tcp6_6to4_frag.c

/*  Copyright (C) 2012-2013  P.D. Buchan (pdbuchan@yahoo.com)

    This program 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 of the License, or
    (at your option) any later version.

    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.  If not, see <http://www.gnu.org/licenses/>.
*/

// Send an IPv6 TCP packet through an IPv4 tunnel (6to4) via raw socket
// at the link layer (ethernet frame) with a large payload requiring fragmentation.
// Includes a TCP header option. Need to have destination MAC address.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>           // close()
#include <string.h>           // strcpy, memset(), and memcpy()

#include <netdb.h>            // struct addrinfo
#include <sys/types.h>        // needed for socket(), uint8_t, uint16_t, uint32_t
#include <sys/socket.h>       // needed for socket()
#include <netinet/in.h>       // IPPROTO_IPV6, IPPROTO_TCP, IPPROTO_FRAGMENT, INET_ADDRSTRLEN, INET6_ADDRSTRLEN
#include <netinet/ip.h>       // struct ip and IP_MAXPACKET (which is 65535)
#define __FAVOR_BSD           // Use BSD format of tcp header
#include <netinet/ip6.h>      // struct ip6_hdr
#include <netinet/tcp.h>      // struct tcphdr
#include <arpa/inet.h>        // inet_pton() and inet_ntop()
#include <sys/ioctl.h>        // macro ioctl is defined
#include <bits/ioctls.h>      // defines values for argument "request" of ioctl. Here, we need SIOCGIFHWADDR.
#include <net/if.h>           // struct ifreq
#include <linux/if_ether.h>   // ETH_P_IP = 0x0800, ETH_P_IPV6 = 0x86DD
#include <linux/if_packet.h>  // struct sockaddr_ll (see man 7 packet)
#include <net/ethernet.h>

#include <errno.h>            // errno, perror()

// Define some constants.
#define ETH_HDRLEN 14        // Ethernet header length
#define IP4_HDRLEN 20        // IPv4 header length
#define IP6_HDRLEN 40        // IPv6 header length
#define FRG_HDRLEN 8         // IPv6 fragment header
#define TCP_HDRLEN 20        // TCP header length, excludes options data

// Function prototypes
uint16_t checksum (uint16_t *, int);
uint16_t tcp6_checksum (struct ip6_hdr, struct tcphdr, int, uint8_t *, int, uint8_t *, int);
char *allocate_strmem (int);
uint8_t *allocate_ustrmem (int);
int *allocate_intmem (int);

int
main (int argc, char **argv)
{
  const int MAX_FRAGS = 1597;  // Maximum number of packet fragments (int) (65535 - TCP_HDRLEN) / (IP6_HDRLEN + 1 data byte))
  int i, n, status, frame_length, sd, bytes, opt_len, opt_pad, *frag_flags;
  int *ip4_flags, *tcp_flags, c, nframes, offset[MAX_FRAGS], len[MAX_FRAGS];
  char *interface, *target4, *target6, *source4, *source6, *src_ip, *dst_ip;
  struct ip ip4hdr;
  struct ip6_hdr ip6hdr;
  struct ip6_frag fraghdr;
  struct tcphdr tcphdr;
  int payloadlen, bufferlen;
  uint8_t *options, *payload, *buffer, *src_mac, *dst_mac, *ether_frame;
  struct addrinfo hints, *res;
  struct sockaddr_in6 *ipv6;
  struct sockaddr_ll device;
  struct ifreq ifr;
  void *tmp;
  FILE *fi;

  // Allocate memory for various arrays.
  target4 = allocate_strmem (INET_ADDRSTRLEN);
  target6 = allocate_strmem (INET6_ADDRSTRLEN);
  source4 = allocate_strmem (INET_ADDRSTRLEN);
  source6 = allocate_strmem (INET6_ADDRSTRLEN);
  src_ip = allocate_strmem (INET6_ADDRSTRLEN);
  dst_ip = allocate_strmem (INET6_ADDRSTRLEN);
  ip4_flags = allocate_intmem (4);
  tcp_flags = allocate_intmem (8);
  src_mac = allocate_ustrmem (6);
  dst_mac = allocate_ustrmem (6);
  ether_frame = allocate_ustrmem (IP_MAXPACKET);
  interface = allocate_strmem (40);
  options = allocate_ustrmem (40);
  frag_flags = allocate_intmem (2);
  payload = allocate_ustrmem (IP_MAXPACKET);

