Hi,
I am trying to get the TCP sequence and ack number of TCP packets. Somehow I
get different values than "tcpdump -vv" does. The numbers are way too big
all the time. Source and destination ports are just fine. Below the relevant
code. I studied the tcpdump source code but can't find why. Please help, I
am stuck!
Thank you,
Andrej
#define ETHER_HDRLEN 14
typedef u_int32_t tcp_seq;
struct tcphdr {
u_int16_t th_sport; /* source port */
u_int16_t th_dport; /* destination port */
tcp_seq th_seq; /* sequence number */
tcp_seq th_ack; /* acknowledgement number */
u_int8_t th_offx2; /* data offset, rsvd */
u_int8_t th_flags;
u_int16_t th_win; /* window */
u_int16_t th_sum; /* checksum */
u_int16_t th_urp; /* urgent pointer */
};
static void handle_packet(unsigned char * ifile, const struct pcap_pkthdr *
h, const u_char * sp)
{
const struct ip * ip = (struct ip *) (sp + ETHER_HDRLEN);
const struct tcphdr * tcp_hdr = (const struct tcphdr *)(sp + ETHER_HDRLEN
+ sizeof(struct iphdr));
tcp_seq seq = htonl(tcp_hdr->th_seq);
tcp_seq ack = htonl(tcp_hdr->th_ack);
fprintf(stdout,"seq %u ack %u", seq, ack);
}
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