The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  Name Systems and TCP/IP Name Registration and Name Resolution            9  TCP/IP Name Systems: Host Tables and Domain Name System (DNS)                 9  TCP/IP Domain Name System (DNS)                      9  DNS Name Servers and Name Resolution                           9  DNS Name Server Concepts and Operation

DNS Name Server Data Storage: Resource Records and Classes 1 2 3 DNS Zone Management, Contacts and Zone Transfers

Reasons for Using Slave Name Servers

The master name server is obviously the most essential server. It is on this name server that the master files for the zone's resource records are maintained, so the master name server is the “final word” for information on the zone. However, there are several reasons why slave servers are also important:

• Redundancy: We've already discussed how important DNS is, especially on the Internet; with only one name server, if it ever failed nobody would be able to resolve names such as “www.xyzindustries.com” into IP addresses, which would be a Bad Thing [tm]. J Slave name servers act as a backup for the masters they support.
  
  
• Maintenance: With more than one server, we can easily take the primary server down for maintenance when needed without name resolution service being disrupted.
  
  
• Load Handling: Busy zones can use multiple servers to spread the load of name resolution requests to improve performance.
  
  
• Efficiency: There are many cases where there is an advantage to positioning a name server in a particular geographical location for the sake of efficiency. For example, a company may have an office in a distant location connected using a low-speed WAN link. To reduce name resolution traffic across that link, it makes sense to have that zone's information available in a name server on both sides of the connection, so one would require two physical servers.

Redundancy is the most important consideration in setting up master and slave name servers. Sticking two machines side by side in a server room, plugged into the same electrical service, both connected to the Internet with the same Internet Service Provider, and making one your master DNS server and the other your slave is not a smart move. Ideally, the primary and secondary servers should be as independent as possible; they should be physically distant and have separate connections to the Internet.

Just as the names “master” and “slave” suggest, the secondary name servers are not the original source of information about a zone. They normally obtain their resource records not from human-edited master files, but from updates obtained from the master server. This is accomplished using a process called a zone transfer. These transfers are performed on a regular basis to ensure that the slave servers are kept up to date. The slaves can then respond to name resolution requests with current information. Both the master and the slave are considered authoritative for the zone.

DNS Name Server Data Storage: Resource Records and Classes 1 2 3 DNS Zone Management, Contacts and Zone Transfers