How is DNS outage determined?

OpenNMS 27.0.2 in running on the same CentOS 8 server that provides DNS caching for the network.

OpenNMS shows DNS as being down although it is working fine. I tested using nslookup from a workstation and from the servers console and both were successful?

I have another CentOS 8 server in a remote location connected by a VPN. It also runs the same DNS caching for the remote network segment. OpenNMS shows the remote servers DNS as being up but this local servers DNS still shows as down.
Where can I look to find the source of the problem? Log files? Config files? I am not sure where to start since DNS seems to be working fine on this network segment. I don’t understand why OpenNMS tells me it is down.

So the solution was to delete the node and run a discovery to add it back. No idea why it just decided to stop reporting the DNS status correctly. :man_shrugging: