The Sign in process can be broken down into parts. First there is the discovery process followed by authentication process. This post will address just the Discovery Process (Authentication Process here TBA)


Discovery process started with DNS SRV records and this is now considered the legacy method, from Lync 2013 (and the mobile clients) lyncdiscover is used. The desktop clients support both methods (currently)

Below is a trace of the current method (Lync 2013)

  • LyncDiscover simply searches for the internal record and if that fails it tries the external record

    Below is a trace of the legacy sign in method (with a sneaky issue to illustrate the resilience)

    • DNS and SRV
      1. SRV _sipinternaltls._tcp
      2. SRV _sip._tls
      3. A sipinternal
      4. A sip
      5. A sipexternal
    In this trace above you will notice that the SRV record although being present (all be it incorrect) didn’t stop the fail over to the next discovery record