/* Tama~o del paquete TCP */
unsigned int buffer_size = sizeof(struct iphdr) + sizeof(struct tcphdr);
 
/* Buffer de tama~o suficiente para un paquete TCP */
unsigned char buffer[buffer_size];
memset (buffer,0,buffer_size);
 
/* Cabecera IP */
struct iphdr *ip = (struct iphdr *)buffer;
 
/* Cabecera TCP */
struct tcphdr *tcp = (struct tcphdr *)(buffer + sizeof(struct iphdr));
/*
   Aqui relleno la cabecera TCP y la de IP
*/
unsigned short CheckSum(unsigned short *buffer, int size)
{
    unsigned long cksum=0;
    while(size >1)
    {
        cksum+=*buffer++;
        size -=sizeof(unsigned short);
    }
    if(size)
        cksum += *(unsigned char*)buffer;
 
    cksum = (cksum >> 16) + (cksum & 0xffff);
    cksum += (cksum >>16);
    return (unsigned short)(~cksum);
}
 
tcp->check=CheckSum((unsigned short int*)buffer,buffer_size);