DNS on Windows Server 2003, 3rd Edition

Changing Other SOA Values

We briefly mentioned increasing the refresh interval as a way of offloading your primary name server. Let’s discuss refresh in a little more detail and go over the remaining SOA values, too.

The refresh value, you’ll remember, controls how often a secondary checks whether its zone data is up-to-date. The retry value becomes the refresh time after the first failure to reach a master name server. The expire value determines how long zone data can be held before it’s discarded when a master is unreachable. Finally, the minimum TTL sets how long zone information may be cached.

Suppose we’ve decided we want the secondaries to pick up new information every hour instead of every three hours. We change the refresh value to one hour in each of the zones. Since retry is related to refresh, we should probably reduce retry, too—to every 15 minutes or so. Typically, retry is less than refresh, but that’s not required. Although lowering the refresh value will speed up the distribution of zone data, it will also increase the load on the server from which data is being loaded, since the secondaries will check more often. The added load isn’t much, though; each secondary makes a single SOA query during each zone’s refresh interval to check its master’s copy of the zone. So with two secondary name servers, changing the refresh time from three hours to one hour will generate only four more queries (per zone) to the primary master in any three-hour span.

If all of your secondaries ...