UDP Health Monitoring with fixed SOURCE port?

The external monitor is a script running on the LTM, called by the external monitor configured to your pool. I suppose with receive-only traffic you will have to be careful with your interval and timeout settings. This script should get you started:

!/usr/bin/perl
use strict;
use warnings;
use IO::Socket::INET;
my $port   = getservbyname 'bootps', 'udp'; 
my $socket = IO::Socket::INET -> new (LocalPort  => $port, 
                                      Broadcast  =>  1,
                                      Proto      => 'udp')
             or die "Failed to bind to socket: $@";
my $mess;     
while ($socket -> recv ($mess, 1024)) {  
    print "Saw a bootp request.\n";
}