DNS and domains
SOA record and zone serial
The record at the top of every DNS zone that names the primary nameserver, the contact address and the serial number that shows which version of the zone a server holds.
Every zone begins with one SOA (start of authority) record. It names the primary nameserver and the administrator's mailbox (written with a dot instead of the @), and carries the serial number plus four timers: refresh, retry, expire and the negative-caching TTL.
The serial is the part you look at. It must go up with every change; secondary nameservers compare it with their copy to decide whether to fetch the zone again. Many hosts use the date plus a counter, 2026102101. Two nameservers reporting different serials are serving different versions of the zone, and some visitors are getting old answers.
The expire timer says how long a secondary keeps serving a zone it cannot refresh from the primary. The negative TTL says how long resolvers cache "this name does not exist", which is why a freshly created record can take a few minutes to appear even on a resolver that never saw it.