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

       setns - reassociate thread with a namespace

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

       int setns(int fd, int nstype);

       Given a file descriptor referring to a namespace, reassociate the call-
       ing 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  reassoci-
       ated  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_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.)

       CLONE_NEWPID behaves somewhat differently from the other nstype values:
       reassociating the calling thread with a PID namespace changes only  the
       PID namespace that child processes of the caller will be created 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_names-

       A process reassociating itself with a  user  namespace  must  have  the
       CAP_SYS_ADMIN  capability  in the target user namespace.  Upon success-
       fully joining a user namespace, a process is granted  all  capabilities
       in  that  namespace,  regardless  of  its user and group IDs.  A multi-
       threaded process may not change user namespace with setns().  It is not
       permitted  to  use  setns() to reenter the caller's current user names-
       pace.  This prevents  a  caller  that  has  dropped  capabilities  from
       regaining  those capabilities via a call to setns().  For security rea-
       sons, a process can't join a  new  user  namespace  if  it  is  sharing
       filesystem-related  attributes  (the  attributes  whose sharing is con-
       trolled by the clone(2) CLONE_FS flag) with another process.  For  fur-
       ther 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.

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

       EBADF  fd is not a valid file descriptor.

       EINVAL fd  refers  to a namespace whose type does not match that speci-
              fied in nstype.

       EINVAL There is problem with reassociating the thread with  the  speci-
              fied 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 names-

       ENOMEM Cannot allocate sufficient memory to change the specified names-

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

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

       The setns() system call is Linux-specific.

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

       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

       The  following shell session demonstrates the use of this program (com-
       piled 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

       We begin by executing the example program  in  clone(2)  in  the  back-
       ground.  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 descriptor for namespace */
           if (fd == -1)

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

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

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

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Linux                             2015-01-10                          SETNS(2)
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