SOA record

An SOA record is a DNS record that marks the start of the official records for a part of DNS called a zone, and stores information used to manage that zone. “SOA” stands for Start of Authority. A zone normally has one SOA record at its top, alongside its NS records.

What information does an SOA record contain?

The record includes the primary source name for the zone, a contact mailbox written in DNS name form, a serial number, and timing values used by secondary servers, which keep copies of the zone data, when checking for updates. The serial number helps secondary servers tell whether the zone data has changed. Refresh, retry, and expire values guide how secondary servers check and maintain their copies. A minimum TTL field is also present, with specific modern uses defined by DNS standards.

For example, when an administrator updates a zone, the serial number is increased. A secondary server that checks the primary can detect the new serial and request updated zone data. The SOA record does not itself contain all the zone records or tell a web browser which page to show.

Why might an SOA appear in a DNS error response?

An authoritative server can include the zone's SOA in a negative answer, such as NXDOMAIN or no data. The SOA information helps DNS lookup services (resolvers) determine how long they may save that answer saying a name or record is missing. The exact response and cache behavior follow DNS rules and the zone's configuration.

Related terms

• DNS zone — the zone whose management data is described
• NS record — lists authoritative servers
• DNS record — general record context
• NXDOMAIN — one negative DNS answer