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path: root/src/transport/gnunet-nat-client-windows.c
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/*
     This file is part of GNUnet.
     (C) 2010 Christian Grothoff (and other contributing authors)

     GNUnet is free software; you can redistribute it and/or modify
     it under the terms of the GNU General Public License as published
     by the Free Software Foundation; either version 3, or (at your
     option) any later version.

     GNUnet is distributed in the hope that it will be useful, but
     WITHOUT ANY WARRANTY; without even the implied warranty of
     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
     General Public License for more details.

     You should have received a copy of the GNU General Public License
     along with GNUnet; see the file COPYING.  If not, write to the
     Free Software Foundation, Inc., 59 Temple Place - Suite 330,
     Boston, MA 02111-1307, USA.
*/

/**
 * @file src/transport/gnunet-nat-client-windows.c
 * @brief Tool to help bypass NATs using ICMP method; must run as
 *        root (SUID will do) or administrator on W32
 *        This code will work under GNU/Linux or W32.
 * @author Nathan Evans
 *
 * This program will send ONE ICMP message using RAW sockets
 * to the IP address specified as the second argument.  Since
 * it uses RAW sockets, it must be installed SUID or run as 'root'.
 * In order to keep the security risk of the resulting SUID binary
 * minimal, the program ONLY opens the RAW socket with root
 * privileges, then drops them and only then starts to process
 * command line arguments.  The code also does not link against
 * any shared libraries (except libc) and is strictly minimal
 * (except for checking for errors).  The following list of people
 * have reviewed this code and considered it safe since the last
 * modification (if you reviewed it, please have your name added
 * to the list):
 *
 * - Christian Grothoff
 * - Nathan Evans
 */
#define _GNU_SOURCE

#include <winsock2.h>
#include <ws2tcpip.h>
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <stdlib.h>
#include <stdint.h>
#include <time.h>


#define ICMP_ECHO 8
#define IPDEFTTL 64
#define ICMP_TIME_EXCEEDED 11

/**
 * Must match IP given in the server.
 */
#define DUMMY_IP "192.0.2.86"

#define NAT_TRAV_PORT 22225

/**
 * IPv4 header.
 */
struct ip_header
{

  /**
   * Version (4 bits) + Internet header length (4 bits)
   */
  uint8_t vers_ihl;

  /**
   * Type of service
   */
  uint8_t tos;

  /**
   * Total length
   */
  uint16_t pkt_len;

  /**
   * Identification
   */
  uint16_t id;

  /**
   * Flags (3 bits) + Fragment offset (13 bits)
   */
  uint16_t flags_frag_offset;

  /**
   * Time to live
   */
  uint8_t ttl;

  /**
   * Protocol
   */
  uint8_t proto;

  /**
   * Header checksum
   */
  uint16_t checksum;

  /**
   * Source address
   */
  uint32_t src_ip;

  /**
   * Destination address
   */
  uint32_t dst_ip;
};


/**
 * Format of ICMP packet.
 */
struct icmp_ttl_exceeded_header
{
  uint8_t type;

  uint8_t code;

  uint16_t checksum;

  uint32_t unused;

  /* followed by original payload */
};

struct icmp_echo_header
{
  uint8_t type;

  uint8_t code;

  uint16_t checksum;

  uint32_t reserved;
};

/**
 * Beginning of UDP packet.
 */
struct udp_header
{
  uint16_t src_port;

  uint16_t dst_port;

  uint16_t length;

  uint16_t crc;
};


/**
 * Socket we use to send our ICMP packets.
 */
static SOCKET rawsock;

/**
 * Target "dummy" address.
 */
static struct in_addr dummy;

/**
 * Port we are listening on (communicated to the server).
 */
static uint16_t port;



/**
 * Convert IPv4 address from text to binary form.
 *
 * @param af address family
 * @param cp the address to print
 * @param buf where to write the address result
 * @return 1 on success
 */
static int
inet_pton (int af,
	   const char *cp,
	   struct in_addr *buf)
{
  buf->s_addr = inet_addr(cp);
  if (buf->s_addr == INADDR_NONE)
    {
      fprintf(stderr,
	      "Error %d handling address %s",
	      WSAGetLastError(),
	      cp);
      return 0;
    }
  return 1;
}


/**
 * CRC-16 for IP/ICMP headers.
 *
 * @param data what to calculate the CRC over
 * @param bytes number of bytes in data (must be multiple of 2)
 * @return the CRC 16.
 */
static uint16_t
calc_checksum(const uint16_t *data,
	      unsigned int bytes)
{
  uint32_t sum;
  unsigned int i;

  sum = 0;
  for (i=0;i<bytes/2;i++)
    sum += data[i];
  sum = (sum & 0xffff) + (sum >> 16);
  sum = htons(0xffff - sum);
  return sum;
}


/**
 * Send an ICMP message to the target.
 *
 * @param my_ip source address
 * @param other target address
 */
static void
send_icmp_udp (const struct in_addr *my_ip,
               const struct in_addr *other)
{
  char packet[sizeof(struct ip_header) * 2 +
	      sizeof(struct icmp_ttl_exceeded_header) +
	      sizeof(struct udp_header)];
  struct ip_header ip_pkt;
  struct icmp_ttl_exceeded_header icmp_pkt;
  struct udp_header udp_pkt;
  struct sockaddr_in dst;
  size_t off;
  int err;

  /* ip header: send to (known) ip address */
  off = 0;
  ip_pkt.vers_ihl = 0x45;
  ip_pkt.tos = 0;
  ip_pkt.pkt_len = htons(sizeof (packet));
  ip_pkt.id = htons(256);
  ip_pkt.flags_frag_offset = 0;
  ip_pkt.ttl = 128;
  ip_pkt.proto = IPPROTO_ICMP;
  ip_pkt.checksum = 0;
  ip_pkt.src_ip = my_ip->s_addr;
  ip_pkt.dst_ip = other->s_addr;
  ip_pkt.checksum = htons(calc_checksum((uint16_t*)&ip_pkt,
					sizeof (struct ip_header)));
  memcpy(&packet[off],
	 &ip_pkt,
	 sizeof(struct ip_header));
  off += sizeof(struct ip_header);

  icmp_pkt.type = ICMP_TIME_EXCEEDED;
  icmp_pkt.code = 0;
  icmp_pkt.checksum = 0;
  icmp_pkt.unused = 0;
  memcpy(&packet[off],
	 &icmp_pkt,
	 sizeof(struct icmp_ttl_exceeded_header));
  off += sizeof(struct icmp_ttl_exceeded_header);

  /* ip header of the presumably 'lost' udp packet */
  ip_pkt.vers_ihl = 0x45;
  ip_pkt.tos = 0;
  ip_pkt.pkt_len = htons(sizeof (struct ip_header) +
			 sizeof (struct udp_header));
  ip_pkt.id = htons(0);
  ip_pkt.flags_frag_offset = 0;
  ip_pkt.ttl = 128;
  ip_pkt.proto = IPPROTO_UDP;
  ip_pkt.checksum = 0;
  ip_pkt.src_ip = other->s_addr;
  ip_pkt.dst_ip = dummy.s_addr;
  ip_pkt.checksum = htons(calc_checksum((uint16_t*)&ip_pkt,
					sizeof (struct ip_header)));
  memcpy(&packet[off],
	 &ip_pkt,
	 sizeof(struct ip_header));
  off += sizeof(struct ip_header);

  /* build UDP header */
  udp_pkt.src_port = htons(NAT_TRAV_PORT);
  udp_pkt.dst_port = htons(NAT_TRAV_PORT);
  udp_pkt.length = htons (port);
  udp_pkt.crc = 0;
  memcpy(&packet[off],
	 &udp_pkt,
	 sizeof(struct udp_header));
  off += sizeof(struct udp_header);

