#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <sys/ioctl.h>
#include <net/if_arp.h>
#include <net/if.h>
#include <string.h>
#include <errno.h>

const unsigned char mac_guest[6] = { 0xfe, 0xfd, 0, 0, 0, 1 };
const unsigned char ip_guest[4] = { 192, 168, 1, 10 };
const unsigned char ip_host[4] = { 192, 168, 1, 1 };

void die (const char *s1, const char *s2)
{
  write(2, s1, strlen(s1));
  write(2, ": ", 2);
  write(2, s2, strlen(s2));
  write(2, "\n", 1);
  exit(111);
}

void usage () { die("usage", "tunconfig <tunX>"); }

int main (int argc, char **argv)
{
  int ret, sock, i;
  struct sockaddr_in *sin;
  struct arpreq arr;
  struct ifreq ifr;

  if (argc != 2) usage();
  if (strlen(argv[1]) != 4) usage();
  if (strncmp(argv[1], "tun", 3)) usage();
  if ((argv[1][3] < '0') || (argv[1][3] > '9')) usage();

  memset(&arr, 0, sizeof(struct arpreq));
  memset(&ifr, 0, sizeof(struct ifreq));

  sock = socket(PF_INET, SOCK_DGRAM, 0);
  if (sock < 0) die("socket()", strerror(errno));

  /* ifconfig up */
  strncpy(ifr.ifr_ifrn.ifrn_name, argv[1], IF_NAMESIZE);
  sin = (struct sockaddr_in*) &ifr.ifr_ifru.ifru_addr;
  sin->sin_family = AF_INET;
  for (i = 3; i >= 0; --i)
    sin->sin_addr.s_addr = (sin->sin_addr.s_addr << 8) + ip_host[i];

  ret = ioctl(sock, SIOCSIFADDR, &ifr);
  if (ret < 0) die("SIOCSIFADDR", strerror(errno));

  ifr.ifr_ifru.ifru_flags = IFF_UP | IFF_PROMISC;
  ret = ioctl(sock, SIOCSIFFLAGS, &ifr);
  if (ret < 0) die("SIOCSIFFLAGS", strerror(errno));

  /* arp */
  arr.arp_flags =  ATF_COM | ATF_PUBL | ATF_PERM;
  strncpy(arr.arp_dev, argv[1], 16);

  sin = (struct sockaddr_in*) &arr.arp_pa;
  sin->sin_family = AF_INET;

  for (i = 3; i >= 0; --i)
    sin->sin_addr.s_addr = (sin->sin_addr.s_addr << 8) + ip_guest[i];
  for (i = 0; i < 6; ++i) arr.arp_ha.sa_data[i] = mac_guest[i];

  ret = ioctl(sock, SIOCSARP, &arr);
  if (ret < 0) die("SIOCSARP", strerror(errno));

  return 0;
}