DNS/RFC/1996について、ここに記述してください。

https://tools.ietf.org/html/rfc1996

A Mechanism for Prompt Notification of Zone Changes (DNS NOTIFY)


3.3. NOTIFY is similar to QUERY in that it has a request message with
   the header QR flag "clear" and a response message with QR "set".  The
   response message contains no useful information, but its reception by
   the master is an indication that the slave has received the NOTIFY
   and that the master can remove the slave from any retry queue for
   this NOTIFY event.