A secondary master server maintains a copy of the data for the zone. The primary server sends its data and delegates authority to the secondary server. Clients can query a secondary server for DNS information. By using secondary servers, you can improve response time and reduce network overhead by spreading the load over multiple machines. Secondary servers also provide redundancy in case the primary server is not available.
When the secondary server starts in.named, it requests all the data for the given zone from the primary. The secondary server then periodically checks with the primary to see if it needs to update its database. The process of sending the most recent zone database from the primary to the secondary is called a zone transfer. Thus, you do not modify data files on a secondary server, you modify the data files on the zone's primary server and the secondary servers update their files from the primary.