  /* no go back to calculate ICMP packet checksum */
  icmp_pkt.checksum = htons(calc_checksum((uint16_t*)&packet[off],
					  sizeof (struct icmp_ttl_exceeded_header) +
					  sizeof (struct ip_header) +
					  sizeof (struct udp_header)));
  memcpy (&packet[sizeof (struct ip_header)],
	  &icmp_pkt,
	  sizeof (struct icmp_ttl_exceeded_header));

  memset (&dst, 0, sizeof (dst));
  dst.sin_family = AF_INET;
  dst.sin_addr = *other;
  err = sendto(rawsock,
               packet,
               sizeof (packet), 0,
               (struct sockaddr*)&dst,
               sizeof(dst));
  if (err < 0)
    {
      fprintf(stderr,
              "sendto failed: %s\n", strerror(errno));
    }
  else if (sizeof (packet) != (size_t) err)
    {
      fprintf(stderr,
              "Error: partial send of ICMP message\n");
    }
}


/**
 * Send an ICMP message to the target.
 *
 * @param my_ip source address
 * @param other target address
 */
static void
send_icmp (const struct in_addr *my_ip,
	   const struct in_addr *other)
{
  struct ip_header ip_pkt;
  struct icmp_ttl_exceeded_header icmp_ttl;
  struct icmp_echo_header icmp_echo;
  struct sockaddr_in dst;
  char packet[sizeof (struct ip_header) * 2 +
	      sizeof (struct icmp_ttl_exceeded_header) +
	      sizeof(struct icmp_echo_header)];
  size_t off;
  int err;

  /* ip header: send to (known) ip address */
  off = 0;
  ip_pkt.vers_ihl = 0x45;
  ip_pkt.tos = 0;
  ip_pkt.pkt_len = htons (sizeof (packet));
  ip_pkt.id = htons(256);
  ip_pkt.flags_frag_offset = 0;
  ip_pkt.ttl = IPDEFTTL;
  ip_pkt.proto = IPPROTO_ICMP;
  ip_pkt.checksum = 0;
  ip_pkt.src_ip = my_ip->s_addr;
  ip_pkt.dst_ip = other->s_addr;
  ip_pkt.checksum = htons(calc_checksum((uint16_t*)&ip_pkt,
					sizeof (struct ip_header)));
  memcpy (&packet[off],
	  &ip_pkt,
	  sizeof (struct ip_header));
  off += sizeof (ip_pkt);

  /* icmp reply: time exceeded */
  icmp_ttl.type = ICMP_TIME_EXCEEDED;
  icmp_ttl.code = 0;
  icmp_ttl.checksum = 0;
  icmp_ttl.unused = 0; 
  memcpy (&packet[off],
	  &icmp_ttl,
	  sizeof (struct icmp_ttl_exceeded_header));
  off += sizeof (struct icmp_ttl_exceeded_header);

  /* ip header of the presumably 'lost' udp packet */
  ip_pkt.vers_ihl = 0x45;
  ip_pkt.tos = 0;
  ip_pkt.pkt_len = htons(sizeof (struct ip_header) + sizeof (struct icmp_echo_header));
  ip_pkt.id = htons (256);
  ip_pkt.flags_frag_offset = 0;
  ip_pkt.ttl = 1; /* real TTL would be 1 on a time exceeded packet */
  ip_pkt.proto = IPPROTO_ICMP;
  ip_pkt.src_ip = other->s_addr;
  ip_pkt.dst_ip = dummy.s_addr;
  ip_pkt.checksum = 0;
  ip_pkt.checksum = htons(calc_checksum((uint16_t*)&ip_pkt,
					sizeof (struct ip_header)));
  memcpy (&packet[off],
	  &ip_pkt,
	  sizeof (struct ip_header));
  off += sizeof (struct ip_header);

  icmp_echo.type = ICMP_ECHO;
  icmp_echo.code = 0;
  icmp_echo.reserved = htonl(port);
  icmp_echo.checksum = 0;
  icmp_echo.checksum = htons(calc_checksum((uint16_t*) &icmp_echo,
					   sizeof (struct icmp_echo_header)));
  memcpy (&packet[off],
	  &icmp_echo,
	  sizeof(struct icmp_echo_header));

