What SOA stores
The SOA record gathers metadata for the whole zone: the name of the primary name server, the administrator's email address, a serial number and a set of timers — refresh, retry, expire and minimum. The primary server is usually one of those listed in the NS records, while the timers define how often secondary servers check with the primary and how long they keep treating stale data as usable.
Serial number and zone transfer
The serial number plays a key role:
- Increment on changes — the serial number is raised on every edit to the zone, often in a date-based YYYYMMDDnn format.
- Signal to secondaries — secondary servers compare serial numbers and start a zone transfer only when the primary's number is higher.
- The minimum timer — sets the TTL for negative caching, that is how long a "record does not exist" answer is stored.
If the serial number is not bumped, secondary servers will not notice the edits and keep serving old data, so a correct SOA is critical for keeping the whole zone in sync.
