c - tun 程序未创 build 备节点

Question

我正在关注http://backreference.org/2010/03/26/tuntap-interface-tutorial/

当我以 root 身份运行以下代码时，它成功获取了一个 fd(通常为 3)，但它没有创建/dev/tun77 设备。

应该吗？

#include <stdio.h>
#include <stdlib.h>
#include <net/if.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <string.h>
#include <linux/if_tun.h>
#include <linux/ioctl.h>

#define IFNAMSIZ 16

int tun_alloc(char *dev, int flags) {

  struct ifreq ifr;
  int fd, err;
  char *clonedev = "/dev/net/tun";

  /* Arguments taken by the function:
   *
   * char *dev: the name of an interface (or '\0'). MUST have enough
   *   space to hold the interface name if '\0' is passed
   * int flags: interface flags (eg, IFF_TUN etc.)
   */

   /* open the clone device */
   if( (fd = open(clonedev, O_RDWR)) < 0 ) {
     return fd;
   }

   /* preparation of the struct ifr, of type "struct ifreq" */
   memset(&ifr, 0, sizeof(ifr));

   ifr.ifr_flags = flags;   /* IFF_TUN or IFF_TAP, plus maybe IFF_NO_PI */

   if (*dev) {
     /* if a device name was specified, put it in the structure; otherwise,
      * the kernel will try to allocate the "next" device of the
      * specified type */
     strncpy(ifr.ifr_name, dev, IFNAMSIZ);
   }

   /* try to create the device */
   if( (err = ioctl(fd, TUNSETIFF, (void *) &ifr)) < 0 ) {
     close(fd);
     return err;
   }

  /* if the operation was successful, write back the name of the
   * interface to the variable "dev", so the caller can know
   * it. Note that the caller MUST reserve space in *dev (see calling
   * code below) */
  strcpy(dev, ifr.ifr_name);

  /* this is the special file descriptor that the caller will use to talk
   * with the virtual interface */
  return fd;
}

int main(void) {
  char tun_name[IFNAMSIZ];
  int nread, tun_fd;
  char buffer[2048];
  /* Connect to the device */
  strcpy(tun_name, "tun77");
  tun_fd = tun_alloc(tun_name, IFF_TUN | IFF_NO_PI);  /* tun interface */

  if (tun_fd < 0){
    perror("Allocating interface");
    exit(1);
  } else {
    printf("connected to %s on fd: %i\n", tun_name, tun_fd);
  }

  /* Now read data coming from the kernel */
  while (1) {
    /* Note that "buffer" should be at least the MTU size of the interface, eg 1500 bytes */
    nread = read(tun_fd, buffer, sizeof(buffer));
    if (nread < 0) {
      perror("Reading from interface");
      close(tun_fd);
      exit(1);
    }

    /* Do whatever with the data */
    printf("Read %d bytes from device %s\n", nread, tun_name);
  }
  return EXIT_SUCCESS;
}

Answer 1

瓦尔德纳在 http://backreference.org/2010/03/26/tuntap-interface-tutorial/ 上回答了这个问题与:

都不是。 Linux 中的网络接口(interface)不会出现在/dev 下；您将看到的唯一内容是/dev/net/tun，这是创建 tun/tap 接口(interface)的第一步应该打开的设备。

如果您运行示例代码，您将能够在程序运行时查看并配置您使用“ip link”创建的接口(interface)；当程序终止时，界面消失。

或者，如所解释的，可以使接口(interface)持久化，在这种情况下，它将在程序终止后继续存在。

无论如何，/dev 下不会创建任何设备(除了已经提到的/dev/net/tun)。