  /* no go back to calculate ICMP packet checksum */
  off = sizeof (struct ip_header);
  icmp_ttl.checksum = htons(calc_checksum((uint16_t*) &packet[off],
					  sizeof (struct icmp_ttl_exceeded_header) +
					  sizeof (struct ip_header) +
					  sizeof (struct icmp_echo_header)));
  memcpy (&packet[off],
	  &icmp_ttl,
	  sizeof (struct icmp_ttl_exceeded_header));

  memset (&dst, 0, sizeof (dst));
  dst.sin_family = AF_INET;
  dst.sin_addr = *other;

  err = sendto(rawsock,
	       packet,
	       sizeof (packet), 0,
	       (struct sockaddr*)&dst,
	       sizeof(dst));

  if (err < 0)
    {
      fprintf(stderr,
	      "sendto failed: %s\n", strerror(errno));
    }
  else if (sizeof (packet) != (size_t) err)
    {
      fprintf(stderr,
	      "Error: partial send of ICMP message\n");
    }
}


/**
 * Create an ICMP raw socket.
 *
 * @return INVALID_SOCKET on error
 */
static SOCKET
make_raw_socket ()
{
  DWORD bOptVal = TRUE;
  int bOptLen = sizeof(bOptVal);
  SOCKET ret;

  ret = socket (AF_INET, SOCK_RAW, IPPROTO_RAW);
  if (INVALID_SOCKET == ret)
    {
      fprintf (stderr,
	       "Error opening RAW socket: %s\n",
	       strerror (errno));
      return INVALID_SOCKET;
    }
  if (0 != setsockopt(ret, SOL_SOCKET, SO_BROADCAST, (char*)&bOptVal, bOptLen))
    {
      fprintf(stderr,
	      "Error setting SO_BROADCAST to ON: %s\n",
	      strerror (errno));
      closesocket(rawsock);
      return INVALID_SOCKET;
    }

  if (0 != setsockopt(ret, IPPROTO_IP, IP_HDRINCL, (char*)&bOptVal, bOptLen))
    {
      fprintf(stderr,
	      "Error setting IP_HDRINCL to ON: %s\n",
	      strerror (errno));
      closesocket(rawsock);
      return INVALID_SOCKET;
    }
  return ret;
}


int
main (int argc, char *const *argv)
{
  struct in_addr external;
  struct in_addr target;
  WSADATA wsaData;

  unsigned int p;

  if (argc != 4)
    {
      fprintf (stderr,
	       "This program must be started with our IP, the targets external IP, and our port as arguments.\n");
      return 1;
    }
  if ( (1 != inet_pton (AF_INET, argv[1], &external)) ||
       (1 != inet_pton (AF_INET, argv[2], &target)) )
    {
      fprintf (stderr,
	       "Error parsing IPv4 address: %s\n",
	       strerror (errno));
      return 1;
    }
  if ( (1 != sscanf (argv[3], "%u", &p) ) ||
       (0 == p) ||
       (0xFFFF < p) )
    {
      fprintf (stderr,
	       "Error parsing port value `%s'\n",
	       argv[3]);
      return 1;
    }
  port = (uint16_t) p;

  if (0 != WSAStartup (MAKEWORD (2, 1), &wsaData))
    {
      fprintf (stderr, "Failed to find Winsock 2.1 or better.\n");
      return 2;
    }
  if (1 != inet_pton (AF_INET, DUMMY_IP, &dummy))
    {
      fprintf (stderr,
	       "Internal error converting dummy IP to binary.\n");
      return 2;
    }
  if (-1 == (rawsock = make_raw_socket()))
    return 3;
  send_icmp (&external,
	     &target);
  send_icmp_udp (&external,
                 &target);
  closesocket (rawsock);
  WSACleanup ();
  return 0;
}

/* end of gnunet-nat-client-windows.c */