RFC 2782:A DNS RR for specifying the location of s...
RFC-Ref

root


Click on the red underlined text to get to the source

... service is added, and makes it possible to move standard services out of the "root-only" port range on unix. ...


... SRV RR, and its Target is "." (the root domain), abort. Else, for all such RR ...


... $ORIGIN example.com. @ SOA server.example.com. root.example.com. ( 1995032001 3600 3600 604800 86400 ) NS ...



Google
Web
RFC-Ref