This article provides Citrix field engineers, consultants, partners, and administrators with details on enabling Lync/Skype Autodiscovery Support within the Citrix HDX RealTime Optimization Pack 1.8. The Autodiscovery feature was not documented in the initial release of 1.8 as it was an experimental capability pending field validation. It is now fully supported as of the 1.8.100 release (September 2015). 

Customers might require Autodiscovery enabled in order to solve challenges within their current Lync/Optimization Pack implementations. One such issue it resolves is if the customer has a mixed Lync 2010 and 2013 environment. Certain users may be redirected from Lync 2010 environments to Lync 2013 and using Autodiscovery enables the HDX RealTime Optimization Pack to handle this properly.

Background of Autodiscovery

The Autodiscovery service (along with other mobility web services) was introduced in the Cumulative Update for Lync Server 2010, in November 2011. While it was primarily used by mobile clients (Android, iOS, etc.) in Lync 2010 to discover their home services, it is now the default method for Lync 2013 and Skype for Business 2015 clients. Some of the reasons for the Autodiscovery service include:

• Because SIP is an extremely chatty protocol, mobile clients are not full SIP clients. To preserve battery, it was much for efficient for mobile clients to obtain information about the Lync service and interact with Lync through web services.

• The Autodiscovery service provides both internal and external URL's in order to provide a consistent user experience.  As users switch between private (internal Wi-Fi) and public (4G) networks on the fly, they will be able to connect to their Lync services.

• Unnecessary SIP traffic never reaches the Lync pool servers (incorrect URI).

Autodiscovery Process

DNS lookup logic for the Lync clients use a specific order of resolution. The Autodiscovery process introduces two new A records which must be added: lyncdiscoverinternal@domain.com and lyncdiscover@domain.com. If these cannot be resolved, full Lync clients will fall back to the SRV/A record method while mobile clients will fail. The following are the steps for the full Lync client.

When the published Lync client is first started and is configured to use automatic configuration, it uses the domain portion of the SIP URI provided by the user (john.doe@citrix.com) to discover which Lync server it should sign into. The client attempts to resolve the following DNS SRV records in order:

lyncdiscoverinternal@<sipdomain> (443)
lyncdiscover@<sipdomain> (443)
_sipinternaltls._tcp.<sipdomain> (5061)
_sipinternal._tcp.<sipdomain>(5060)
_sip._tls.<sipdomain> (443)
_sip._tcp.<sipdomain> (5060)

If the client fails on any of the preceding records, it then tries the following A records:
sipinternal.<sipdomain> (5061 and 443)
sip.<sipdomain> (5061 and 443)
Sipexternal.<sipdomain> (5061 and 443)

Enabling Autodiscovery on the HDX RealTime Optimization Pack

In 1.8 Citrix introduced web service-based Lync server autodiscovery. The new autodiscovery mechanism is not currently enabled by default; in the 1.8.100 release it needs to be enabled using a registry setting.

Value name: EnableHTTPAutodiscovery 
Type: REG_DWORD 
Data: 1 to enable; 0 or remove value to disable