespaces using clone(2) and unshare(2) in most cases
   requires  the  CAP_SYS_ADMIN  capability.   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 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   inode   numbers   of  their
   /proc/[pid]/ns/xxx symbolic links will be the same; an application  can
   check  this  using  the  stat.st_ino  field  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.

   /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).

   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
   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)
   Network namespaces provide isolation of the system resources associated
   with  networking:  network  devices,  IPv4 and IPv6 protocol stacks, IP
   routing tables, firewalls, the /proc/net directory, the  /sys/class/net
   directory,  port  numbers  (sockets),  and  so  on.  A physical network
   device can live in exactly one network namespace.   A  virtual  network
   device  ("veth") pair provides a pipe-like abstraction that can be used
   to create tunnels between network namespaces, and can be used to create
   a bridge to a physical network device in another namespace.

   When  a  network namespace is freed (i.e., when the last process in the
   namespace terminates), its physical network devices are moved  back  to
   the initial network namespace (not to the parent of the process).

   Use of network namespaces requires a kernel that is configured with the
   CONFIG_NET_NS option.

   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).

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

   Introspecting namespace relationships
   Since  Linux  4.9,  two  ioctl(2)  operations  are  provided  to  allow
   introspection  of  namespace  relationships (see user_namespaces(7) and
   pid_namespaces(7)).  The form of the calls is:

       new_fd = ioctl(fd, request);

   In each case, fd refers to a /proc/[pid]/ns/*  file.   Both  operations
   return a new file descriptor on success.

   NS_GET_USERNS
          Returns  a  file  descriptor  that  refers  to  the  owning user
          namespace for the namespace referred to by fd.

   NS_GET_PARENT
          Returns a file descriptor that refers to the parent namespace of
          the  namespace  referred to by fd.  This operation is valid only
          for hierarchical namespaces (i.e.,  PID  and  user  namespaces).
          For   user   namespaces,   NS_GET_PARENT   is   synonymous  with
          NS_GET_USERNS.

   The new file descriptor returned by these operations is opened with the
   O_RDONLY and O_CLOEXEC (close-on-exec; see fcntl(2))flags.

   By  applying  fstat(2)  to  the returned file descriptor, one obtains a
   stat structure whose st_dev (resident device) and st_ino (inode number)
   fields  together  identify  the  owning/parent  namespace.   This inode
   number  can   be   matched   with   the   inode   number   of   another
   /proc/[pid]/ns/{pid,user}   file  to  determine  whether  that  is  the
   owning/parent namespace.

   Either of these ioctl(2) operations can fail with the following errors:

   EPERM  The requested namespace is outside  of  the  caller's  namespace
          scope.   This  error  can occur if, for example, the owning user
          namespace is an ancestor of the caller's current user namespace.
          It  can  also  occur  on  attempts  to  obtain the parent of the
          initial user or PID namespace.

   ENOTTY The operation is not supported by this kernel version.

   Additionally, the NS_GET_PARENT operation can fail with  the  following
   error:

   EINVAL fd refers to a nonhierarchical namespace.

   See the EXAMPLE section for an example of the use of these operations.

CONFORMING TO

   Namespaces are a Linux-specific feature.

EXAMPLE

   For one example, user_namespaces(7).

   The example shown below uses the ioctl(2) operations described above to
   perform simple introspection of namespace relationships.  The following
   shell sessions show various examples of the use of this program.

   Trying  to  get the parent of the initial user namespace fails, for the
   reasons explained earlier:

       $ ./ns_introspect /proc/self/ns/user p
       The parent namespace is outside your namespace scope

   Create a process running sleep(1) that resides  in  new  user  and  UTS
   namespaces,  and show that new UTS namespace is associated with the new
   user namespace:

       $ unshare -Uu sleep 1000 &
       [1] 23235
       $ ./ns_introspect /proc/23235/ns/uts
       Device/Inode of owning user namespace is: [0,3] / 4026532448
       $ readlink /proc/23235/ns/user
       user:[4026532448]

   Then show that the parent of the new user namespace  in  the  preceding
   example is the initial user namespace:

       $ readlink /proc/self/ns/user
       user:[4026531837]
       $ ./ns_introspect /proc/23235/ns/user
       Device/Inode of owning user namespace is: [0,3] / 4026531837

   Start  a  shell in a new user namespace, and show that from within this
   shell, the parent user namespace can't be discovered.   Similarly,  the
   UTS  namespace  (which  is  associated with the initial user namespace)
   can't be discovered.

