NOTIFY
Originally, slave DNS servers would poll only the master server as set by the zone's refresh parameter. Some people have been unhappy with that, so the NOTIFY extension to the DNS protocol was defined in RFC 1996. Using this protocol, a zone's master can notify the zone's slaves of changes in the zone. So, instead of waiting for a few hours for a refresh cycle to start, the zone update propagates to the slave servers pretty quickly. I say "pretty quickly" because after a zone has been updated, your DNS server waits for a random number of seconds before sending the NOTIFY. This avoids being bombarded by SOA and zone transfer requests all at the same time, and the delay distributes the queries from the slaves in time.
A master server notifies ...
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