SOA Lookup

An SOA (Start of Authority) record is a mandatory DNS record that contains administrative information about a DNS zone. Every DNS zone must have exactly one SOA record. It specifies the primary nameserver, the email address of the zone administrator (with @ replaced by a dot), a serial number used for zone transfers, and timing parameters that control how secondary nameservers refresh their zone data.

The SOA serial number is a version identifier for the DNS zone. Secondary nameservers compare their serial number against the primary's — if the primary has a higher number, they initiate a zone transfer to fetch updated records. A common format is YYYYMMDDNN (year, month, day, two-digit increment), for example 2024061501. Every time you update DNS records, the serial must be incremented for zone transfers to trigger.

Refresh tells secondary nameservers how often (in seconds) to check the primary for zone changes. Retry specifies how long to wait before retrying a failed refresh. Expire is how long a secondary will serve zone data without successfully contacting the primary before it stops answering queries. A typical configuration: refresh=3600, retry=900, expire=604800. With NOTIFY (RFC 1996), many modern setups use longer refresh values since the primary actively notifies secondaries of changes.

The minimum TTL field in an SOA record was originally the default TTL for all records in the zone, but RFC 2308 redefined it as the negative caching TTL — the duration resolvers should cache a NXDOMAIN (non-existent domain) response. A value of 300–3600 is common. Setting it too low increases query load; setting it too high means deleted records take longer to stop resolving.