Recently upgraded a UC environement from 10.5 to 11.5. After the upgrade was complete, the CUCM Publisher server displayed a “DNS Unreachable” error upon logging into the Administration webpage.
This warning message only appeared on the CUCM Publisher and not the subscriber etc. I confirmed DNS had been configured identically for both the publisher and subscriber. I read that CUCM Version 11.x no does a couple of more network tests that have a direct impact on warning/error messages being displayed on the GUI. One of these tests is against the DNS Reverse Lookup Zone. Pre 11.x versions.. The CUCM server would check to see if a PTR record existed for the hostname, in fact this is required for CUCM installation process to succeed. In version 11.x the CUCM now also continuously checks against the Reverse Lookup Zone for mismatches and displays a warning message on the GUI. In my case, the customer had created an additional A Record for user friendly purposes called myphone.domain.com.au, this automatically created a PTR record. So when CUCM ran checks against the Reverse Lookup Zone, the results displayed a mismatch. Hence the warning was raised and displayed. (Although DNS was functioning perfectly)
Removing this PTR Record resolved the issue with the warning message.
I ran the command “utils diagnose test” to pin point the issue.