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.