#include <sys/types.h>
#include <sys/select.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/ioctl.h>

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <linux/if.h>
#include <linux/if_tun.h>

int tun_alloc(char *dev);
void usage(const char *msg);

const char* argv0;
const int debug = 1;

int main(int argc, char **argv)
{
  int fd_tun;
  char null_intf[260];
  char scratch[1024];
  int intf_no;

  argv0 = argv[0];

  if (argc > 1)
    usage(0);

  if (geteuid())
    usage("Must be run by root");

  strcpy(null_intf, "null%d");
  if ((fd_tun = tun_alloc(null_intf)) < 0) {
    perror("Could not alloc tun device");
    exit(1);
  }

  intf_no = atoi(null_intf+4);

  printf("Using interface %s with address 169.254.%d.1.\n", null_intf, intf_no);
  /* Bring up the interface with a link local address */
  sprintf(scratch, "/sbin/ip link set %s up", null_intf);
  system(scratch);
  sprintf(scratch, "/sbin/ip addr add 169.254.%d.1/32 dev %s", intf_no, null_intf);
  system(scratch);
  if (debug) {
    sprintf(scratch, "/sbin/ip addr ls dev %s", null_intf);
    system(scratch);
  }

  debug && puts("waiting for packets...");
  while (1) {
    if (read(fd_tun, scratch, sizeof(scratch))<0) {
      perror("Error reading from tun device");
      exit(1);
    }
    if (debug) {
      printf(".");
      fflush(stdout);
    }
  }    

  /* never get here */
  close(fd_tun);
  exit(0);
}


void usage(const char *msg)
{
  if (msg)
    printf("Error: %s\n\n", msg);

  printf("Null interface, discard everything sent here.\n"
	 "\n"
	 "usage: %s <interface> <route> [<route> ...]\n", argv0);
  exit(1);
}



int tun_alloc(char *dev)
{
  struct ifreq ifr;
  int fd, err;

  if( (fd = open("/dev/net/tun", O_RDWR)) < 0 ) {
    /*    return tun_alloc_old(dev); */
    perror("Cannot open tun allocation device");
    exit(1);
  }

  memset(&ifr, 0, sizeof(ifr));

  /* Flags: IFF_TUN   - TUN device (no Ethernet headers)
   *        IFF_TAP   - TAP device
   *
   *        IFF_NO_PI - Do not provide packet information
   */
  ifr.ifr_flags = IFF_TUN | IFF_NO_PI;
  if( *dev )
    strncpy(ifr.ifr_name, dev, IFNAMSIZ);

  if( (err = ioctl(fd, TUNSETIFF, (void *) &ifr)) < 0 ){
    close(fd);
    return err;
  }
  strcpy(dev, ifr.ifr_name);
  return fd;
}