When changes are made to a zone and its records on the primary server, a zone transfer is used to update DNS records on secondary DNS servers.
A primary server has the "master" copy of a zone, and secondary servers keep copies of the zone for redundancy.
When changes are made to zone data on the primary, they must be distributed to the secondary servers.
Most DNS servers (including Simple DNS Plus) automatically notifies secondary servers whenever changes are made, and most DNS servers (again including Simple DNS Plus) will request a Zone Transfer whenever such a notification is received.
For this to work correctly, NS-records (and corresponding A-records) for each secondary DNS server must exist in the zone.
Secondary servers also periodically check for changes, by polling the SOA-record of the zone from the primary server, and checking the serial number.
In addition to whatever other changes are made to a zone and its records, the serial number of the SOA-record is always incremented.
The periodic polling by the secondary servers is controlled by the refresh, retry, and expire parameters of the SOA-record.
The secondary server waits the "refresh" interval before checking with the primary for a new serial number. If this check cannot be completed, new checks are started every "retry" interval.
If the secondary finds it impossible to perform a serial check within the "expire" interval, it discards the zone.
When the poll shows that the zone has changed (higher serial number), the secondary server will request a zone transfer.
The actual zone transfer operation transfers all the records in the zone from the primary to the secondary server (similar to FTP).
Simple DNS Plus supports a special optimized "incremental zone transfer" method which saves bandwidth by only transferring those changes made since the last zone transfer.
Simple DNS Plus will by default request incremental zone transfers when getting zone updates from another (primary) DNS server. If that primary server does not support this and returns an error, Simple DNS Plus will then revert to doing a regular zone transfer.
If you know that your primary DNS server does not support incremental zone transfers, you can prevent Simple DNS Plus from first trying this with the "UseIXFR" setting in the "sdnsplus.ini" file.