  // Interface to send packet through.
  strcpy (interface, "eth0");

  // Submit request for a socket descriptor to look up interface.
  if ((sd = socket (PF_PACKET, SOCK_RAW, htons (ETH_P_ALL))) < 0) {
    perror ("socket() failed to get socket descriptor for using ioctl() ");
    exit (EXIT_FAILURE);
  }

  // Use ioctl() to look up interface name and get its MAC address.
  memset (&ifr, 0, sizeof (ifr));
  snprintf (ifr.ifr_name, sizeof (ifr.ifr_name), "%s", interface);
  if (ioctl (sd, SIOCGIFHWADDR, &ifr) < 0) {
    perror ("ioctl() failed to get source MAC address ");
    return (EXIT_FAILURE);
  }
  close (sd);

  // Copy source MAC address.
  memcpy (src_mac, ifr.ifr_hwaddr.sa_data, 6 * sizeof (uint8_t));

  // Report source MAC address to stdout.
  printf ("MAC address for interface %s is ", interface);
  for (i=0; i<5; i++) {
    printf ("%02x:", src_mac[i]);
  }
  printf ("%02x\n", src_mac[5]);

  // Find interface index from interface name and store index in
  // struct sockaddr_ll device, which will be used as an argument of sendto().
  if ((device.sll_ifindex = if_nametoindex (interface)) == 0) {
    perror ("if_nametoindex() failed to obtain interface index ");
    exit (EXIT_FAILURE);
  }
  printf ("Index for interface %s is %i\n", interface, device.sll_ifindex);

  // Set destination MAC address: you need to fill these out
  dst_mac[0] = 0xff;
  dst_mac[1] = 0xff;
  dst_mac[2] = 0xff;
  dst_mac[3] = 0xff;
  dst_mac[4] = 0xff;
  dst_mac[5] = 0xff;

  // Source IPv4 address: you need to fill this out
  strcpy (source4, "192.168.1.132");

  // Source IPv6 address: you need to fill this out
  strcpy (source6, "2001:db8::214:51ff:fe2f:1556");

  // IPv4 target as the 6to4 anycast address (do not change)
  strcpy (target4, "192.88.99.1");

  // Target URL or IPv6 address: you need to fill this out
  strcpy (target6, "ipv6.google.com");

  // Fill out hints for getaddrinfo().
  memset (&hints, 0, sizeof (struct addrinfo));
  hints.ai_family = AF_INET6;
  hints.ai_socktype = SOCK_RAW;
  hints.ai_flags = hints.ai_flags | AI_CANONNAME;

  // Resolve source using getaddrinfo().
  if ((status = getaddrinfo (source6, NULL, &hints, &res)) != 0) {
    fprintf (stderr, "getaddrinfo() failed: %s\n", gai_strerror (status));
    return (EXIT_FAILURE);
  }
  ipv6 = (struct sockaddr_in6 *) res->ai_addr;
  tmp = &(ipv6->sin6_addr);
  if (inet_ntop (AF_INET6, tmp, src_ip, INET6_ADDRSTRLEN) == NULL) {
    status = errno;
    fprintf (stderr, "inet_ntop() failed.\nError message: %s", strerror (status));
    exit (EXIT_FAILURE);
  }
  freeaddrinfo (res);

  // Resolve target using getaddrinfo().
  if ((status = getaddrinfo (target6, NULL, &hints, &res)) != 0) {
    fprintf (stderr, "getaddrinfo() failed: %s\n", gai_strerror (status));
    return (EXIT_FAILURE);
  }
  ipv6 = (struct sockaddr_in6 *) res->ai_addr;
  tmp = &(ipv6->sin6_addr);
  if (inet_ntop (AF_INET6, tmp, dst_ip, INET6_ADDRSTRLEN) == NULL) {
    status = errno;
    fprintf (stderr, "inet_ntop() failed.\nError message: %s", strerror (status));
    exit (EXIT_FAILURE);
  }
  freeaddrinfo (res);

  // Fill out sockaddr_ll.
  device.sll_family = AF_PACKET;
  memcpy (device.sll_addr, src_mac, 6 * sizeof (uint8_t));
  device.sll_halen = htons (6);

  // TCP Options
  // Here we introduce one TCP option.
  opt_len = 0;
  options[0] = 2u; opt_len++;  // Option kind 2 = maximum segment size
  options[1] = 4u; opt_len++;  // This option kind is 4 bytes long
  options[2] = 0x1u; opt_len++;  // Set maximum segment size to 0x100 = 256
  options[3] = 0x0u; opt_len++;

