Accessing TCP flags in TCP packets on Linux using C !!

This is a discussion on Accessing TCP flags in TCP packets on Linux using C !! within the Linux Programming forums, part of the Platform Specific Boards category; Hi to all, i would like some help regarding the following: I'm trying myself at network programming and I want ...

  1. #1
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    Accessing TCP flags in TCP packets on Linux using C !!

    Hi to all,

    i would like some help regarding the following:

    I'm trying myself at network programming and I want to access the TCP flags in a TCP packet. I know that in C programming language, the TCP header can be accessed in the following manner:

    Code:
    const struct ethernet_header *ethernet;
    const struct tcp_header *tcp;
    const u_char *packet;
    ethernet = (struct ethernet_header*) (packet)
    tcp = (struct tcp_header*) (packet + size_ethernet + size_ip)
    We kind of skip past the ethernet frame and IP packet headers. But I'm having problems with how to access the TCP flags (SYN, ACK, URG, FIN...).

    Can anyone help me with this? Any help will be very much appreciated.

    Warm regards,
    Visham

  2. #2
    Just Lurking Dave_Sinkula's Avatar
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    Generally, use bit masking -- something like this.
    Code:
    if ( tcp->th_flags & TH_SYN )
    7. It is easier to write an incorrect program than understand a correct one.
    40. There are two ways to write error-free programs; only the third one works.*

  3. #3
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    Many thx for the reply Dave_Sinkula..i'll try it and see how it goes.

    Warm regards,
    Visham

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