The master name servers maintain all the data corresponding to the zone, making them the authority for that zone. These are commonly called authoritative name servers. The data corresponding to any given zone should be available on at least two authoritative servers. You should designate one name server as the primary master server and at least one more as a secondary master server, to act as a backup if the primary is unavailable or overloaded.
A server may function as a master for multiple zones: as a primary for some zones, and as a secondary for others.
The primary master server is the DNS name server that loads the master copy of its data from disk when it starts in.named. A zone's primary master server is where you make changes for the zone. The primary master is the source for DNS information regarding its zone. The primary server may also delegate authority to secondary servers in its zone as well as to servers outside its zone.
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.