/PRIMARY=([NO]DOMAIN:do,[NO]FILE:file)

    Optional. Default: None.

    Specifies the primary server for the specified zone. Multiple
    primary servers are allowed if each is associated with a
    different domain.

    o  DOMAIN keys to a particular domain.

       NODOMAIN deletes the entry.

    o  FILE specifies the domain to be served.

       If you do not specify a file, the default file name is created
       from the value that you supply with the DOMAIN option.

       NOFILE specifies that no file is created.