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

       Tim Potter <tpot@samba.org>

       Stephanie Wehner <atrak@itsx.com>

perl v5.36.0                                       2022-11-19                                NetPacket::TCP(3pm)

       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.

       "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.

   Instancedata
       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.

       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);
          }

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

         use NetPacket::TCP;

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

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

       version 1.7.2