Calculated tcp checksum and original tcp checksum don't match

Question

I am trying to modify ip packets coming to my pc. I am using iptables to queue the packet and libnetfilter_queue library to modify the packet, actually some http-header content. Even if i change a single character, the packet is rejected, most probably due to change in tcp checksum. So, i am trying to recalculate the tcp checksum of modified packet. Initially, to test checksum function, i am not modifying the packet but simply recalculating the checksum. But the calculated checksum, does not match with the original one. Here is the checksum function:`

unsigned short tcp_sum_calc(unsigned short len_tcp, unsigned short src_addr[],unsigned short dest_addr[], unsigned short buff[])
{
unsigned char prot_tcp=6;
unsigned long sum;
int nleft;
unsigned short *w;

sum = 0;
nleft = len_tcp;

w=buff;

/* calculate the checksum for the tcp header and payload */
while(nleft > 1)
{
sum = sum + ntohs(*w);
w++;
nleft = nleft - 2;
}

/* if nleft is 1 there ist still on byte left. We add a padding byte (0xFF) to build a 16bit word */
if(nleft>0)
{
    cout<<"check out";
// sum += *w&0xFF; 
sum += ntohs(*w&0xFF00);            // is this the correct way of doing
}

/* add the pseudo header */
sum += ntohs(src_addr[0]);
sum += ntohs(src_addr[1]);
sum += ntohs(dest_addr[0]);
sum += ntohs(dest_addr[1]);
sum += len_tcp;
sum += prot_tcp;

// keep only the last 16 bits of the 32 bit calculated sum and add the carries
sum = (sum >> 16) + (sum & 0xFFFF);
sum += (sum >> 16);

// Take the one's complement of sum
sum = ~sum;

return ((unsigned short) sum);
}

Here is the callback function of libnetfilter_queue module:

static int analyzeResponse(struct nfq_q_handle *qh, struct nfgenmsg *nfmsg,struct nfq_data *nfa, void *data)
{
int len=0,id=0;
struct iphdr *ip;
struct tcphdr *tcp;
    char *pktData;
string tempPkt;
unsigned short chksum=0,ip_hdr_len,tcp_len;
unsigned int src_ip, des_ip;
struct nfqnl_msg_packet_hdr *pktHeader;
pktHeader = nfq_get_msg_packet_hdr(nfa);

if (pktHeader) 
{
    id = ntohl(pktHeader->packet_id);
}
len = nfq_get_payload ( ( struct nfq_data * ) nfa, (char**)&ip );

if(len) 
{
    pktData=(char*)ip;
    ip_hdr_len=(unsigned short)(pktData[0]&0x7);
    src_ip=ip->saddr;
    des_ip=ip->daddr;
    int pos;
    tempPkt.assign(pktData,len);
    pos=tempPkt.find("teststring",0);
        tempPkt.replace("teststring");
    pktData=(char*)tempPkt.c_str();  
            tcp = (struct tcphdr*) (pktData + (4*ip_hdr_len));

    cout<<"*********************************************************************************";
    cout<<ip_hdr_len;
    cout<<"\ntcp checksum: "<<tcp->check;
    cout<<"\nip packet length: "<<ip->tot_len;
    cout<<"\nip packet length_calc: "<<len;
    cout<<"\nip source address: "<<src_ip;
    cout<<"\nip destination address: "<<des_ip;
    cout<<"*********************************************************************************";
    tcp->check=0; 
tcp_len=len - (4*ip_hdr_len);
    chksum = tcp_sum_calc(tcp_len, (unsigned short *) &src_ip, (unsigned short *) &des_ip, (unsigned short *) &tcp);
    tcp->check=chksum;
    cout<<"\nnew checkksum: "<<chksum;
return nfq_set_verdict(qh, id, NF_ACCEPT, len, (unsigned char*)pktData);

}   
return nfq_set_verdict(qh, id, NF_ACCEPT, len, (unsigned char*)pktData);
}

Here is the output:

5
tcp checksum: 11687
ip packet length: 54017
ip packet length_calc: 467
ip source address: 1719453657
ip destination address: 2569775296
new checkksum: 36507

`

Answer 1

It looks like you're not correcting the endian-ness of the data nor the pseudo-header. The raw data is big-endian but Intel architectures are little-endian. Thus, you need to swap the bytes of the words -- you can use ntohs() -- before doing a direct add.

See this example .