NetPacket::TCP(3)

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>