SOA Record

A SOA (Start of Authority) record is a mandatory DNS record that exists exactly once per zone and carries the authoritative metadata for it. It names the primary name server, the responsible contact address for the zone, and a set of timer values that control how secondary servers keep their copy in sync.

The record fields include a serial number (SERIAL) that is incremented on every change, REFRESH which tells secondaries how often to check for updates, RETRY which sets the wait after a failed contact, EXPIRE after which a secondary stops answering without reaching the primary, and MINIMUM which historically defined the negative-cache duration for NXDOMAIN answers.

The SOA record is the anchor for zone delegation and zone transfers (AXFR/IXFR). Recursive resolvers use it for negative caching, while authoritative servers use it to coordinate replication between the primary and its secondaries.