Update: I did not realize SNMP is a front end for Net::SNMP. The code below probably won't help.

Run tcpdump on the machine running the SNMP server to see if the NAT is really working and that the traffic really is getting through to port 161.

Check that snmpd is listening and allowing connections from outside localhost. (On Ubuntu/Debian check /etc/default/snmp to see what IP it is binding to. Also check snmpd.conf.)

Original post:

If you can do it just open UDP port 161 on the firewall from the IP of the server doing the monitoring.

I have not tried using a different port but Net::SNMP has functionality to specify the port number. Code I have run (based on the docs as I recall) is below.

use strict;
use Data::Dumper;
use Net::SNMP;

my ($session, $error) = Net::SNMP->session(
      -hostname  => shift || 'localhost',
      -community => shift || 'public',
      -port      => shift || 161 
);

if (!defined($session)) {
    printf("ERROR: %s.\n", $error);
    exit 1;
}

my $sysUpTime = '1.3.6.1.2.1.1.3.0';

my $result = $session->get_request(
      -varbindlist => [$sysUpTime]
);

if (!defined($result)) {
    printf("ERROR: %s.\n", $session->error);
    $session->close;
    exit 1;
}

printf("sysUpTime for host '%s' is %s\n",
       $session->hostname, $result->{$sysUpTime} );

#print Dumper($result);
$session->close;

exit 0;
[download]

Example output is:

$ ./snmp-uptime.pl 172.19.1.2 my_community_string
sysUpTime for host '172.19.1.2' is 263 days, 14:47:49.58
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