  // Pad TCP options to the next 4-byte boundary.
  opt_pad = 0;
  while (((opt_len + opt_pad)%4) != 0) {
    opt_pad++;
  }

  // Get TCP data.
  i = 0;
  fi = fopen ("data", "r");
  if (fi == NULL) {
    printf ("Can't open file 'data'.\n");
    exit (EXIT_FAILURE);
  }
  while ((n=fgetc (fi)) != EOF) {
    payload[i] = n;
    i++;
  }
  fclose (fi);
  payloadlen = i;
  printf ("IPv6 header length (bytes): %i\n", IP6_HDRLEN);
  printf ("Upper layer protocol header (TCP) length (bytes): %i\n", TCP_HDRLEN);
  printf ("TCP options length (bytes): %i\n", opt_len);
  printf ("TCP options padding length (bytes): %i\n", opt_pad);
  printf ("Payload length (bytes): %i\n", payloadlen);

  // Length of fragmentable portion of packet.
  bufferlen = TCP_HDRLEN + opt_len + opt_pad + payloadlen;
  printf ("Total fragmentable data (bytes): %i\n", bufferlen);

  // Allocate memory for a buffer for fragmentable portion.
  buffer = allocate_ustrmem (bufferlen);

  // Determine how many ethernet frames we'll need.
  // Fragment IPv6 packet with fragmentation extension headers as usual,
  // but use an MTU of 1280 bytes as per Section 3.2.1 of RFC 4213. 
  // Then prepend an IPv4 header (with appropriate 6to4 settings) on each fragment later.
  memset (len, 0, MAX_FRAGS * sizeof (int));
  memset (offset, 0, MAX_FRAGS * sizeof (int));
  i = 0;
  c = 0;  // Variable c is index to buffer, which contains upper layer protocol header and data.
  while (c < bufferlen) {

    // Do we still need to fragment remainder of fragmentable portion?
    if ((bufferlen - c) > (1280 - IP4_HDRLEN - IP6_HDRLEN - FRG_HDRLEN)) {  // Yes
      len[i] = 1280 - IP4_HDRLEN - IP6_HDRLEN - FRG_HDRLEN;  // len[i] is amount of fragmentable part we can include in this frame.

    } else {  // No
      len[i] = bufferlen - c;  // len[i] is amount of fragmentable part we can include in this frame.
    }
    c += len[i];

    // If not last fragment, make sure we have an even number of 8-byte blocks.
    // Reduce length as necessary.
    if (c < (bufferlen - 1)) {
      while ((len[i]%8) > 0) {
        len[i]--;
        c--;
      }
    }
    printf ("Frag: %i,  Data (bytes): %i,  Data Offset (8-byte blocks): %i\n", i, len[i], offset[i]);
    i++;
    offset[i] = (len[i-1] / 8) + offset[i-1];
  }
  nframes = i;
  printf ("Total number of frames to send: %i\n", nframes);

  // IPv4 header (Section 3.5 of RFC 4213)

  // IPv4 header length (4 bits): Number of 32-bit words in header = 5
  ip4hdr.ip_hl = sizeof (struct ip) / sizeof (uint32_t);

  // Internet Protocol version (4 bits): IPv4
  ip4hdr.ip_v = 4;

  // Type of service (8 bits)
  ip4hdr.ip_tos = 0;

  // Total length of datagram (16 bits)
  // ip4hdr.ip_len is set for each fragment in loop below.

  // ID sequence number (16 bits)
  ip4hdr.ip_id = htons (31415);

  // Flags, and Fragmentation offset (3, 13 bits)

  // Zero (1 bit)
  ip4_flags[0] = 0;

  // Do not fragment flag (1 bit)
  // Must not be set, as per Section 3.2.1 of RFC 4213.
  ip4_flags[1] = 0;

  // More fragments following flag (1 bit): zero, since we fragment at IPv6 level instead.
  ip4_flags[2] = 0u;

  // Fragmentation offset (13 bits)
  ip4_flags[3] = 0;

  // Flags, and Fragmentation offset (3, 13 bits)
  ip4hdr.ip_off = htons ((ip4_flags[0] << 15)
                       + (ip4_flags[1] << 14)
                       + (ip4_flags[2] << 13)
                       +  ip4_flags[3]);

