Another “Cannot Communicate with Server” error when logging into Cisco Jabber. This error can lead to many possibilities of where the actual error lies. You will need to do some digging. I come across a doozy and have documented below. But this relates to the newer version on Expressway 8.8+ which now also queries reverse DNS zones to confirm operations. If PTR records are missing or mis-configured you will know about it..

The symptoms I received were as follows:

- I could resolve the SRV record and A records
- I could log into Jabber from within the Corporate LAN
- If I typed an incorrect password, I would receive “Username or Password is incorrect” message
- If I typed in the correct password, I would receive the infamous “Cannot Communication with Server” error.

Versions I’m running:

- CUCM/IMP version 11.0.1.21900
- Expressway version 8.8.1

Processes and Solution (to this particular scenario):

- Expressway-E Event Log showing Service Unavailable – 503 error.
- Set the ‘network’, ‘network tcp’ and ‘network sip’ diagnostic logging to DEBUG.
- Attempted to log into Jabber. Captured and Downloaded logs.
- Analysed log file and found this line. “Detail=Certificate verification failed for host=X.X.X.X, additional info: Invalid Hostname cor8-ppp2557.per.dsl.connect.net.au”
- Discovered a PTR record for my exact Expressway-E’s A Record had been created somewhere on the Web, and when the Expressway-C was actioning a Reverse DNS Lookup Query, this PTR record was being found. What are the chances hey??
- I had to create a Reverse Lookup Zoen for the Expressway-E Public IP Address, add the correct PTR record. Once I flushed DNS.. Everything started to work fine and I could log into Jabber remote.

**Leaving Thoughts.. As I’ve also found in CUCM version 11.5, reverse DNS plays a very important role now.. Make sure all A records have PTR records and only one!