利用setns()将进程加入一个新的network namespace

1、首先使用docker创建一个容器，并获取该容器的network namespace

monster@monster-Z:~$ docker run -itd --name test ubuntu /bin/bash
3926b5b272107843602fae240fdda929c0e5830dbadc0122b10cd8e66cf71f31
monster@monster-Z:~$ docker ps 
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES
3926b5b27210        ubuntu              "/bin/bash"         3 seconds ago       Up 2 seconds                            test
monster@monster-Z:~$ docker inspect --format '{{ .State.Pid }}' test
3656

由上可知，该容器的network namespace在/proc/3656/ns/net

2、启动一个程序，加入该network namespace，并且exec为bash，查看运行效果

程序文件如下所示：

#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...
", 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 namspace
        errExit("execvp");
}

3、操作步骤如下所示，已经进入了该network namespace

root@monster-Z:/home/monster/TEST/c# ./ns_exec /proc/3656/ns/net /bin/bash 
root@monster-Z:/home/monster/TEST/c# ifconfig 
eth0      Link encap:Ethernet  HWaddr 02:42:ac:11:00:02  
          inet addr:172.17.0.2  Bcast:0.0.0.0  Mask:255.255.0.0
          inet6 addr: fe80::42:acff:fe11:2/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:28 errors:0 dropped:0 overruns:0 frame:0
          TX packets:8 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:4324 (4.3 KB)  TX bytes:648 (648.0 B)

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)