start of authority (SOA) record

Definition of start of authority (SOA) record in The Network Encyclopedia.

What is start of authority (SOA) record?

The first record in a zone file, which defines the general properties of the zone for a name server.

Here is an example of a start of authority (SOA) record:

@   IN  SOA     nameserver.place.dom.  postmaster.place.dom. (
1 ; serial number
3600 ; refresh [1h]
600 ; retry [10m]
86400 ; expire [1d]
3600 ) ; min TTL [1h]

This SOA record contains the following information:

  • The name of the subdomain over which a particular name server has authority (nameserver.place.dom)
  • The name of the host on which this zone file resides
  • The e-mail address of the person responsible for administering the subdomain (postmaster@nameserver.place.dom)
  • A serial number, which increases when the zone data is updated and which is used in zone transfers to determine whether the secondary name server needs a new version of the zone file
  • A refresh interval, in seconds, that informs the secondary name server how frequently it should check with the master name server to see whether its zone information is current
  • A retry interval, in seconds, that tells the secondary name server how often to contact the master name server if the initial contact is unsuccessful
  • An expiry interval, in seconds, that informs the secondary name server how long to keep trying to contact the master name server for a refresh of the zone data before the data expires and the secondary name server no longer responds to name queries
  • The Time to Live (TTL), which is a value returned by the name server to resolvers when a name is resolved, informing the resolver how long it can cache the resolved name and IP address

See also: