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Document how secondaries refresh a zone in the ARM

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We have a KB article that describes this, put a condensed version into
the ARM.
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Matthijs Mekking 2025-01-20 16:47:25 +01:00
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doc/arm/reference.rst
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@ -6676,8 +6676,43 @@ Zone Types
specifies one or more IP addresses of primary servers that the secondary
contacts to update its copy of the zone.
If a file is
specified, then the replica is written to this file whenever the zone
A zone may refresh on timer or on receipt of a notify. If a valid notify is
received where the notify carries a serial number larger than the one in the
SOA currently served, then the secondary will schedule a zone refresh.
A notify is considered valid if the sender is one of the servers in the NS
RRset for the zone, has been explicitly allowed using an :any:`allow-notify`
clause, or is from an address listed in the primary servers clause.
If no notifies have been received, the server will try to refresh the zone.
The REFRESH field in the SOA record determines how long after the last zone
update it should query the primaries for the SOA record. Again, if the
SOA record contains a serial number larger than the one in the SOA currently
served, a zone refresh is scheduled. If a notify is received while a
refresh is in progress, the serial number of the notify is checked and if
it is larger, another refresh for the zone is queued. There will at most
be one zone refresh queued.
The primary servers are queried in turn, :any:`named` will move on to the
next server in the list if either it is unable to get a valid response from
the server it is currently querying, or the primary being queried returns
the same or smaller SOA than the secondary is currently serving. On the
first SOA received that has a serial bigger than the one currently served,
:any:`named` will initiate a zone transfer with that server. Once the zone
transfer has been received and the zone has been updated, then this zone
refresh is complete, and no other servers are tried.
When receiving a notify, :any:`named` does not first query the sender of
the notify. It will continue with the next server in the list that
transferred the zone, skipping over unreachable servers. A primary is
considered unreachable if the secondary cannot get a response from the
server. This state will be cached for 10 minutes, or until a notify is
received from that address.
Furthermore, a zone is refreshed when the secondary server is restarted,
or when a :option:`rndc refresh <rndc refresh>` command is received.
If a file is specified, then the replica is written to this file whenever the zone
is changed, and reloaded from this file on a server restart. Use of a file
is recommended, since it often speeds server startup and eliminates a
needless waste of bandwidth. Note that for large numbers (in the tens or