Unable to log into Lync Server 2013 Edge server with the tracing error: “SIP/2.0 504 Server time-out”

Problem

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) [0]0DD4.0988::10/31/2014-17:50:04.235.0000549b (SIPStack,SIPAdminLog::ProtocolRecord::Flush:ProtocolRecord.cpp(196))[1125246962] $$begin_recordTrace-Correlation-Id: 1125246962
Instance-Id: 28
Direction: outgoing;source=”local”;destination=”external edge”
Peer: 99.17.994.228:39692
Message-Type: response
Start-Line: SIP/2.0 504 Server time-out
From: <sip:a-tluk@contoso.com>;tag=fee610818a;epid=f836970f32
To: <sip:a-tluk@contoso.com>;tag=A333237A7528FFD6CFCB2CA74178CF12
Call-ID: f1860a5c80e841288095c15c59f6bb0e
CSeq: 1 REGISTER
Via: SIP/2.0/TLS 172.16.1.124:53555;received=69.17.194.228;ms-received-port=39692;ms-received-cid=E00
Content-Length: 0

$$end_record

Reviewing the event logs also displays the following warning:

Log Name: Lync Server
Event ID: 14518
Source: LS Protocol Stack
Level: Warning

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.

Solution

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.