NetPacket::UDP(3p)

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($ip_obj)"
    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
    udp_strip

tags
    The following tags group together related exportable items.

    ":strip"
        Import the strip function "udp_strip".

    ":ALL"
        All the above exportable items.

EXAMPLE
The following example prints the source IP address and port, the destination IP address and port, and the UDP packet length:

  #!/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>