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Recommended Refresh, Retry, Expire, and TTL settings

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We recommend the below settings for your SOA record. All times are in seconds.

Refresh  21600
Retry  10800
Expire  604800
TTL  43200

We recommend that the Refresh parameter is less than or equal to half the TTL of the SOA record but it must be longer than the Retry parameter. We also recommend the Retry parameter to be half the Refresh parameter.

The Refresh, Retry, and Expire parameters of the SOA record are used only if you have secondary DNS servers. The periodic polling by the secondary DNS servers is controlled by these parameters. A primary DNS server has the "master copy" of a zone, and secondary DNS 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.

The secondary DNS server waits for the Refresh interval before checking with the primary DNS server for zone changes (signified by a new SOA 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.

In addition to this method of updating secondary DNS servers, primary DNS servers can send Notify requests to secondary DNS servers to update their DNS information. This is handled automatically by the DNS servers.

The TTL parameter tells other DNS servers on the Internet how long to cache a zone file.
Click here for more information about TTL.

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