NAME | DESCRIPTION | CONFORMING TO | EXAMPLE | 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.

       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         top

       Namespaces are a Linux-specific feature.

EXAMPLE         top

       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         top

       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         top

       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/.

Linux                            2016-12-12                    NAMESPACES(7)