SETNS(2)                  Linux Programmer's Manual                 SETNS(2)

NAME         top

       setns - reassociate thread with a namespace

SYNOPSIS         top

       #define _GNU_SOURCE             /* See feature_test_macros(7) */
       #include <sched.h>

       int setns(int fd, int nstype);

DESCRIPTION         top

       Given a file descriptor referring to a namespace, reassociate the
       calling thread with that namespace.

       The fd argument is a file descriptor referring to one of the
       namespace entries in a /proc/[pid]/ns/ directory; see namespaces(7)
       for further information on /proc/[pid]/ns/.  The calling thread will
       be reassociated with the corresponding namespace, subject to any
       constraints imposed by the nstype argument.

       The nstype argument specifies which type of namespace the calling
       thread may be reassociated with.  This argument can have one of the
       following values:

       0      Allow any type of namespace to be joined.

       CLONE_NEWCGROUP (since Linux 4.6)
              fd must refer to a cgroup namespace.

       CLONE_NEWIPC (since Linux 3.0)
              fd must refer to an IPC namespace.

       CLONE_NEWNET (since Linux 3.0)
              fd must refer to a network namespace.

       CLONE_NEWNS (since Linux 3.8)
              fd must refer to a mount namespace.

       CLONE_NEWPID (since Linux 3.8)
              fd must refer to a descendant PID namespace.

       CLONE_NEWUSER (since Linux 3.8)
              fd must refer to a user namespace.

       CLONE_NEWUTS (since Linux 3.0)
              fd must refer to a UTS namespace.

       Specifying nstype as 0 suffices if the caller knows (or does not
       care) what type of namespace is referred to by fd.  Specifying a
       nonzero value for nstype is useful if the caller does not know what
       type of namespace is referred to by fd and wants to ensure that the
       namespace is of a particular type.  (The caller might not know the
       type of the namespace referred to by fd if the file descriptor was
       opened by another process and, for example, passed to the caller via
       a UNIX domain socket.)

       If fd refers to a PID namespaces, the semantics are somewhat
       different from other namespace types: reassociating the calling
       thread with a PID namespace changes only the PID namespace that
       subsequently created child processes of the caller will be placed in;
       it does not change the PID namespace of the caller itself.
       Reassociating with a PID namespace is allowed only if the PID
       namespace specified by fd is a descendant (child, grandchild, etc.)
       of the PID namespace of the caller.  For further details on PID
       namespaces, see pid_namespaces(7).

       A process reassociating itself with a user namespace must have the
       CAP_SYS_ADMIN capability in the target user namespace.  Upon
       successfully joining a user namespace, a process is granted all
       capabilities in that namespace, regardless of its user and group IDs.
       A multithreaded process may not change user namespace with setns().
       It is not permitted to use setns() to reenter the caller's current
       user namespace.  This prevents a caller that has dropped capabilities
       from regaining those capabilities via a call to setns().  For
       security reasons, a process can't join a new user namespace if it is
       sharing filesystem-related attributes (the attributes whose sharing
       is controlled by the clone(2) CLONE_FS flag) with another process.
       For further details on user namespaces, see user_namespaces(7).

       A process may not be reassociated with a new mount namespace if it is
       multithreaded.  Changing the mount namespace requires that the caller
       possess both CAP_SYS_CHROOT and CAP_SYS_ADMIN capabilities in its own
       user namespace and CAP_SYS_ADMIN in the target mount namespace.  See
       user_namespaces(7) for details on the interaction of user namespaces
       and mount namespaces.

       Using setns() to change the caller's cgroup namespace does not change
       the caller's cgroup memberships.

RETURN VALUE         top

       On success, setns() returns 0.  On failure, -1 is returned and errno
       is set to indicate the error.

ERRORS         top

       EBADF  fd is not a valid file descriptor.

       EINVAL fd refers to a namespace whose type does not match that
              specified in nstype.

       EINVAL There is problem with reassociating the thread with the
              specified namespace.

       EINVAL The caller tried to join an ancestor (parent, grandparent, and
              so on) PID namespace.

       EINVAL The caller attempted to join the user namespace in which it is
              already a member.

       EINVAL The caller shares filesystem (CLONE_FS) state (in particular,
              the root directory) with other processes and tried to join a
              new user namespace.

       EINVAL The caller is multithreaded and tried to join a new user

       ENOMEM Cannot allocate sufficient memory to change the specified

       EPERM  The calling thread did not have the required capability for
              this operation.

VERSIONS         top

       The setns() system call first appeared in Linux in kernel 3.0;
       library support was added to glibc in version 2.14.

CONFORMING TO         top

       The setns() system call is Linux-specific.

NOTES         top

       Not all of the attributes that can be shared when a new thread is
       created using clone(2) can be changed using setns().

EXAMPLE         top

       The program below takes two or more arguments.  The first argument
       specifies the pathname of a namespace file in an existing
       /proc/[pid]/ns/ directory.  The remaining arguments specify a command
       and its arguments.  The program opens the namespace file, joins that
       namespace using setns(), and executes the specified command inside
       that namespace.

       The following shell session demonstrates the use of this program
       (compiled as a binary named ns_exec) in conjunction with the
       CLONE_NEWUTS example program in the clone(2) man page (complied as a
       binary named newuts).

       We begin by executing the example program in clone(2) in the
       background.  That program creates a child in a separate UTS
       namespace.  The child changes the hostname in its namespace, and then
       both processes display the hostnames in their UTS namespaces, so that
       we can see that they are different.

           $ su                   # Need privilege for namespace operations
           # ./newuts bizarro &
           [1] 3549
           clone() returned 3550
           uts.nodename in child:  bizarro
           uts.nodename in parent: antero
           # uname -n             # Verify hostname in the shell

       We then run the program shown below, using it to execute a shell.
       Inside that shell, we verify that the hostname is the one set by the
       child created by the first program:

           # ./ns_exec /proc/3550/ns/uts /bin/bash
           # uname -n             # Executed in shell started by ns_exec

   Program source
       #define _GNU_SOURCE
       #include <fcntl.h>
       #include <sched.h>
       #include <unistd.h>
       #include <stdlib.h>
       #include <stdio.h>

       #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                               } while (0)

       main(int argc, char *argv[])
           int fd;

           if (argc < 3) {
               fprintf(stderr, "%s /proc/PID/ns/FILE cmd args...\n", argv[0]);

           fd = open(argv[1], O_RDONLY);  /* Get file descriptor for namespace */
           if (fd == -1)

           if (setns(fd, 0) == -1)        /* Join that namespace */

           execvp(argv[2], &argv[2]);     /* Execute a command in namespace */

SEE ALSO         top

       clone(2), fork(2), unshare(2), vfork(2), namespaces(7), unix(7)

COLOPHON         top

       This page is part of release 4.12 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

Linux                            2016-03-15                         SETNS(2)

Pages that refer to this page: nsenter(1)clone(2)syscalls(2)unshare(2)proc(5)systemd.exec(5)capabilities(7)cgroup_namespaces(7)mount_namespaces(7)namespaces(7)pid_namespaces(7)user_namespaces(7)ip-netns(8)