Question

I’m following http://backreference.org/2010/03/26/tuntap-interface-tutorial/

The following code successfully gets a fd (usually 3) when I run it as root, but it does not create a /dev/tun77 device.

Should it?

#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

Waldner answered this on http://backreference.org/2010/03/26/tuntap-interface-tutorial/ with:

Neither. Network interfaces in Linux don’t appear under /dev; the only thing you’ll see there is /dev/net/tun, which is the device that should be opened as the first step to create a tun/tap interface.

If you run the sample code, you’ll be able to see and configure the interface you create by using “ip link” while the program is running; when the program terminates, the interface disappears.

Alternatively, the interface can be made persistent, as explained, and in that case it will survive program termination.

In any case, no device is created under /dev (apart from the already mentioned /dev/net/tun).