#!/usr/bin/perl -Tw

use constant EMAIL  =>  'email@yourdomain.com';
use constant SERVER =>  'mail.yourdomain.com';

use Net::PcapUtils;
use Net::SMTP;
use NetPacket::ARP;
use NetPacket::Ethernet qw/:types/;
use strict;

my %ether;

my $mail = Net::SMTP->new(SERVER);

Net::PcapUtils::loop( 
    sub {
        my ($arg, $header, $packet) = @_;
        my $ethernet = NetPacket::Ethernet->decode($packet);
        if ($ethernet->{'type'} == ETH_TYPE_ARP) {
            my $arp = NetPacket::ARP->decode($ethernet->{'data'}, $ethernet);
            my $ip = join '.', map { hex } ($arp->{'spa'} =~ /([[:xdigit:]]{2})/g);
            my $mac = join ':', ($arp->{'sha'} =~ /([[:xdigit:]]{2})/g);
            if ((exists $ether{$ip}) && ($ether{$ip} ne $mac)) {
                $mail->to(EMAIL);
                $mail->mail('root@yourdomain.com');
                $mail->data;
                $mail->datasend("There has been a change in the MAC address associated with IP address ", $ip, "!\n\n");
                $mail->datasend("    Previous hardware MAC address -> ", $ether{$ip}, "\n");
                $mail->datasend("    Current hardware MAC address -> ", $mac, "\n");
                $mail->dataend;
                $mail->quit;
            }
            $ether{$ip} = $mac;
        }
    }, 
    'DEV'   =>  'eth0'
);