I have a small install of PRTG I have been running in a lab environment for evaluation for potential purchase for a much larger network. I have approximately 20 or so sensors.

In a workgroup, using Linux DNS server. DNS is working, has worked, and works well on the network.

Started to get Ping alerts basically due to DNS Lookup failures and I have no idea why. It isn't all the time, but sometimes. I finally decided to look into it more today. For those sensors I am currently getting active DNS lookup failures (Error 11004), I can execute an nslookup without issue directly from the command line on the server PRTG.

As I was troubleshooting, the ONLY thing I changed was the local probe IP from auto to the actual static IP address. Within about 5 minutes, I got SNMP errors on all of my switchports that have been working just fine up until now.

Getting no response errors. So I changed IP Address for probe back to auto, no change.

Downloaded the SNMP tester and here is the interesting thing - I am able to walk the switch when I use the IP Address, but it says hostname lookup failed when I try to walk by hostname. I am running this from the PRTG server and when it fails lookup, I can again open CMD and complete a successful nslookup.

Does anyone know why this is happening?

Edit: Update - I realized on the tester, I was only trying to connect using hostname - when I added the suffix (not on a domain, on a workgroup, but with the suffix wrkgrp.miller) it walked just fine.

So, what am I missing here, why is SNMP not working then? I have checked the community string, firewall isn't an issue - pretty basic setup.

Article Comments

Hi there,

PRTG uses the DNS cashes of Windows, so does the SNMP Tester. It's indeed unexpected that it works after adding the suffixes. I'd recommend to start a Wireshark capture on the PRTG host with a filter for UDP packets on port 161. In case of an error it would be interesting to which target IP address the packets are sent and if there are any responds packets arriving.

Best regards, Felix


May, 2018 - Permalink

Yeah, it is very strange. I was able to figure out that if I changed the community string back to public (which I hate to do), everything starts working suddenly. I'm not sure why the problem suddenly happened though as I had been using a custom community string for 6 months without any issue. Also, the custom St worked just fine in the tester so long as I added the suffix.


May, 2018 - Permalink