The SOA record is from RFC 1035 and includes:
- serial number
- refresh interval
All are unsigned 32 bit numbers in seconds. In normal operation, the important fields are the refresh interval and the minimium.
Refresh refers to the interval that secondary and caching name servers will cache a dns (RR) record.
Minimium refers to the minimium ttl (time to live) for a particular record. Shorter ttl values can be specified for particular records.
The retry and expire fields refer to failed refreshes, if a refresh fails it will be retried every retry seconds, until the expire period is reached.