SOA lookup - start of authority

What the SOA record is

Every DNS zone has exactly one SOA (Start of Authority) record, and it is administrative rather than something a visitor's browser ever asks for directly: it names the zone's primary nameserver, an administrative contact, and four timers that govern how secondary nameservers keep themselves in sync.

The fields

• Serial - a version number for the zone. Secondary nameservers compare it to their own copy to decide whether they are out of date; it must increase on every change or secondaries will not notice an update happened.
• Refresh - how often a secondary checks the primary for a new serial.
• Retry - how soon to try again if that check failed.
• Expire - how long a secondary may keep serving stale data if it cannot reach the primary at all before it must stop answering for the zone.
• Minimum - historically the default TTL for negative answers (NXDOMAIN caching); modern DNS uses it specifically for that.

When this actually matters

Almost only when diagnosing why a change is not propagating consistently across a domain's own secondary nameservers - the serial number is the first thing to compare against what you expect.