       $ PS1="sh2$ " unshare -U bash
       sh2$ ./ns_introspect /proc/self/ns/user p
       The parent namespace is outside your namespace scope
       sh2$ ./ns_introspect /proc/self/ns/uts u
       The owning user namespace is outside your namespace scope

   Program source

   /* ns_introspect.c

      Licensed under the GNU General Public License v2 or later.
   */
   #include <stdlib.h>
   #include <unistd.h>
   #include <stdio.h>
   #include <fcntl.h>
   #include <string.h>
   #include <sys/stat.h>
   #include <sys/ioctl.h>
   #include <errno.h>
   #include <sys/sysmacros.h>

   #ifndef NS_GET_USERNS
   #define NSIO    0xb7
   #define NS_GET_USERNS   _IO(NSIO, 0x1)
   #define NS_GET_PARENT   _IO(NSIO, 0x2)
   #endif

   int
   main(int argc, char *argv[])
   {
       int fd, userns_fd, parent_fd;
       struct stat sb;

       if (argc < 2) {
           fprintf(stderr, "Usage: %s /proc/[pid]/ns/[file] [p|u]\n",
                   argv[0]);
           fprintf(stderr, "\nDisplay the result of one or both "
                   "of NS_GET_USERNS (u) or NS_GET_PARENT (p)\n"
                   "for the specified /proc/[pid]/ns/[file]. If neither "
                   "'p' nor 'u' is specified,\n"
                   "NS_GET_USERNS is the default.\n");
           exit(EXIT_FAILURE);
       }

       /* Obtain a file descriptor for the 'ns' file specified
          in argv[1] */

       fd = open(argv[1], O_RDONLY);
       if (fd == -1) {
           perror("open");
           exit(EXIT_FAILURE);
       }

       /* Obtain a file descriptor for the owning user namespace and
          then obtain and display the inode number of that namespace */

       if (argc < 3 || strchr(argv[2], 'u')) {
           userns_fd = ioctl(fd, NS_GET_USERNS);

           if (userns_fd == -1) {
               if (errno == EPERM)
                   printf("The owning user namespace is outside "
                           "your namespace scope\n");
               else
                  perror("ioctl-NS_GET_USERNS");
               exit(EXIT_FAILURE);
            }

           if (fstat(userns_fd, &sb) == -1) {
               perror("fstat-userns");
               exit(EXIT_FAILURE);
           }
           printf("Device/Inode of owning user namespace is: "
                   "[%lx,%lx] / %ld\n",
                   (long) major(sb.st_dev), (long) minor(sb.st_dev),
                   (long) sb.st_ino);

           close(userns_fd);
       }

       /* Obtain a file descriptor for the parent namespace and
          then obtain and display the inode number of that namespace */

       if (argc > 2 && strchr(argv[2], 'p')) {
           parent_fd = ioctl(fd, NS_GET_PARENT);

           if (parent_fd == -1) {
               if (errno == EINVAL)
                   printf("Can' get parent namespace of a "
                           "nonhierarchical namespace\n");
               else if (errno == EPERM)
                   printf("The parent namespace is outside "
                           "your namespace scope\n");
               else
                   perror("ioctl-NS_GET_PARENT");
               exit(EXIT_FAILURE);
           }

           if (fstat(parent_fd, &sb) == -1) {
               perror("fstat-parentns");
               exit(EXIT_FAILURE);
           }
           printf("Device/Inode of parent namespace is: [%lx,%lx] / %ld\n",
                   (long) major(sb.st_dev), (long) minor(sb.st_dev),
                   (long) sb.st_ino);

           close(parent_fd);
       }

       exit(EXIT_SUCCESS);
   }

SEE ALSO

   nsenter(1), readlink(1), unshare(1),  clone(2),  setns(2),  unshare(2),
   proc(5),     capabilities(7),     cgroup_namespaces(7),     cgroups(7),
   credentials(7),   pid_namespaces(7),    user_namespaces(7),    lsns(8),
   switch_root(8)

COLOPHON

   This  page  is  part of release 4.09 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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