  // Time-to-Live (8 bits): use maximum value
  ip4hdr.ip_ttl = 255;

  // Transport layer protocol (8 bits): 41 for IPv6 (Section 3.5 of RFC 4213)
  ip4hdr.ip_p = IPPROTO_IPV6;

  // Source IPv4 address (32 bits)
  if ((status = inet_pton (AF_INET, source4, &(ip4hdr.ip_src))) != 1) {
    fprintf (stderr, "inet_pton() failed.\nError message: %s", strerror (status));
    exit (EXIT_FAILURE);
  }

  // Destination IPv4 address (32 bits)
  if ((status = inet_pton (AF_INET, target4, &(ip4hdr.ip_dst))) != 1) {
    fprintf (stderr, "inet_pton() failed.\nError message: %s", strerror (status));
    exit (EXIT_FAILURE);
  }

  // IPv4 header checksum (16 bits) - set to 0 when calculating checksum
  ip4hdr.ip_sum = 0;
  ip4hdr.ip_sum = checksum ((uint16_t *) &ip4hdr, IP4_HDRLEN);

  // IPv6 header

  // IPv6 version (4 bits), Traffic class (8 bits), Flow label (20 bits)
  ip6hdr.ip6_flow = htonl ((6 << 28) | (0 << 20) | 0);

  // Payload length (16 bits): fragmentable portion of packet. i.e., TCP header + TCP options + TCP option padding + TCP payload data
  ip6hdr.ip6_plen = htons (TCP_HDRLEN + opt_len + opt_pad + payloadlen);

  // Next header (8 bits) - 6 for TCP
  // This will be changed if we need to fragment but we'll change this to
  // 44 only in ether_frame because otherwise TCP checksum will be wrong.
  ip6hdr.ip6_nxt = IPPROTO_TCP;

  // Hop limit  (8 bits) - use 255 (RFC 4861)
  ip6hdr.ip6_hops = 255;

  // Source IPv6 address (128 bits)
  if ((status = inet_pton (AF_INET6, src_ip, &(ip6hdr.ip6_src))) != 1) {
    fprintf (stderr, "inet_pton() failed.\nError message: %s", strerror (status));
    exit (EXIT_FAILURE);
  }

  // Destination IPv6 address (128 bits)
  if ((status = inet_pton (AF_INET6, dst_ip, &(ip6hdr.ip6_dst))) != 1) {
    fprintf (stderr, "inet_pton() failed.\nError message: %s", strerror (status));
    exit (EXIT_FAILURE);
  }

  // TCP header

  // Source port number (16 bits)
  tcphdr.th_sport = htons (60);

  // Destination port number (16 bits)
  tcphdr.th_dport = htons (80);

  // Sequence number (32 bits)
  tcphdr.th_seq = htonl (0);

  // Acknowledgement number (32 bits)
  tcphdr.th_ack = htonl (0);

  // Reserved (4 bits): should be 0
  tcphdr.th_x2 = 0;

  // Data offset (4 bits): size of TCP header in 32-bit words
  tcphdr.th_off = (TCP_HDRLEN + opt_len + opt_pad) / 4;

  // Flags (8 bits)

  // FIN flag (1 bit)
  tcp_flags[0] = 0;

  // SYN flag (1 bit)
  tcp_flags[1] = 0;

  // RST flag (1 bit)
  tcp_flags[2] = 0;

  // PSH flag (1 bit)
  tcp_flags[3] = 1;

  // ACK flag (1 bit)
  tcp_flags[4] = 1;

  // URG flag (1 bit)
  tcp_flags[5] = 0;

  // ECE flag (1 bit)
  tcp_flags[6] = 0;

  // CWR flag (1 bit)
  tcp_flags[7] = 0;

  tcphdr.th_flags = 0;
  for (i=0; i<8; i++) {
    tcphdr.th_flags += (tcp_flags[i] << i);
  }

  // Window size (16 bits)
  tcphdr.th_win = htons (65535);

  // Urgent pointer (16 bits): 0 (only valid if URG flag is set)
  tcphdr.th_urp = htons (0);

  // TCP checksum (16 bits)
  tcphdr.th_sum = tcp6_checksum (ip6hdr, tcphdr, opt_len, options, opt_pad, payload, payloadlen);

