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Suppose that the zone statement contains a list of nameservers from which zone data can be obtained.
These need not be masters in the sense defined above: it is possible (and sometimes necessary) for a slave to obtain zone data from another slave.
It will not then be able to start serving the zone again until it has performed a zone transfer, and if the master is unavailable for any reason then the period of downtime could be substantial.
The example shows DDNS for three zones: the first disables DDNS explicitly, the second uses an IP-based list, and the third references a key clause.
A notify is deemed valid if the sender is one of the servers in the NS RRset for the zone, has been explicitly allowed using an 'allow-notify' clause, or is from an address listed in the masters' clause.
This seems unintuitive to many when they learn this for the first time, but it vastly simplifies the code/algorithm for handling refreshes on notify and also ensures, by using the same sequence of SOA checks each time, that the slaves will always converge their SOA serial numbers to the most up-to-date version.
If a valid notify is received where the notify carries a serial number larger than the one in the SOA currently served by the slave then the slave zone will again schedule a zone refresh, following exactly the same process it would use if the zone refresh was initiated on timer - i.e.
it behaves exactly as if the zone refresh timer has expired.