SETNS(2)

NAME
setns - reassociate thread with a namespace

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

int setns(int fd, int nstype);

DESCRIPTION
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 proc(5) 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_NEWIPC
    fd must refer to an IPC namespace.

CLONE_NEWNET
    fd must refer to a network namespace.

CLONE_NEWUTS
    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.)

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

ERRORS

EBADF
    fd is not a valid file descriptor.

EINVAL
    fd refers to a namespace whose type does not match that specified in nstype, or there is problem with reassociating the the thread with the specified namespace.

ENOMEM
    Cannot allocate sufficient memory to change the specified namespace.

EPERM
    The calling thread did not have the required privilege (CAP_SYS_ADMIN) for this operation.

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

CONFORMING TO
The setns() system call is Linux-specific.

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

EXAMPLE
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
Password:
# ./newuts bizarro &
[1] 3549
clone() returned 3550
uts.nodename in child:  bizarro
uts.nodename in parent: antero
# uname -n             # Verify hostname in the shell
antero

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
bizarro

  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)

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

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

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

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

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

SEE ALSO
clone(2), fork(2), vfork(2), proc(5), unix(7)

COLOPHON
This page is part of release 3.50 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.