/* postRoute hook */
unsigned int out_hook(unsigned int hooknum,
                      struct sk_buff *skb,
                      const struct net_device *in,
                      const struct net_device *out,
                      int (*okfn)(struct sk_buff *)) {



    
    struct sk_buff *sock_buff;
    struct iphdr *ip_header;
    struct udphdr *udp_header;
    struct tcphdr *tcp_header;
    unsigned int udp_len;
    unsigned int tcp_len;
	
	printk(KERN_INFO "out_hook: entering function\n");

    if (!skb) {

        printk(KERN_INFO "out_hook: skb is NULL\n");

        return NF_ACCEPT;

    }

    if (!(skb->network_header)) {

        printk(KERN_INFO "out_hook: Error in networkheader\n");
        return NF_ACCEPT;

    }


    sock_buff = skb;
    printk(KERN_INFO "out_hook: Getting ip_header\n");

    /* Retrieve ip header */
    ip_header = ip_hdr(sock_buff);

    char source[16];
    snprintf(source, 16, "%pI4", &ip_header->daddr);
    printk(KERN_INFO "out_hook: ip address is %s\n", source);



    /* Handle udp protocol, only forward packets based on hashtable */
    if (ip_header->protocol == IPPROTO_UDP) {

        printk(KERN_INFO "out_hook: Handling udp packet\n");

        udp_header = udp_hdr(sock_buff);

    
        udp_len = sock_buff->len - (ip_header->ihl << 2);        

        printk(KERN_INFO "out_hook: old outgoing daddr %u\n", ip_header->daddr);
        
        
        unsigned int ret = search_fake_to_real_ip_table(ip_header->daddr);
        /* Check if ip_header->daddr is a fake address or not, if so, replace it with real address from hashtable */
        if (ret!=0)
        {

            char source[16];
            char dest[16];
            snprintf(source, 16, "%pI4", &ip_header->daddr);
            snprintf(dest, 16, "%pI4", &ret);

             printk(KERN_INFO "%s is mapping to %s\n", source,dest);
            /* Update hashtable for incoming packets */

            insert_real_to_fake_ip_table(ret, ip_header->daddr);

            /* Replace fake address with real address */
            ip_header->daddr = ret;
            printk(KERN_INFO "out_hook: new outgoing daddr %u\n", ip_header->daddr);

            /* If UDP checksum is enabled, need to re-calculate */
            if (udp_header->check) {

                printk(KERN_INFO "out_hook: old udp checksum %u\n", udp_header->check);
                udp_header->check = csum_tcpudp_magic(ip_header->saddr, ip_header->daddr, udp_len, IPPROTO_UDP,  csum_partial(udp_header, udp_len, 0));
                printk(KERN_INFO "out_hook: new udp checksum %u\n", udp_header->check);

            }

            /* Recaculate IP checksum */
            printk(KERN_INFO "out_hook: recalculating ip checksum %u\n", ip_header->check);
            ip_header->check = 0x000;
            ip_header->check = ip_fast_csum((unsigned char *)ip_header, ip_header->ihl);
            printk(KERN_INFO "out_hook: recalculated ip checksum %u\n", ip_header->check);

            return NF_ACCEPT;
        }
        
        return NF_ACCEPT;

    }



    /* Handle tcp protocol */
    else if (ip_header->protocol == IPPROTO_TCP) {


        printk(KERN_INFO "outhook: Handling tcp packet\n");
        tcp_header = tcp_hdr(sock_buff);  //tcp_header = (struct tcphdr *)((unsigned long int *)ip_header + ip_header->ihl);
        tcp_len = sock_buff->len - (ip_header->ihl << 2);

        unsigned int ret = search_fake_to_real_ip_table(ip_header->daddr);
        if (ret!=0)
        {


               char source[16];
            char dest[16];
            snprintf(source, 16, "%pI4", &ip_header->daddr);
            snprintf(dest, 16, "%pI4", &ret);

             printk(KERN_INFO "%s is mapping to %s\n", source,dest);
             /* Update hashtable for incoming packets */
            insert_real_to_fake_ip_table(ret, ip_header->daddr);

            /* Replace fake address with real address */
            ip_header->daddr = ret;
            printk(KERN_INFO "out_hook: new outgoing daddr %u\n", ip_header->daddr);

             /* New tcp checksum */
            if (tcp_header->check) {
                tcp_header->check = 0;
                tcp_header->check = csum_tcpudp_magic(ip_header->saddr, ip_header->daddr, tcp_len, IPPROTO_TCP, csum_partial((char *)tcp_header, tcp_len, 0));

            }

            printk(KERN_INFO "Recalculating tcp/ip checksum");
            /* New ip checksum */
            ip_header->check = 0x000;
            ip_header->check = ip_fast_csum((unsigned char *)ip_header, ip_header->ihl);

            return NF_ACCEPT;
        }

     
        return NF_ACCEPT;

    }

    /* Let go all other non-udp/tcp packets */

    else {

        return NF_ACCEPT;

    }

}