Most of the DNS queries sent by Windows clients during the location process are for SRV (service location) records. The SRV record, introduced in RFC 2052 and updated in RFC 2782, is a general mechanism for locating services. Before we can talk in detail about exactly how a Windows client finds its domain controller using SRV records, we need to describe the SRV record itself.
Locating a service or a particular type of server within a zone is a difficult problem if you don't know which host it runs on. Some administrators have attempted to solve this problem by using service-specific aliases in their zones. For example, at Movie U. we created the alias ftp.movie.edu and pointed it to the domain name of the host that runs our FTP archive:
ftp.movie.edu. IN CNAME plan9.fx.movie.edu.
This makes it easy for people to guess a domain name for our FTP archive and separates the domain name people use to access the archive from the domain name of the host on which it runs. If we were to move the archive to a different host, we could simply change the CNAME record.
Another option, for clients that understand it, is the SRV record. In addition to simply allowing a client to locate the host on which a particular service runs, SRV provides powerful features for load balancing and backup services, similar to what the MX record provides.
A unique aspect of the SRV record is the format of the domain name to which it's attached. Like the service-specific alias ftp.movie.edu ...