Modifying Packets (continued)

• When modifying TCP packets, the size can't change

  • TCP is a reliable protocol

  • Modifying the data size without modifying the synchronization counters introduces a discrepancy

  • It's possible to track a stream and modify counters as well as the payload,

  • ...but it's non-trivial, and there are no existing modules to help

• Modifying packets, not streams, and will fail on pattern matches that span multiple packets

• The slow-write server:

    use IO::Socket;

    my $server = IO::Socket::INET->new
        (LocalPort => 4242, Reuse => 1, Listen => 1);

    my $client = $server->accept;

    for (split //, "Hello, FOO\n") {
        print $client $_;
        $client->flush;
        sleep 1;
    }

• The client:

    [mct@ellesmere ~]$ telnet quint 4242
    Trying 207.8.132.210...
    Connected to quint.netisland.net.
    Escape character is '^]'.
    Hello, FOO