In my last DNS tip, DNS Tips #2, I talked a bit about how Active Directory relies on DNS to function properly. DNS servers must provide support for Service Location (SRV) resource records to specify the location of services.

SRV records are used to map the name of a service to the name of a server offering that service. Active Directory clients and domain controllers will use the SRV records to determine the IP addresses of domain controllers. I also mentioned a file called Netlogon.dns. When you perform DCPROMO on a server, the process creates a text file that contains all the DNS resource records for the domain controller. This file is called Netlogon.dns and is located in the %systemroot%\System32\config folder. Netlogon.dns contains the records needed to register the resource records of the DC. The following will describe the resource records created and what they are used for. Knowing this can help troubleshoot DNS related problems you may encounter in your Active Directory domain.

• _ldap._tcp.[DNSDomainName] - This record is used by clients to locate a DC in the domain [DNSDomainName]. For example my client would look to the DNS server for domain controller in my domain by looking for _ldap._tcp.dortoh.ca.
• _ldap._tcp.[SiteName]._sites.[DNSDomainName] - This record is used by clients to locate a DC in the domain and site specified. For example _ldap._tcp.testsite._sites.dortoh.ca would point to a DC in the Testsite site of my domain.
• _ldap._tcp.pdc._ms-dcs.[DNSDomainName] - This record is used by clients to locate the PDC Emulator FSMO role holder. Only the PDC Emulator FSMO role holder will have a record.
• _ldap._tcp.gc._msdcs.[DNSTreeName] - This record is used by clients to find a GC. Only GC's tree will register this name. When you remove the GC from a DC, the server will unregister this resource record.
• _ldap._tcp. ._sites.gc._msdcs.[DNSTreeName] - This record is used by clients to find a GC in the site.
• _ldap._tcp.[DomainGuid].domains._msdcs.[DNSTreeName] - This record is used by clients to find a DC based on the DC's GUID.  
• [DNSDomainName] - This record is used by clients to find a DC with its A(HOST) record.