This topic gives a brief description of how DNS queries work in Office Communications Server 2007 R2.

During DNS lookup, SRV records are queried in parallel and returned in the following order to the client:

  1. _sipinternaltls._tcp. <domain>— for internal TLS connections.

  2. _sipinternal._tcp. <domain>— for internal TCP connections (performed only if TCP is allowed).

  3. _sip._tls. <domain>— for external TLS connections.

Where <domain>is the SIP domain used by your internal clients.

The last query is useful when clients are connecting from outside your internal network. For details about remote user access, see Planning for External User Accessin the Planning and Architecture documentation.

The client uses the SRV record that is returned and is successful. The client does not try any other SRV records.

After the SRV record is returned, a query is performed for the DNS A record for the host name that is returned by the SRV record. If no records are found during the DNS SRV query, the client performs an explicit lookup of sipinternal. <domain>. If the lookup does not produce results, the client performs a lookup for sip. <domain>. If the client does not find sip. <domain>, it performs a lookup for sipexternal. <domain>.

If your DNS infrastructure prohibits configuration of these DNS records, you can manually edit the client registry to point to the appropriate home server. For details about editing the client registry and configuring policy settings for the client, see Deploying Communicatorin the Client Planning and Deployment documentation.