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Inbox Theory

DNS

SOA Lookup

Resolves a domain's SOA (Start of Authority) record: primary nameserver, zone serial number, and refresh/retry/expire timers.

Start of Authority: the zone's primary nameserver, admin contact, and the timers secondary nameservers use to stay in sync.

The serial number should increment on every zone change: secondaries use it to know a zone transfer is needed. A stale serial means secondaries may be serving outdated records.