// thanx to http://seclists.org/lists/bugtraq/1999/Mar/0057.html
struct tcp_pseudo /*the tcp pseudo header*/
{
  __u32 src_addr;
  __u32 dst_addr;
  __u8 zero;
  __u8 proto;
  __u16 length;
} pseudohead;



long get_tcp_checksum(struct iphdr * myip, struct tcphdr * mytcp) {

        u16 total_len = ntohs(myip->tot_len);

        int tcpopt_len = mytcp->doff*4 - 20;
        int tcpdatalen = total_len - (mytcp->doff*4) - (myip->ihl*4);

        pseudohead.src_addr=myip->saddr;
        pseudohead.dst_addr=myip->daddr;
        pseudohead.zero=0;
        pseudohead.proto=IPPROTO_TCP;
        pseudohead.length=htons(sizeof(struct tcphdr) + tcpopt_len + tcpdatalen);

        int totaltcp_len = sizeof(struct tcp_pseudo) + sizeof(struct tcphdr) + tcpopt_len + tcpdatalen;
        unsigned short * tcp = new unsigned short[totaltcp_len];


        memcpy((unsigned char *)tcp,&pseudohead,sizeof(struct tcp_pseudo));
        memcpy((unsigned char *)tcp+sizeof(struct tcp_pseudo),(unsigned char *)mytcp,sizeof(struct tcphdr));
        memcpy((unsigned char *)tcp+sizeof(struct tcp_pseudo)+sizeof(struct tcphdr), (unsigned char *)myip+(myip->ihl*4)+(sizeof(struct tcphdr)), tcpopt_len);
        memcpy((unsigned char *)tcp+sizeof(struct tcp_pseudo)+sizeof(struct tcphdr)+tcpopt_len, (unsigned char *)mytcp+(mytcp->doff*4), tcpdatalen);

/*      printf("pseud length: %d\n",pseudohead.length);
        printf("tcp hdr length: %d\n",mytcp->doff*4);
        printf("tcp hdr struct length: %d\n",sizeof(struct tcphdr));
        printf("tcp opt length: %d\n",tcpopt_len);
        printf("tcp total+psuedo length: %d\n",totaltcp_len);

        fflush(stdout);

        printf("tcp data len: %d, data start %u\n", tcpdatalen,mytcp + (mytcp->doff*4));
*/


         return checksum(tcp,totaltcp_len);

}

long checksum(unsigned short *addr, unsigned int count) {
          /* Compute Internet Checksum for "count" bytes
            *         beginning at location "addr".
            */
       register long sum = 0;


        while( count > 1 )  {
           /*  This is the inner loop */
               sum += * addr++;
               count -= 2;
       }
           /*  Add left-over byte, if any */
       if( count > 0 )
               sum += * (unsigned char *) addr;

           /*  Fold 32-bit sum to 16 bits */
       while (sum>>16)
           sum = (sum & 0xffff) + (sum >> 16);

       return ~sum;
}