I am writing a heartbeat script to test whether or not our remote systems are up and running. The system consists of a firewall with two public IP's and a server behind the firewall. I need to be able to determine the status of each connection and if the server behind the firewall is responsive. Since the firewall is set to drop ICMP packets I had to get a little creative with the check. So I used the Net::Ping module to 'ping' the firewall by attempting a connection to the firewall's management website over https. If the connection is refused, the connection is up (vs no reply = down). I attempted this same thing using http, which should be forwarded through the firewall to the server, but this always came back as alive, even on connections I know are down. Since I also use SNMP to monitor the server, I thought I'd just have try to pull a simple OID from the server. If it works, the server is responsive, if not then it isn't. The problem I'm hitting is that because of a limitation with the firewall, I can't pass udp packets over port 161 through the firewall, so I have to use port 163 then nat them over to 161 on the backside of the firewall. This is all well and good, but I can't seem to get Net::SNMP to use port 163, or if it is, It's just not working. Here is my code:

#!/usr/bin/perl

use warnings;
use strict;
use SNMP;
use Socket;


my $host = 'xxx.xxx.xxx.xxx';
my $oid = '1.3.6.1.2.1.1.4';
my $sver = '1';
my $comm = 'secret';

my $session;
my $reply;


&SNMP::initMib();


my %snmpparms;
$snmpparms{Community} = $comm;
$snmpparms{DestHost} = inet_ntoa(inet_aton($host));
$snmpparms{Version} = $sver;
$snmpparms{UseSprintValue} = '1';
$snmpparms{RemotePort} = '163';

$session = new SNMP::Session(%snmpparms);

$reply = $session->get($oid);

if ($session->{ErrorNum}) {
  die "DOOM!: ".$session->{ErrorStr}."\n";
}

print "$reply\n";
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running this gives:

DOOM!: Timeout
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I know the server is reachable over port 163, as I'm running other monitoring software and I've been able to get this to work in php. So I guess my question is two-fold. How do I get snmp to work over port 163 and is there a better way to approach this? Possibly make a socket connection with the server over port 80, send some headers and check the reply?