You’ve noticed that users are unable to sign into their Lync Client on their laptops when not in the internal network but the Edge server services are up. Performing a trace and reviewing the snooper logs reveals the following:
SIP/2.0 504 Server time-out
TL_INFO(TF_PROTOCOL) 0DD4.0988::10/31/2014-17:50:04.235.0000549b (SIPStack,SIPAdminLog::ProtocolRecord::Flush:ProtocolRecord.cpp(196)) $$begin_recordTrace-Correlation-Id: 1125246962
Direction: outgoing;source="local";destination="external edge"
Start-Line: SIP/2.0 504 Server time-out
CSeq: 1 REGISTER
Via: SIP/2.0/TLS 172.16.1.124:53555;received=188.8.131.52;ms-received-port=39692;ms-received-cid=E00
Reviewing the event logs also displays the following warning:
Log Name: Lync Server
Event ID: 14518
Source: LS Protocol Stack
The server configuration validation mechanism detected some potential problems. The server might not behave as expected.
1 warnings were detected:
The FQDN [lyncstd01.contoso.com] is a configured server but does not resolve by DNS host name query. This may be a temporary condition.
To confirm the issue is DNS, log onto the Edge server and perform a nslookup to try resolving the Lync back-end server:
If you are unable to resolve the Lync back-end server, either configure your Edge server(s) to be able to resolve the Lync back-end server via DNS or create a record in the host file on the Edge server.
I understand that the warning error makes it pretty obvious this is a Edge unable to resolve the back-end Lync server IP address but thought I’d blog this in case someone overlooks this warning but manages to perform a trace and see the same error as shown at the beginning of this post.