NAME

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

SYNOPSIS

  use NetPacket::TCP;

  $tcp_obj = NetPacket::TCP->decode($raw_pkt);
  $tcp_pkt = NetPacket::TCP->encode($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.

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.

  This package is free software and is provided "as is" without express
  or implied warranty.  It may be used, redistributed and/or modified
  under the terms of the Perl Artistic License (see
  http://www.perl.com/perl/misc/Artistic.html)

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

  ACSys makes this software and all associated data and documentation
  ('Software') available free of charge.  You may make copies of the
  Software but you must include all of this notice on any copy.

  The Software was developed for research purposes and ACSys does not
  warrant that it is error free or fit for any purpose.  ACSys
  disclaims any liability for all claims, expenses, losses, damages
  and costs any user may incur as a result of using, copying or
  modifying the Software.

AUTHOR

Tim Potter <tpot@samba.org>

Stephanie Wehner <atrak@itsx.com>

C - Assemble and disassemble TCP (Transmission Control Protocol) packets.