  // Build fragmentable portion of packet in buffer array.
  memcpy (buffer, &tcphdr, TCP_HDRLEN * sizeof (uint8_t));  // TCP header
  memcpy (buffer + TCP_HDRLEN, options, opt_len * sizeof (uint8_t));  // TCP options
  memcpy (buffer + TCP_HDRLEN + opt_len + opt_pad, payload, payloadlen * sizeof (uint8_t));  // TCP data (note the offset for padding)

  // IPv6 next header (8 bits)
  if (nframes == 1)  {
    ip6hdr.ip6_nxt = IPPROTO_TCP;  // 6 for TCP
  } else {
    ip6hdr.ip6_nxt = IPPROTO_FRAGMENT;  // 44 for Fragmentation extension header
  }

  // Submit request for a raw socket descriptor.
  if ((sd = socket (PF_PACKET, SOCK_RAW, htons (ETH_P_ALL))) < 0) {
    perror ("socket() failed ");
    exit (EXIT_FAILURE);
  }

  // Loop through fragments.
  for (i=0; i<nframes; i++) {

    // Set ethernet frame contents to zero initially.
    memset (ether_frame, 0, IP_MAXPACKET * sizeof (uint8_t));

    // Fill out ethernet frame header.

    // Copy destination and source MAC addresses to ethernet frame.
    memcpy (ether_frame, dst_mac, 6 * sizeof (uint8_t));
    memcpy (ether_frame + 6, src_mac, 6 * sizeof (uint8_t));

    // Next is ethernet type code (ETH_P_IP for IPv4).
    // http://www.iana.org/assignments/ethernet-numbers
    ether_frame[12] = ETH_P_IP / 256;
    ether_frame[13] = ETH_P_IP % 256;

    // Next is ethernet frame data (IPv4 header + IPv6 header + [fragmentation extension header] + fragment).

    // Total length of datagram (16 bits): IPv4 header + IPv6 header + [fragmentation extension header] + fragment
    if (nframes == 1) {
      ip4hdr.ip_len = htons (IP4_HDRLEN + IP6_HDRLEN + len[i]);
    } else {
      ip4hdr.ip_len = htons (IP4_HDRLEN + IP6_HDRLEN + FRG_HDRLEN + len[i]);
    }

    // IPv4 header checksum (16 bits)
    ip4hdr.ip_sum = 0;
    ip4hdr.ip_sum = checksum ((uint16_t *) &ip4hdr, IP4_HDRLEN);

    // Copy IPv4 header to ethernet frame.
    memcpy (ether_frame + ETH_HDRLEN, &ip4hdr, IP4_HDRLEN * sizeof (uint8_t));

    // Payload length (16 bits): See 4.5 of RFC 2460.
    if (nframes == 1) {
      ip6hdr.ip6_plen = htons (len[i]);
    } else {
      ip6hdr.ip6_plen = htons (FRG_HDRLEN + len[i]);
    }

    // Copy IPv6 header to ethernet frame.
    memcpy (ether_frame + ETH_HDRLEN + IP4_HDRLEN, &ip6hdr, IP6_HDRLEN * sizeof (uint8_t));

    // Fill out and copy fragmentation extension header to ethernet frame.
    if (nframes > 1) {
      fraghdr.ip6f_nxt = IPPROTO_TCP;  // Upper layer protocol
      fraghdr.ip6f_reserved = 0;  // Reserved
      frag_flags[1] = 0;  // Reserved
      if (i < (nframes - 1)) {
        frag_flags[0] = 1;  // More fragments to follow
      } else {
        frag_flags[0] = 0;  // This is the last fragment
      }
      fraghdr.ip6f_offlg = htons ((offset[i] << 3) + frag_flags[0] + (frag_flags[1] <<1));
      fraghdr.ip6f_ident = htonl (31415);
      memcpy (ether_frame + ETH_HDRLEN + IP4_HDRLEN + IP6_HDRLEN, &fraghdr, FRG_HDRLEN * sizeof (uint8_t));
    }

    // Copy fragmentable portion of packet to ethernet frame.
    if (nframes == 1) {
      memcpy (ether_frame + ETH_HDRLEN + IP4_HDRLEN + IP6_HDRLEN, buffer, bufferlen * sizeof (uint8_t));
    } else {
      memcpy (ether_frame + ETH_HDRLEN + IP4_HDRLEN + IP6_HDRLEN + FRG_HDRLEN, buffer + (offset[i] * 8), len[i] * sizeof (uint8_t));
    }

