SOA

networking

Stands for: Start of Authority record

The DNS record holding a zone's authoritative metadata.

A Start of Authority record (RFC 1035) carries the primary nameserver, the zone's serial number, and the timers that govern how secondaries refresh. Every zone has exactly one.

The SOA record is the administrative header of a DNS zone, naming the primary nameserver and the responsible contact, and carrying the timers that govern how secondary servers keep in step with the primary. Every zone has exactly one, and it is the record that defines a zone as a zone.

The serial number is the field with operational consequence. Secondaries compare it to decide whether to transfer a fresh copy, so a zone edited without incrementing it will not propagate: the primary knows about the change and every secondary continues serving the old data indefinitely. Automation usually handles this now, and hand-edited zone files are where the mistake still happens.

The remaining timers say how long a secondary waits before checking, how soon it retries a failed check, when it stops answering for a zone it can no longer refresh, and the minimum, which modern practice reads as how long a negative answer may be cached. That last one is subtle: a record that does not exist yet can stay non-existent in caches for as long as the negative TTL says, which is why a newly created record sometimes appears not to work.

Also known as: soa, SOA record

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