Just record.

iproute2

在分析 ip netns 的实现时，发现它是通过对文件系统的操作实现操纵 netns，如

ip netns list

就是遍历目录 /var/run/netns/

ip netns add 1

• 创建 /var/run/netns/1
• 隔离进程执行环境 unshare

• 挂载到 /proc/self/ns/net

  ip netns monitor

监听目录 /var/run/netns/ 创建和删除事件，具体怎么实现的，非常简单

inotify

inotify - monitoring filesystem events.
The inotify API provides a mechanism for monitoring filesystem events. Inotify can be used to monitor individual files, or to monitor directories. When a directory is monitored, inotify will return events for the directory itself, and for files inside the directory.

  #include <stdio.h>
  #include <stdlib.h>
  #include <errno.h>
  #include <sys/inotify.h>

  #define MONITOR_PATH    "/var/onestraw/"
  #define MONITOR_MASK    IN_CREATE | IN_DELETE | IN_ACCESS | IN_MODIFY

  inline void _err(const char *str)
  {
    perror(str);
    exit(1);
  }

  inline void inotify_loop(int fd)
  {
    char buf[4096];
    size_t len;
    struct inotify_event *event;
    while (1) {
        len = read(fd, buf, sizeof(buf));
        if (len < 0) {
            _err("read() failed");
        }
        for (event = (struct inotify_event *)buf;
             (char *)event < &buf[len];
             event =
             (struct inotify_event *)((char *)event + sizeof(*event) +
                          event->len)) {
            if (event->mask & IN_CREATE)
                printf("add %s\n", event->name);
            if (event->mask & IN_DELETE)
                printf("delete %s\n", event->name);
            if (event->mask & IN_ACCESS)
                printf("access %s\n", event->name);
            if (event->mask & IN_MODIFY)
                printf("modify %s\n", event->name);
        }
    }
  }

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

    if ((fd = inotify_init()) < 0) {
        _err("inotify_init() failed");
    }
    //if (inotify_add_watch(fd, argv[1], MONITOR_MASK) < 0) {
    if (inotify_add_watch(fd, MONITOR_PATH, MONITOR_MASK) < 0) {
        _err("inotify_add_watch() failed");
    }

    inotify_loop(fd);
    return 0;
  }

参考

• iproute2
• man7.org