dns soa rr timers

from rfc1912 - http:_www.faws.org/rfcs/rfc1912.html

Refresh: How often a secondary will poll the primary server to see if the serial number for the zone has increased (so it knows to request a new copy of the data for the zone). Set this to how long your secondaries can comfortably contain out-of-date data. You can keep it short (20 mins to 2 hours) if you aren’t worried about a small increase in bandwidth used, or longer (2-12 hours) if your Internet connection is slow or is started on demand. Recent BIND versions (4.9.3) have optional code to automatically notify secondaries that data has changed, allowing you to set this TTL to a long value (one day, or more).

Retry: If a secondary was unable to contact the primary at the last refresh, wait the retry value before trying again. This value isn’t as important as others, unless the secondary is on a distant network from the primary or the primary is more prone to outages. It’s typically some fraction of the refresh interval.

Expire: How long a secondary will still treat its copy of the zone data as valid if it can’t contact the primary. This value should be greater than how long a major outage would typically last, and must be greater than the minimum and retry intervals, to avoid having a secondary expire the data before it gets a chance to get a new copy. After a zone is expired a secondary will still continue to try to contact the primary, but it will no longer provide nameservice for the zone. 2-4 weeks are suggested values.

Minimum: The default TTL (time-to-live) for resource records —  how long data will remain in other nameservers' cache. ([RFC 1035] defines this to be the minimum value, but servers seem to always implement this as the default value) This is by far the most important timer. Set this as large as is comfortable given how often you update your nameserver. If you plan to make major changes, it’s a good idea to turn this value down temporarily beforehand. Then wait the previous minimum value, make your changes, verify their correctness, and turn this value back up. 1-5 days are typical values. Remember this value can be overridden on individual resource records.

a "typical" soa record looks something like:

@ IN SOA grox.net. hostmaster.grox.net. ( 2013010100 ; serial 172800 ; refresh (2 days) 86400 ; retry (1 day) 2419200 ; expire (4 weeks) 604800 ; minimum (1 week) )