Cisco CUCM v12 – DNS Unreachable

How to fix DNS Unreachable on Cisco CUCM v12

The OS administration dashborad of CUCM v12 will notify you if DNS is not reachable, see screenshot below.

Where to find the DNS IP Address?

Using the web GUI of CUCM v12 you must first log into OS Administration, then go to Show/Network.

Cisco CUCM v12 – OS Administration – Show Network

All the DNS related information (Primary/Secondary IP addresses, Domian Name) will be shown at the bottom of the page, as shown as follows.

Cisco CUCM v12 – DNS details

DNS settings from the CLI

As you can imagine there is also a useful CLI command that can be used to retrieve information related to DNS on your Cisco CUCM v12.

show network eth0 detail
CUCM v12 – show network eth0 detail

Verify your DNS server is properly working

There are two tests you must do on your DNS server, one to test the direct entry and another to test the reverse entry, see screenshots below.

nslookup cucm.tdtlab.com
Linux Centos 7 – BIND DNS server – direct entry test
nslookup 192.168.0.169
Linux Centos 7 – BIND DNS server – reverse entry test

Verify connectivity between CUCM and DNS

To be absolutely sure that connectivity is fine you must test DNS on both directions, from DNS to CUCM and the other way around.

Ping from DNS server to CUCM

ping from DNS to CUCM
utils network ping 192.168.0.21

Ping from CUCM to DNS via CLI

Ping from CUCM to DNS from the Web GUI

Cisco CUCM v12 – OS Administration – Services/Ping
Cisco CUCM v12 – Ping to DNS server using the web GUI

Change IP Address of the Primary DNS

In this particular case, the fix comes down simply to changing the IP Address of the DNS server.

set network dns primary 192.168.0.21
Cisco CUCM v12 – changing DNS server IP Address

Verify new DNS IP Address was successfully changed

show network eth0 detail
Cisco CUCM v12 – New IP address

Test DNS from the CUCM web GUI

utils network host 192.168.0.169
Test DNS direct entry
utils network host cucm.tdtlab.com
Test DNS reverse entry

Problem Solved

The OS Administration will automatically remove the red warning as soon a the new DNS is configured and your are now good to go.

Cisco CUCM v12 – OS Administration clean dashboard