openvpn/dhcp.c

/*
 *  OpenVPN -- An application to securely tunnel IP networks
 *             over a single TCP/UDP port, with support for SSL/TLS-based
 *             session authentication and key exchange,
 *             packet encryption, packet authentication, and
 *             packet compression.
 *
 *  Copyright (C) 2002-2010 OpenVPN Technologies, Inc. <sales@openvpn.net>
 *
 *  This program is free software; you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License version 2
 *  as published by the Free Software Foundation.
 *
 *  This program 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 this program (see the file COPYING included with this
 *  distribution); if not, write to the Free Software Foundation, Inc.,
 *  59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 */

#include "syshead.h"

#include "dhcp.h"
#include "socket.h"
#include "error.h"

#include "memdbg.h"

static int
get_dhcp_message_type (const struct dhcp *dhcp, const int optlen)
{
  const uint8_t *p = (uint8_t *) (dhcp + 1);
  int i;

  for (i = 0; i < optlen; ++i)
    {
      const uint8_t type = p[i];
      const int room = optlen - i;
      if (type == DHCP_END)           /* didn't find what we were looking for */
	return -1;
      else if (type == DHCP_PAD)      /* no-operation */
	;
      else if (type == DHCP_MSG_TYPE) /* what we are looking for */
	{
	  if (room >= 3)
	    {
	      if (p[i+1] == 1)        /* option length should be 1 */
		return p[i+2];        /* return message type */
	    }
	  return -1;
	}
      else                            /* some other option */
	{
	  if (room >= 2)
	    {
	      const int len = p[i+1]; /* get option length */
	      i += (len + 1);         /* advance to next option */
	    }
	}
    }
  return -1;
}

static in_addr_t
do_extract (struct dhcp *dhcp, int optlen)
{
  uint8_t *p = (uint8_t *) (dhcp + 1);
  int i;
  in_addr_t ret = 0;

  for (i = 0; i < optlen; )
    {
      const uint8_t type = p[i];
      const int room = optlen - i;
      if (type == DHCP_END)
	break;
      else if (type == DHCP_PAD)
	++i;
      else if (type == DHCP_ROUTER)
	{
	  if (room >= 2)
	    {
	      const int len = p[i+1]; /* get option length */
	      if (len <= (room-2))
		{
		  /* get router IP address */
		  if (!ret && len >= 4 && (len & 3) == 0)
		    {
		      memcpy (&ret, p+i+2, 4);
		      ret = ntohl (ret);
		    }
		  {
		    /* delete the router option */
		    uint8_t *dest = p + i;
		    const int owlen = len + 2;            /* len of data to overwrite */
		    uint8_t *src = dest + owlen;
		    uint8_t *end = p + optlen;
		    const int movlen = end - src;
		    if (movlen > 0)
		      memmove(dest, src, movlen);         /* overwrite router option */
		    memset(end - owlen, DHCP_PAD, owlen); /* pad tail */		    
		  }
		}
	      else
		break;
	    }
	  else
	    break;
	}
      else                              /* some other option */
	{
	  if (room >= 2)
	    {
	      const int len = p[i+1];   /* get option length */
	      i += (len + 2);           /* advance to next option */
	    }
	  else
	    break;
	}
    }
  return ret;
}

static uint16_t
udp_checksum (const uint8_t *buf,
	      const int len_udp,
	      const uint8_t *src_addr,
	      const uint8_t *dest_addr)
{
  uint16_t word16;
  uint32_t sum = 0;
  int i;
	
  /* make 16 bit words out of every two adjacent 8 bit words and  */
  /* calculate the sum of all 16 bit words */
  for (i = 0; i < len_udp; i += 2){
    word16 = ((buf[i] << 8) & 0xFF00) + ((i + 1 < len_udp) ? (buf[i+1] & 0xFF) : 0);
    sum += word16;
  }

  /* add the UDP pseudo header which contains the IP source and destination addresses */
  for (i = 0; i < 4; i += 2){
    word16 =((src_addr[i] << 8) & 0xFF00) + (src_addr[i+1] & 0xFF);
    sum += word16;
  }
  for (i = 0; i < 4; i += 2){
    word16 =((dest_addr[i] << 8) & 0xFF00) + (dest_addr[i+1] & 0xFF);
    sum += word16; 	
  }

  /* the protocol number and the length of the UDP packet */
  sum += (uint16_t) OPENVPN_IPPROTO_UDP + (uint16_t) len_udp;

  /* keep only the last 16 bits of the 32 bit calculated sum and add the carries */
  while (sum >> 16)
    sum = (sum & 0xFFFF) + (sum >> 16);
		
  /* Take the one's complement of sum */
  return ((uint16_t) ~sum);
}

in_addr_t
dhcp_extract_router_msg (struct buffer *ipbuf)
{
  struct dhcp_full *df = (struct dhcp_full *) BPTR (ipbuf);
  const int optlen = BLEN (ipbuf) - (sizeof (struct openvpn_iphdr) + sizeof (struct openvpn_udphdr) + sizeof (struct dhcp));

  if (optlen >= 0
      && df->ip.protocol == OPENVPN_IPPROTO_UDP
      && df->udp.source == htons (BOOTPS_PORT)
      && df->udp.dest == htons (BOOTPC_PORT)
      && df->dhcp.op == BOOTREPLY)
    {
      const int message_type = get_dhcp_message_type (&df->dhcp, optlen);
      if (message_type == DHCPACK || message_type == DHCPOFFER)
	{
	  /* get the router IP address while padding out all DHCP router options */
	  const in_addr_t ret = do_extract (&df->dhcp, optlen);

	  /* recompute the UDP checksum */
	  df->udp.check = 0;
	  df->udp.check = htons (udp_checksum ((uint8_t *) &df->udp, 
					       sizeof (struct openvpn_udphdr) + sizeof (struct dhcp) + optlen,
					       (uint8_t *)&df->ip.saddr,
					       (uint8_t *)&df->ip.daddr));

	  /* only return the extracted Router address if DHCPACK */
	  if (message_type == DHCPACK)
	    {
	      if (ret)
		{
		  struct gc_arena gc = gc_new ();
		  msg (D_ROUTE, "Extracted DHCP router address: %s", print_in_addr_t (ret, 0, &gc));
		  gc_free (&gc);
		}

	      return ret;
	    }
	}
    }
  return 0;
}