    // Ethernet frame length = ethernet header (MAC + MAC + ethernet type) + ethernet data (IPv4 header + IPv6 header + [fragment header] + fragment)
    if (nframes == 1) {
      frame_length = ETH_HDRLEN + IP4_HDRLEN + IP6_HDRLEN + len[i];
    } else {
      frame_length = ETH_HDRLEN + IP4_HDRLEN + IP6_HDRLEN + FRG_HDRLEN + len[i];
    }

    // Send ethernet frame to socket.
    printf ("Sending fragment: %i\n", i);
    if ((bytes = sendto (sd, ether_frame, frame_length, 0, (struct sockaddr *) &device, sizeof (device))) <= 0) {
      perror ("sendto() failed");
      exit (EXIT_FAILURE);
    }
  }  // End loop nframes

  // Close socket descriptor.
  close (sd);

  // Free allocated memory.
  free (target4);
  free (target6);
  free (source4);
  free (source6);
  free (src_mac);
  free (dst_mac);
  free (ether_frame);
  free (interface);
  free (src_ip);
  free (dst_ip);
  free (ip4_flags);
  free (tcp_flags);
  free (options);
  free (frag_flags);
  free (payload);
  free (buffer);

  return (EXIT_SUCCESS);
}

// Checksum function
uint16_t
checksum (uint16_t *addr, int len)
{
  int nleft = len;
  int sum = 0;
  uint16_t *w = addr;
  uint16_t answer = 0;

  while (nleft > 1) {
    sum += *w++;
    nleft -= sizeof (uint16_t);
  }

  if (nleft == 1) {
    *(uint8_t *) (&answer) = *(uint8_t *) w;
    sum += answer;
  }

  sum = (sum >> 16) + (sum & 0xFFFF);
  sum += (sum >> 16);
  answer = ~sum;

  return (answer);
}

// Build IPv6 TCP pseudo-header and call checksum function (Section 8.1 of RFC 2460).
uint16_t
tcp6_checksum (struct ip6_hdr iphdr, struct tcphdr tcphdr, int opt_len, uint8_t *options, int opt_pad, uint8_t *payload, int payloadlen)
{
  uint32_t lvalue;
  char buf[IP_MAXPACKET], cvalue;
  char *ptr;
  int chksumlen = 0;
  int i;

  memset (buf, 0, IP_MAXPACKET * sizeof (uint8_t));

  ptr = &buf[0];  // ptr points to beginning of buffer buf

  // Copy source IP address into buf (128 bits)
  memcpy (ptr, &iphdr.ip6_src.s6_addr, sizeof (iphdr.ip6_src.s6_addr));
  ptr += sizeof (iphdr.ip6_src.s6_addr);
  chksumlen += sizeof (iphdr.ip6_src.s6_addr);

  // Copy destination IP address into buf (128 bits)
  memcpy (ptr, &iphdr.ip6_dst.s6_addr, sizeof (iphdr.ip6_dst.s6_addr));
  ptr += sizeof (iphdr.ip6_dst.s6_addr);
  chksumlen += sizeof (iphdr.ip6_dst.s6_addr);

  // Copy TCP length to buf (32 bits)
  lvalue = htonl (sizeof (tcphdr) + opt_len + opt_pad + payloadlen);
  memcpy (ptr, &lvalue, sizeof (lvalue));
  ptr += sizeof (lvalue);
  chksumlen += sizeof (lvalue);

  // Copy zero field to buf (24 bits)
  *ptr = 0; ptr++;
  *ptr = 0; ptr++;
  *ptr = 0; ptr++;
  chksumlen += 3;

  // Copy next header field to buf (8 bits)
  memcpy (ptr, &iphdr.ip6_nxt, sizeof (iphdr.ip6_nxt));
  ptr += sizeof (iphdr.ip6_nxt);
  chksumlen += sizeof (iphdr.ip6_nxt);

  // Copy TCP source port to buf (16 bits)
  memcpy (ptr, &tcphdr.th_sport, sizeof (tcphdr.th_sport));
  ptr += sizeof (tcphdr.th_sport);
  chksumlen += sizeof (tcphdr.th_sport);

  // Copy TCP destination port to buf (16 bits)
  memcpy (ptr, &tcphdr.th_dport, sizeof (tcphdr.th_dport));
  ptr += sizeof (tcphdr.th_dport);
  chksumlen += sizeof (tcphdr.th_dport);

