SNMP traps are now showing UNKNOWN for the sender IP

Issue

SNMP traps that had previously worked fine are now showing UNKNOWN for the sender IP.

Solution

There is a bug (https://sourceforge.net/p/snmptt/bugs/46/) in version 1.4.2 of the snmptt daemon that your server is running which is causing it to not forward the traps to Nagios.

If this is the cause of your issue, here is how to fix it.

Edit this file on your server: /usr/sbin/snmptt
change line 3291 from:

if ($tempvar[0] =~ /<UNKNOWN>/) {

to:

if ($tempvar[0] =~ /UNKNOWN/) {

Save it and restart the snmptt daemon.

After this, the traps should be received and display the IP address.

Another possible solution is to replace $r with $aA - Trap agent IP address.