DNS lookup

Check the SOA record of a DNS zone and its refresh timers

The SOA record opens every DNS zone. It names the primary server and holds the timers that secondary servers use to stay in sync.

What the SOA record shows

The serial number grows with each change of the zone. Secondary name servers compare it with their copy and download the zone when the number goes up.

The admin mailbox is written with a dot instead of @, so hostmaster.example.com means hostmaster@example.com. The last field sets how long resolvers remember that a name does not exist.

When you need it

Compare serials on all name servers to see whether a change reached them
Find how long a missing record stays cached as missing
Find the primary server and the DNS provider of a zone

Questions about DNS records

01What format should the serial number use?

Many admins use the date plus a counter, like 2026092601. DNS only requires that the number grows with each change.

02Why does a new subdomain still show as missing?

Resolvers cached the earlier NXDOMAIN answer for the negative TTL from the SOA. The new record appears once that time passes.

03What do refresh, retry and expire mean?

Refresh is how often secondaries check the serial, retry is the pause after a failed check, and expire is when a secondary stops answering if it cannot reach the primary.

04Where is the zone admin address in the SOA?

In the RNAME field. Its first dot stands for the @, so hostmaster.example.com means hostmaster@example.com.

05Why do the NS servers show different serial numbers?

The secondary servers have not pulled the new zone version yet. They ask the primary once per refresh or right after a NOTIFY message. If the numbers stay apart longer than refresh, check the zone transfer.