The SOA means Start Of Authority. The SOA record defines the beginning of the authority DNS zone and specifies the global parameters for the zone. These parameters include the primary name server, the email of the domain administrator, the domain serial number, and several timers relating to refreshing the zone.
An example of an SOA record can be seen below:
$ dig SOA 5centscdn.net +short
pns1.5centscdn.com. support.5centscdn.com. 2020080526 7200 3600 1209600 60
The SOA record contains core information about your zone. It is not possible for your zone to work without that information. Therefore, it is mandatory to have an SOA record for each of your zones.
The SOA record has the following structure:
- Serial number – The revision number of this zone file. Increment this number each time the zone file is changed. It is important to increment this value each time when a change is made so that the changes will be distributed to any secondary DNS servers.
- Primary name server (NS) – The hostname for the primary DNS server for the zone. If you enter an invalid primary name server, it will be changed back to default.
- DNS admin e-mail – The e-mail address of the person, responsible for administering the domain’s zone file.
- Refresh Rate – The time in seconds that a secondary DNS server waits before querying the primary DNS server’s SOA record to check for changes. The refresh rate varies from 1200 to 43200 seconds.
- Retry Rate – The time in seconds that a secondary server waits before retrying a failed zone transfer. Usually, the retry rate is less than the refresh rate. The default value is 1800 seconds. The retry rate varies from 180 to 2419200 seconds.
- Expiry time – The time in seconds that a secondary server will keep trying to complete a zone transfer. If this time expires before a successful zone transfer, the secondary server will expire its zone file. The secondary will stop answering queries, as it considers its data too old to be reliable. The default value is 1209600 seconds.
- Default TTL – The minimum time-to-live value applies to all resource records in the zone file. This value is supplied in query responses to inform other servers how long they should keep the data in the cache. The default value is 3600 seconds (1 Hour).