  // Copy sequence number to buf (32 bits)
  memcpy (ptr, &tcphdr.th_seq, sizeof (tcphdr.th_seq));
  ptr += sizeof (tcphdr.th_seq);
  chksumlen += sizeof (tcphdr.th_seq);

  // Copy acknowledgement number to buf (32 bits)
  memcpy (ptr, &tcphdr.th_ack, sizeof (tcphdr.th_ack));
  ptr += sizeof (tcphdr.th_ack);
  chksumlen += sizeof (tcphdr.th_ack);

  // Copy data offset to buf (4 bits) and
  // copy reserved bits to buf (4 bits)
  // NOTE: It is assumed here that the data offset value
  // already accounts for any options padding.
  cvalue = (tcphdr.th_off << 4) + tcphdr.th_x2;
  memcpy (ptr, &cvalue, sizeof (cvalue));
  ptr += sizeof (cvalue);
  chksumlen += sizeof (cvalue);

  // Copy TCP flags to buf (8 bits)
  memcpy (ptr, &tcphdr.th_flags, sizeof (tcphdr.th_flags));
  ptr += sizeof (tcphdr.th_flags);
  chksumlen += sizeof (tcphdr.th_flags);

  // Copy TCP window size to buf (16 bits)
  memcpy (ptr, &tcphdr.th_win, sizeof (tcphdr.th_win));
  ptr += sizeof (tcphdr.th_win);
  chksumlen += sizeof (tcphdr.th_win);

  // Copy TCP checksum to buf (16 bits)
  // Zero, since we don't know it yet
  *ptr = 0; ptr++;
  *ptr = 0; ptr++;
  chksumlen += 2;

  // Copy urgent pointer to buf (16 bits)
  memcpy (ptr, &tcphdr.th_urp, sizeof (tcphdr.th_urp));
  ptr += sizeof (tcphdr.th_urp);
  chksumlen += sizeof (tcphdr.th_urp);

  // Copy TCP options into buf.
  memcpy (ptr, options, opt_len * sizeof (uint8_t));
  ptr += opt_len;
  chksumlen += opt_len;

  // Pad options to the next 32-bit boundary.
  ptr += opt_pad;
  chksumlen += opt_pad;

  // Copy payload to buf
  memcpy (ptr, payload, payloadlen * sizeof (uint8_t));
  ptr += payloadlen;
  chksumlen += payloadlen;

  // Pad to the next 16-bit boundary
  i = 0;
  while (((payloadlen+i)%2) != 0) {
    i++;
    chksumlen++;
    ptr++;
  }

  return checksum ((uint16_t *) buf, chksumlen);
}

// Allocate memory for an array of chars.
char *
allocate_strmem (int len)
{
  void *tmp;

  if (len <= 0) {
    fprintf (stderr, "ERROR: Cannot allocate memory because len = %i in allocate_strmem().\n", len);
    exit (EXIT_FAILURE);
  }

  tmp = (char *) malloc (len * sizeof (char));
  if (tmp != NULL) {
    memset (tmp, 0, len * sizeof (char));
    return (tmp);
  } else {
    fprintf (stderr, "ERROR: Cannot allocate memory for array allocate_strmem().\n");
    exit (EXIT_FAILURE);
  }
}

// Allocate memory for an array of unsigned chars.
uint8_t *
allocate_ustrmem (int len)
{
  void *tmp;

  if (len <= 0) {
    fprintf (stderr, "ERROR: Cannot allocate memory because len = %i in allocate_ustrmem().\n", len);
    exit (EXIT_FAILURE);
  }

  tmp = (uint8_t *) malloc (len * sizeof (uint8_t));
  if (tmp != NULL) {
    memset (tmp, 0, len * sizeof (uint8_t));
    return (tmp);
  } else {
    fprintf (stderr, "ERROR: Cannot allocate memory for array allocate_ustrmem().\n");
    exit (EXIT_FAILURE);
  }
}

// Allocate memory for an array of ints.
int *
allocate_intmem (int len)
{
  void *tmp;

  if (len <= 0) {
    fprintf (stderr, "ERROR: Cannot allocate memory because len = %i in allocate_intmem().\n", len);
    exit (EXIT_FAILURE);
  }

  tmp = (int *) malloc (len * sizeof (int));
  if (tmp != NULL) {
    memset (tmp, 0, len * sizeof (int));
    return (tmp);
  } else {
    fprintf (stderr, "ERROR: Cannot allocate memory for array allocate_intmem().\n");
    exit (EXIT_FAILURE);
  }
}