The following knowledgebase will tell you the procedure you can use to retrieve the DNS name of a domain from registry.
The following registry location is the best place to find the DNS name of a domain controller.
In the right pane, you will see an entry by name DCName=
The above entry will contain the DNS name of the domain. This DNS name of domain is stored in registry key after Winlogon retrieves the Domain controller by using the DcGetDCName API call.