text: /* packet.c
  
     Packet assembly code, originally contributed by Archie Cobbs. */
  
  /*
   * Copyright (c) 2004-2007 by Internet Systems Consortium, Inc. ("ISC")
   * Copyright (c) 1996-2003 by Internet Software Consortium
   *
   * Permission to use, copy, modify, and distribute this software for any
   * purpose with or without fee is hereby granted, provided that the above
   * copyright notice and this permission notice appear in all copies.
   *
   * THE SOFTWARE IS PROVIDED "AS IS" AND ISC DISCLAIMS ALL WARRANTIES
   * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
   * MERCHANTABILITY AND FITNESS.  IN NO EVENT SHALL ISC BE LIABLE FOR
   * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
   * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
   * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT
   * OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
   *
   *   Internet Systems Consortium, Inc.
   *   950 Charter Street
   *   Redwood City, CA 94063
   *   <info@isc.org>
   *   http://www.isc.org/
   *
   * This code was originally contributed by Archie Cobbs, and is still
   * very similar to that contribution, although the packet checksum code
   * has been hacked significantly with the help of quite a few ISC DHCP
   * users, without whose gracious and thorough help the checksum code would
   * still be disabled.
   */
  
  #ifndef lint
  static char copyright[] =
  "$Id: packet.c,v 1.40.2.8 2007/05/01 20:42:55 each Exp $ Copyright (c) 2004-2005 Internet Systems Consortium.  All rights reserved.\n";
  #endif /* not lint */
  
  #include "dhcpd.h"
  
  #if defined (PACKET_ASSEMBLY) || defined (PACKET_DECODING)
  #include "includes/netinet/ip.h"
  #include "includes/netinet/udp.h"
  #include "includes/netinet/if_ether.h"
  #endif /* PACKET_ASSEMBLY || PACKET_DECODING */
  
  /* Compute the easy part of the checksum on a range of bytes. */
  
  u_int32_t checksum (buf, nbytes, sum)
  	unsigned char *buf;
  	unsigned nbytes;
  	u_int32_t sum;
  {
  	unsigned i;
  
  #ifdef DEBUG_CHECKSUM
  	log_debug ("checksum (%x %d %x)", buf, nbytes, sum);
  #endif
  
  	/* Checksum all the pairs of bytes first... */
  	for (i = 0; i < (nbytes & ~1U); i += 2) {
  #ifdef DEBUG_CHECKSUM_VERBOSE
  		log_debug ("sum = %x", sum);
  #endif
  		sum += (u_int16_t) ntohs(*((u_int16_t *)(buf + i)));
  		/* Add carry. */
  		if (sum > 0xFFFF)
  			sum -= 0xFFFF;
  	}	
  
  	/* If there's a single byte left over, checksum it, too.   Network
  	   byte order is big-endian, so the remaining byte is the high byte. */
  	if (i < nbytes) {
  #ifdef DEBUG_CHECKSUM_VERBOSE
  		log_debug ("sum = %x", sum);
  #endif
  		sum += buf [i] << 8;
  		/* Add carry. */
  		if (sum > 0xFFFF)
  			sum -= 0xFFFF;
  	}
  	
  	return sum;
  }
  
  /* Finish computing the checksum, and then put it into network byte order. */
  
  u_int32_t wrapsum (sum)
  	u_int32_t sum;
  {
  #ifdef DEBUG_CHECKSUM
  	log_debug ("wrapsum (%x)", sum);
  #endif
  
  	sum = ~sum & 0xFFFF;
  #ifdef DEBUG_CHECKSUM_VERBOSE
  	log_debug ("sum = %x", sum);
  #endif
  	
  #ifdef DEBUG_CHECKSUM
  	log_debug ("wrapsum returns %x", htons (sum));
  #endif
  	return htons(sum);
  }
  
  #ifdef PACKET_ASSEMBLY
  void assemble_hw_header (interface, buf, bufix, to)
  	struct interface_info *interface;
  	unsigned char *buf;
  	unsigned *bufix;
  	struct hardware *to;
  {
  #if defined (HAVE_TR_SUPPORT)
  	if (interface -> hw_address.hbuf [0] == HTYPE_IEEE802)
  		assemble_tr_header (interface, buf, bufix, to);
  	else
  #endif
  #if defined (DEC_FDDI)
  	     if (interface -> hw_address.hbuf [0] == HTYPE_FDDI)
  		     assemble_fddi_header (interface, buf, bufix, to);
  	else
  #endif
  		assemble_ethernet_header (interface, buf, bufix, to);
  
