Stephanie Wehner > NetPacket-0.04 > NetPacket::UDP

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Module Version: 0.04   Source  

NAME ^

NetPacket::UDP - Assemble and disassemble UDP (User Datagram Protocol) packets.

SYNOPSIS ^

  use NetPacket::UDP;

  $udp_obj = NetPacket::UDP->decode($raw_pkt);
  $udp_pkt = NetPacket::UDP->encode($ip_obj);
  $udp_data = NetPacket::UDP::strip($raw_pkt);

DESCRIPTION ^

NetPacket::UDP provides a set of routines for assembling and disassembling packets using UDP (User Datagram Protocol).

Methods

NetPacket::UDP->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::UDP->encode(param => value)

Return a UDP packet encoded with the instance data specified. Needs parts of the ip header contained in $ip_obj, the IP object, in order to calculate the UDP checksum. The length field will also be set automatically.

Functions

NetPacket::UDP::strip([RAW PACKET])

Return the encapsulated data (or payload) contained in the UDP 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::UDP object consists of the following fields.

src_port

The source UDP port for the datagram.

dest_port

The destination UDP port for the datagram.

len

The length (including length of header) in bytes for this packet.

cksum

The checksum value for this packet.

data

The encapsulated data (payload) for this packet.

Exports

default

none

exportable

igmp_strip IGMP_VERSION_RFC998 IGMP_VERSION_RFC1112 IGMP_MSG_HOST_MQUERY IGMP_MSG_HOST_MREPORT IGMP_IP_NO_HOSTS IGMP_IP_ALL_HOSTS IGMP_IP_ALL_ROUTERS

tags

The following tags group together related exportable items.

:strip

Import the strip function igmp_strip.

:versions

IGMP_VERSION_RFC998 IGMP_VERSION_RFC1112

:msgtypes

IGMP_HOST_MQUERY IGMP_HOST_MREPORT

:group_addrs

IGMP_IP_NO_HOSTS IGMP_IP_ALL_HOSTS IGMP_IP_ALL_ROUTERS

:ALL

All the above exportable items.

EXAMPLE ^

The following script dumps IGMP the contents of IGMP frames to standard output.

  #!/usr/bin/perl -w

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

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

      my $ip_obj = NetPacket::IP->decode(eth_strip($pkt));
      my $udp_obj = NetPacket::UDP->decode($ip_obj->{data});

      print("$ip_obj->{src_ip}:$udp_obj->{src_port} -> ",
            "$ip_obj->{dest_ip}:$udp_obj->{dest_port} ",
            "$udp_obj->{len}\n");
  }

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

The following is an example use in combination with Net::Divert to alter the payload of packets that pass through. All occurences of foo will be replaced with bar. This example is easy to test with netcat, but otherwise makes little sense. :) Adapt to your needs:

#!/usr/bin/perl

use Net::Divert; use NetPacket::IP qw(IP_PROTO_UDP); use NetPacket::UDP;

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

$divobj->getPackets(\&alterPacket);

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

    $ip_obj = NetPacket::IP->decode($data);

    if($ip_obj->{proto} == IP_PROTO_UDP) {

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

        # replace foo in the payload with bar
        $udp_obj->{data} =~ s/foo/bar/g;

        # reencode the packet
        $ip_obj->{data} = $udp_obj->encode($ip_obj);
        $data = $ip_obj->encode;

    }

    $divobj->putPacket($data,$fwtag);
}

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>

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