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>