  }
  
  /* UDP header and IP header assembled together for convenience. */
  
  void assemble_udp_ip_header (interface, buf, bufix,
  			     from, to, port, data, len)
  	struct interface_info *interface;
  	unsigned char *buf;
  	unsigned *bufix;
  	u_int32_t from;
  	u_int32_t to;
  	u_int32_t port;
  	unsigned char *data;
  	unsigned len;
  {
  	struct ip ip;
  	struct udphdr udp;
  
  	/* Fill out the IP header */
  	IP_V_SET (&ip, 4);
  	IP_HL_SET (&ip, 20);
  	ip.ip_tos = IPTOS_LOWDELAY;
  	ip.ip_len = htons(sizeof(ip) + sizeof(udp) + len);
  	ip.ip_id = 0;
  	ip.ip_off = 0;
  	ip.ip_ttl = 16;
  	ip.ip_p = IPPROTO_UDP;
  	ip.ip_sum = 0;
  	ip.ip_src.s_addr = from;
  	ip.ip_dst.s_addr = to;
  	
  	/* Checksum the IP header... */
  	ip.ip_sum = wrapsum (checksum ((unsigned char *)&ip, sizeof ip, 0));
  	
  	/* Copy the ip header into the buffer... */
  	memcpy (&buf [*bufix], &ip, sizeof ip);
  	*bufix += sizeof ip;
  
  	/* Fill out the UDP header */
  	udp.uh_sport = local_port;		/* XXX */
  	udp.uh_dport = port;			/* XXX */
  	udp.uh_ulen = htons(sizeof(udp) + len);
  	memset (&udp.uh_sum, 0, sizeof udp.uh_sum);
  
  	/* Compute UDP checksums, including the ``pseudo-header'', the UDP
  	   header and the data. */
  
  	udp.uh_sum =
  		wrapsum (checksum ((unsigned char *)&udp, sizeof udp,
  				   checksum (data, len, 
  					     checksum ((unsigned char *)
  						       &ip.ip_src,
  						       2 * sizeof ip.ip_src,
  						       IPPROTO_UDP +
  						       (u_int32_t)
  						       ntohs (udp.uh_ulen)))));
  
  	/* Copy the udp header into the buffer... */
  	memcpy (&buf [*bufix], &udp, sizeof udp);
  	*bufix += sizeof udp;
  }
  #endif /* PACKET_ASSEMBLY */
  
  #ifdef PACKET_DECODING
  /* Decode a hardware header... */
  /* XXX currently only supports ethernet; doesn't check for other types. */
  
  ssize_t decode_hw_header (interface, buf, bufix, from)
       struct interface_info *interface;
       unsigned char *buf;
       unsigned bufix;
       struct hardware *from;
  {
  #if defined (HAVE_TR_SUPPORT)
  	if (interface -> hw_address.hbuf [0] == HTYPE_IEEE802)
  		return decode_tr_header (interface, buf, bufix, from);
  	else
  #endif
  #if defined (DEC_FDDI)
  	     if (interface -> hw_address.hbuf [0] == HTYPE_FDDI)
  		     return decode_fddi_header (interface, buf, bufix, from);
  	else
  #endif
  		return decode_ethernet_header (interface, buf, bufix, from);
  }
  
  /* UDP header and IP header decoded together for convenience. */
  
  ssize_t
  decode_udp_ip_header(struct interface_info *interface,
  		     unsigned char *buf, unsigned bufix,
  		     struct sockaddr_in *from, unsigned buflen,
  		     unsigned *rbuflen)
  {
    unsigned char *data;
    struct ip ip;
    struct udphdr udp;
    unsigned char *upp, *endbuf;
    u_int32_t ip_len, ulen, pkt_len;
    u_int32_t sum, usum;
    static int ip_packets_seen;
    static int ip_packets_bad_checksum;
    static int udp_packets_seen;
    static int udp_packets_bad_checksum;
    static int udp_packets_length_checked;
    static int udp_packets_length_overflow;
    unsigned len;
  
    /* Designate the end of the input buffer for bounds checks. */
    endbuf = buf + bufix + buflen;
  
