The SOA record contains important information about a domain and who is responsible for it.
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The DNS ‘start of authority’ (SOA) record stores important information about a domain or zone such as the email address of the administrator, when the domain was last updated, and how long the server should wait between refreshes.
All DNS zones need an SOA record in order to conform to IETF standards. SOA records are also important for zone transfers.
Example of an SOA record:
The 'RNAME' value here represents the administrator's email address, which can be confusing because it is missing the ‘@’ sign, but in an SOA record admin.example.com is the equivalent of firstname.lastname@example.org.
In the DNS, a 'zone' is an area of control over namespace. A zone can include a single domain name, one domain and many subdomains, or many domain names. In some cases, 'zone' is essentially equivalent with 'domain,' but this is not always true.
A zone serial number is a version number for the SOA record. In the example above, the serial number is listed next to 'SERIAL.' When the serial number changes in a zone file, this alerts secondary nameservers that they should update their copies of the zone file via a zone transfer.
A DNS zone transfer is the process of sending DNS record data from a primary nameserver to a secondary nameserver. The SOA record is transferred first. The serial number tells the secondary server if its version needs to be updated. Zone transfers take place over the TCP protocol.
Learn more about various DNS records.