in reply to Re: Piggybacking data on TCP ACK
in thread Piggybacking data on TCP ACK

Thanks for replying, Let me explain why i want to simulate the data along with Ack packet.I have seen some web clients which Piggyback HTTP GET request in the Ack packet, I want to simulate that to test a proxy.I am very sure this is allowed as per TCP RFC.If anybody knows any trick other than Raw sockets and creating packets please let me know

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Re^3: Piggybacking data on TCP ACK
by ikegami (Patriarch) on Jun 16, 2011 at 22:08 UTC

    If anybody knows any trick other than Raw sockets and creating packets please let me know

    You *will* need to use a raw socket since you don't want to use your system's TCP driver. The only question is whether there exists something that will create the packets for you or not.

Re^3: Piggybacking data on TCP ACK
by zek152 (Pilgrim) on Jun 17, 2011 at 18:52 UTC

    I think we are using the same terminology to refer to 2 different things. After a connection is made:

    A B SYN------> <--------SYN+ACK ACK------>

    Then the ACK flag (or bit) is kept high for all subsequent packets. I used the term ACK to mean specifically the packet sent in response to a SYN+ACK packet, I did not mean to use it to refer to any packet with the ACK flag set.

    You are correct that data is allowed to be sent during the 3 way handshaking provided that it remains in a buffer until the connection is established (from RFC 793). So the data does (should) not be passed up the stack until the connection is made.

    I currently do not have access to a computer with the libpcap library but this code should help get you started towards your goal.

    use Net::RawIP; $ack_with_data = Net::RawIP->new({ ip => { saddr => '111.111.111.111', daddr => '111.111.111.112' }, }); tcp => { source => 111, dest => 111, ack => 1, #insert the correct seq number and ack seq number seq => 11111, ack_seq => 11111, #insert http data here (i am not very familiar with http) data => "GET /asdlfk HTTP/1.1\nHost: asd.asd.com\n", }, }); $ack_with_data->send;

    Disclaimer: This code might be missing some things. I was not able to test it but it should help you see how to form a packet and send it. The specific fields will need to be correctly set and I might be missing a few.