After upgrading your UC environment from 10.5 to 11.5 or 12 you may receive an error when logging into your CUCM Publisher server dashboard “DNS Unreachable”.

DNS Unreachable

After upgrading, your CUCM Version will do 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. In previous versions CUCM server a check would be made 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 and above the CUCM now also continuously checks against the Reverse Lookup Zone for mismatches and displays a warning message on the GUI.

To resolve the DNS unreachable issue you will need to access the Reverse Lookup zone on your DNS server for the CUCM subnet. Look for any PTR records that have the CUCM FQDN. There should only be one. However, if there are two PTR records with the same FQDN Name, CUCM will throw a DNS warning message. Simply remove the PTR record that has the incorrect IP Address associated with it.

There could be a few other reasons, but all to do with DNS configuration. Things to check –

  • Confirm A records are in FQDN format
  • Confirm PTR records are in FQDN format
  • Confirm DNS servers have replicated and all records are up to date
  • Confirm correct DNS servers are being queried
  • Ensure only one a single type of record exists for each UC server

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