    /* Assure there is at least an IP header there. */
    if ((buf + bufix + sizeof(ip)) > endbuf)
  	  return -1;
  
    /* Copy the IP header into a stack aligned structure for inspection.
     * There may be bits in the IP header that we're not decoding, so we
     * copy out the bits we grok and skip ahead by ip.ip_hl * 4.
     */
    upp = buf + bufix;
    memcpy(&ip, upp, sizeof(ip));
    ip_len = (*upp & 0x0f) << 2;
    upp += ip_len;
  
    /* Check the IP packet length. */
    pkt_len = ntohs(ip.ip_len);
    if (pkt_len > buflen)
  	return -1;
  
    /* Assure after ip_len bytes that there is enough room for a UDP header. */
    if ((upp + sizeof(udp)) > endbuf)
  	  return -1;
  
    /* Copy the UDP header into a stack alined structure for inspection. */
    memcpy(&udp, upp, sizeof(udp));
  
  #ifdef USERLAND_FILTER
    /* Is it a UDP packet? */
    if (ip.ip_p != IPPROTO_UDP)
  	  return -1;
  
    /* Is it to the port we're serving? */
    if (udp.uh_dport != local_port)
  	  return -1;
  #endif /* USERLAND_FILTER */
  
    ulen = ntohs(udp.uh_ulen);
    if (ulen < sizeof(udp))
  	return -1;
  
    udp_packets_length_checked++;
    if ((upp + ulen) > endbuf) {
  	udp_packets_length_overflow++;
  	if ((udp_packets_length_checked > 4) &&
  	    ((udp_packets_length_checked /
  	      udp_packets_length_overflow) < 2)) {
  		log_info("%d udp packets in %d too long - dropped",
  			 udp_packets_length_overflow,
  			 udp_packets_length_checked);
  		udp_packets_length_overflow = 0;
  		udp_packets_length_checked = 0;
  	}
  	return -1;
    }
  
    if ((ulen < sizeof(udp)) || ((upp + ulen) > endbuf))
  	return -1;
  
    /* Check the IP header checksum - it should be zero. */
    ++ip_packets_seen;
    if (wrapsum (checksum (buf + bufix, ip_len, 0))) {
  	  ++ip_packets_bad_checksum;
  	  if (ip_packets_seen > 4 &&
  	      (ip_packets_seen / ip_packets_bad_checksum) < 2) {
  		  log_info ("%d bad IP checksums seen in %d packets",
  			    ip_packets_bad_checksum, ip_packets_seen);
  		  ip_packets_seen = ip_packets_bad_checksum = 0;
  	  }
  	  return -1;
    }
  
    /* Copy out the IP source address... */
    memcpy(&from->sin_addr, &ip.ip_src, 4);
  
    /* Compute UDP checksums, including the ``pseudo-header'', the UDP
       header and the data.   If the UDP checksum field is zero, we're
       not supposed to do a checksum. */
  
    data = upp + sizeof(udp);
    len = ulen - sizeof(udp);
  
    usum = udp.uh_sum;
    udp.uh_sum = 0;
  
    /* XXX: We have to pass &udp, because we have to zero the checksum
     * field before calculating the sum...'upp' isn't zeroed.
     */
    sum = wrapsum(checksum((unsigned char *)&udp, sizeof(udp),
  			 checksum(data, len,
  				  checksum((unsigned char *)&ip.ip_src,
  					   8, IPPROTO_UDP + ulen))));
  
    udp_packets_seen++;
    if (usum && usum != sum) {
  	  udp_packets_bad_checksum++;
  	  if (udp_packets_seen > 4 &&
  	      (udp_packets_seen / udp_packets_bad_checksum) < 2) {
  		  log_info ("%d bad udp checksums in %d packets",
  			    udp_packets_bad_checksum, udp_packets_seen);
  		  udp_packets_seen = udp_packets_bad_checksum = 0;
  	  }
  	  return -1;
    }
  
    /* Copy out the port... */
    memcpy (&from -> sin_port, &udp.uh_sport, sizeof udp.uh_sport);
  
    /* Save the length of the UDP payload. */
    if (rbuflen != NULL)
  	*rbuflen = len;
  
    /* Return the index to the UDP payload. */
    return ip_len + sizeof udp;
  }
  #endif /* PACKET_DECODING */
  