SNMP traps that had previously worked fine are now showing UNKNOWN for the sender IP.
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.
Article ID: 886
Created On: Tue, Jan 19, 2021 at 9:51 AM
Last Updated On: Tue, Jan 19, 2021 at 10:15 AM
Authored by: rspielman
Online URL: https://support.nagios.com/kb/article/snmp-traps-are-now-showing-unknown-for-the-sender-ip-886.html