How to Locate an Active Directory Domain Controller
To find an Active Directory domain controller in a domain or forest, a client must query the Domain Name Service (DNS) for the SRV and DNS resource records of the domain controller. The resource records provide the client with the names and IP addresses of the domain controllers. In this context, the SRV and resource records are referred to as Locator DNS resource records.
For more information on DNS support for Active Directory, see How DNS Support for Active Directory Works.