I am stuck trying to access the TCP header from the callback function
of my nfq_q_handle. I have packets coming in, and can access the IP
header without any problem, but I am not getting the correct TCP
destination port. Here is the section of code I am having problems
with. I am guessing I am setting the pointer tchp wrong. I have done
this before in the kernel with sk_buffs, but the method for accessing
the TCP header was a little different.
struct iphdr *iph = NULL;
struct tcphdr *tcph = NULL;
int ret;
char *payload;
ret = nfq_get_payload(nfa, &payload);
if (ret >= 0)
printf("payload_len=%d ", ret);
iph = (struct iphdr *) payload;
if ((iph->protocol == IPPROTO_TCP) ){
printf("packet is a TCP packet\n ");
}
tcph = (struct tcphdr *) (((u_int32_t *)payload) + (iph->ihl * 4));
//this must initialize to the wrong byte
printf("iph length is %u\n",iph->ihl); //this works
printf("dest port is %u\n",tcph->dest); //this displays the wrong ports
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