create_tun_device.h

#ifndef __CREATE_TUN_DEVICE_H
#define __CREATE_TUN_DEVICE_H

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

/** Arguments taken by the function:
 *
 * @param char *dev the name of an interface (or '\0'). MUST have enough
 *   space to hold the interface name if '\0' is passed
 * @param int flags interface flags (eg, IFF_TUN etc.)
 * @return int file descriptor (if positive) or error code (negative)
 */
static int create_tun_device( char *dev, int flags ) {
  struct ifreq ifr;
  int fd, err;
  char *clonedev = "/dev/net/tun";
  
  /* 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;
}

#endif