...

struct pseudo_header
{
   unsigned int s_addr;
   unsigned int d_addr;
   unsigned char zer0;
   unsigned char protocol;
   unsigned short length;
};

...

u_short in_cksum(const u_short *addr, register u_int len)
{
   int nleft = len;
   const u_short *w = addr;
   u_short answer;
   int sum = 0;
 
        while (nleft > 1)  {
                sum += *w++;
                nleft -= 2;
        }
        if (nleft == 1)
                sum += htons(*(u_char *)w<<8);

        sum = (sum >> 16) + (sum & 0xffff);
        sum += (sum >> 16);             
        answer = ~sum;
        return (answer);
}

...

pseudo->protocol = IPPROTO_TCP;
pseudo->length = htons(sizeof(struct tcphdr));
pseudo->s_addr = inet_addr("192.168.168.251");
pseudo->d_addr = inet_addr("194.109.192.114");

...

tcp->th_sport = htons(sport);
tcp->th_dport = htons(dport);
tcp->th_seq = htonl(random()%time(NULL));
tcp->th_ack = htonl(random()%time(NULL));
tcp->th_off = 5;
tcp->th_flags = TH_FIN;
tcp->th_win = htons(12000);
tcp->th_sum = 0;
tcp->th_sum = in_cksum((unsigned short *)(outPacket+sizeof (struct ethdr)+sizeof (struct iphdr)), sizeof(struct tcphdr) + sizeof(struct pseudo_header));

...