how to print flags in TCP header of raw packets using libpcap

Question

sniffex.c is a program that is based on libpcap , to sniff and display some packet information. How do i modify it so as to print the values of TCP flags - urg , ack , psh , rst , syn and fin ? please help..

Answer 1

If you check this code where sniff_tcp is used, make sure to print out th_flags member of this structure which contians flags you need.

/* TCP header */
    struct sniff_tcp {
        u_short th_sport;   /* source port */
        u_short th_dport;   /* destination port */
        tcp_seq th_seq;     /* sequence number */
        tcp_seq th_ack;     /* acknowledgement number */

        u_char th_offx2;    /* data offset, rsvd */
    #define TH_OFF(th)  (((th)->th_offx2 & 0xf0) >> 4)
        u_char th_flags;
    #define TH_FIN 0x01
    #define TH_SYN 0x02
    #define TH_RST 0x04
    #define TH_PUSH 0x08
    #define TH_ACK 0x10
    #define TH_URG 0x20
    #define TH_ECE 0x40
    #define TH_CWR 0x80
    #define TH_FLAGS (TH_FIN|TH_SYN|TH_RST|TH_ACK|TH_URG|TH_ECE|TH_CWR)
        u_short th_win;     /* window */
        u_short th_sum;     /* checksum */
        u_short th_urp;     /* urgent pointer */
};

Generally you will have something like this (a bit ugly, you can do it better):

//this is already there in the code:
    printf("   Src port: %d\n", ntohs(tcp->th_sport));
    printf("   Dst port: %d\n", ntohs(tcp->th_dport))

//you add:
    if (tcp->th_flags & TH_ECE){
        printf("   Flag: TH_ECE");
    }
    if (tcp->th_flags & TH_RST){
        printf("   Flag: TH_RST");
    }