A primary server is authoritative for one or more DNS zones. Each server maintains its data in database files, and may transfer this data periodically to a secondary server if one exists in the zone.
When a server answers a client request, it attaches an expiration (specified by the time-to-live variable) for the data after which the data is considered invalid. The time-to-live variable is specified in the SOA (start of authority) resource record contained in the zone's database file.
Primary servers should be machines that are reliable and have excess memory and disk storage space.
A server may serve as a primary for one zone and a secondary for one or more others.
The primary server reads the configuration file, /etc/named.conf, to determine which zones are served and in which zone files the data is stored. Root servers are used to help the server find other zones.