|
|
A secondary server functions as a backup to the primary server, should the primary be unreachable or down. It maintains a copy of the zone file (the host database for the zone) and, at a specified interval defined by the refresh variable, retrieves the contents of the primary's file. If it cannot reach the primary for a refresh, it tries again at intervals defined by the retry variable and, if it cannot reach the primary within the time specified by the expire variable, the data is flushed.
When a secondary 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 value is specified in the SOA (start of authority) resource record contained in the zone's database file.
A server may serve as a secondary for one or more zones and a primary for others.
Root servers are used to help the secondary server find routes to other zones.
See also: