NetPacket::TCP(3p)

NAME
NetPacket::TCP - Assemble and disassemble TCP (Transmission Control Protocol) packets.

VERSION
version 1.7.2

SYNOPSIS
  use NetPacket::TCP;

  $tcp_obj = NetPacket::TCP->decode($raw_pkt);
  $tcp_pkt = $tcp_obj->encode($ip_pkt);
  $tcp_data = NetPacket::TCP::strip($raw_pkt);

DESCRIPTION
"NetPacket::TCP" provides a set of routines for assembling and disassembling packets using TCP (Transmission Control Protocol).

  Methods

"NetPacket::TCP->decode([RAW PACKET])"
    Decode the raw packet data given and return an object containing instance data. This method will quite happily decode garbage input. It is the responsibility of the programmer to ensure valid packet data is passed to this method.

"NetPacket::TCP->encode($ip_obj)"
    Return a TCP packet encoded with the instance data specified. Needs parts of the ip header contained in $ip_obj in order to calculate the TCP checksum.

"$packet->parse_tcp_options"
    Returns a hash (or a hash ref in scalar context) containing the packet's options.

    For now the method only recognizes well-known and widely used options (MSS, noop, windows scale factor, SACK permitted, SACK, timestamp). If the packet contains options unknown to the method, it may fail.

  Functions

"NetPacket::TCP::strip([RAW PACKET])"
    Return the encapsulated data (or payload) contained in the TCP packet. This data is suitable to be used as input for other "NetPacket::*" modules.

    This function is equivalent to creating an object using the decode() constructor and returning the "data" field of that object.

  Instance data
The instance data for the "NetPacket::TCP" object consists of the following fields.

src_port
    The source TCP port for the packet.

dest_port
    The destination TCP port for the packet.

seqnum
    The TCP sequence number for this packet.

acknum
    The TCP acknowledgement number for this packet.

hlen
    The header length for this packet.

reserved
    The 6-bit "reserved" space in the TCP header.

flags
    Contains the urg, ack, psh, rst, syn, fin, ece and cwr flags for this packet.

winsize
    The TCP window size for this packet.

cksum
    The TCP checksum.

urg The TCP urgent pointer.

options
    Any TCP options for this packet in binary form.

data
    The encapsulated data (payload) for this packet.

  Exports

default
    FIN SYN RST PSH ACK URG ECE CWR Can be used to set the appropriate flag.

exportable
    tcp_strip

tags
    The following tags group together related exportable items.

    ":strip"
        Import the strip function "tcp_strip".

    ":ALL"
        All the above exportable items.

EXAMPLE
The following script is a primitive pop3 sniffer.

  #!/usr/bin/perl -w

  use strict;
  use Net::PcapUtils;
  use NetPacket::Ethernet qw(:strip);
  use NetPacket::IP qw(:strip);
  use NetPacket::TCP;

  sub process_pkt {
      my($arg, $hdr, $pkt) = @_;

      my $tcp_obj = NetPacket::TCP->decode(ip_strip(eth_strip($pkt)));

      if (($tcp_obj->{src_port} == 110) or ($tcp_obj->{dest_port} == 110)) {
          print($tcp_obj->{data});
      }
  }

  Net::PcapUtils::loop(\&process_pkt, FILTER => 'tcp');

The following uses NetPacket together with Net::Divert to add a syn flag to all TCP packets passing through:

  #!/usr/bin/perl

  use Net::Divert;
  use NetPacket::IP qw(IP_PROTO_TCP);
  use NetPacket::TCP;

  $divobj = Net::Divert->new('yourhostname',9999);

  $divobj->getPackets(\&alterPacket);

  sub alterPacket {
      my($packet,$fwtag) = @_;

      # decode the IP header
      $ip_obj = NetPacket::IP->decode($packet);

      # check if this is a TCP packet
      if($ip_obj->{proto} == IP_PROTO_TCP) {

          # decode the TCP header
          $tcp_obj = NetPacket::TCP->decode($ip_obj->{data});

          # set the syn flag
          $tcp_obj->{flags} |= SYN;

          # construct the new ip packet
          $ip_obj->{data} = $tcp_obj->encode($ip_obj);
          $packet = $ip_obj->encode;

      }

      # write it back out
      $divobj->putPacket($packet,$fwtag);
   }

TODO

Assembly of TCP fragments into a data stream
Option processing
Nicer processing of TCP flags

COPYRIGHT
Copyright (c) 2001 Tim Potter and Stephanie Wehner.

Copyright (c) 1995,1996,1997,1998,1999 ANU and CSIRO on behalf of the participants in the CRC for Advanced Computational Systems ('ACSys').

This module is free software. You can redistribute it and/or modify it under the terms of the Artistic License 2.0.

This program is distributed in the hope that it will be useful, but without any warranty; without even the implied warranty of merchantability or fitness for a particular purpose.

AUTHOR
Tim Potter <tpot@samba.org>

Stephanie Wehner <atrak@itsx.com>