See also How to setup primary / secondary

 

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 DNS server, these changes must be distributed to the secondary DNS servers for the zone.

This is done through zone transfers.

 

Most DNS servers automatically notifies secondary servers whenever changes are made through a NOTIFY request, and most DNS servers will request a Zone Transfer whenever such a notification is received.
You can specify if Simple DNS Plus should send these NOTIFY requests to secondary DNS servers in the Options dialog / DNS / Miscellaneous section.

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 querying the primary server for the SOA-record of the zone, 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 must always be incremented.
NOTE: Simple DNS Plus does this for you automatically as long as you do not change the serial number yourself.

 

The periodic polling by the secondary servers is controlled by the refresh, retry, and expire parameters of the SOA-record.

The secondary server waits for 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 fetch a fresh copy of the zone through a zone transfer request.

 

The actual zone transfer operation transfers all the records in the zone from the primary to the secondary server.

 

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. If the primary server does not support this and returns an error, Simple DNS Plus will then revert to doing a full zone transfer.

If you know that your primary DNS server does not support incremental zone transfers, you can prevent Simple DNS Plus from using this with a setting in the Options dialog / DNS / Secondary Zones section.