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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 Resolution Concepts and Resolver Operations

DNS Resolution Concepts and Resolver Operations 1 2 DNS Basic Name Resolution Techniques: Iterative and Recursive Resolution

The DNS name servers we explored in the preceding section are arguably the most important part of the system as a whole. After all, they store all the data on the system and actually provide the addresses we need when names are given to them; without these servers, there would be no DNS at all. Of course, what use is a server if nobody is asking for service? The clients in the system, called resolvers, are also important, because they initiate the process of name resolution; resolvers are where the “rubber meets the road”, so to speak.

The operation of DNS resolvers is explained in the two main DNS standards. RFC 1034 describes the functions performed by resolvers, and how they work in general terms. This includes a discussion of the algorithm used to conduct name resolution. RFC 1035 deals more with the implementation details of resolvers, and the fine points of how they do their jobs. Several subsequent standards have of course modified these base standards, changing some of the ways that resolvers work in different ways.

Just as the main job of a DNS server is to store DNS name data and “serve” it when it receives requests, the main job of a DNS resolver is to, well, resolve. J While most people only think of name resolution as the process of transforming a DNS name into an IP address, this is but one of several types of resolution services performed by DNS. A few of the most typical types of DNS resolution are:

• Standard Name Resolution: Taking a DNS name as input and determining its corresponding IP address.
  
  
• Reverse Name Resolution: Taking an IP address and determining what name is associated with it.
  
  
• Electronic Mail Resolution: Determining where to send electronic mail (e-mail) messages based on the e-mail address used in a message.

There are other types of resolution activities as well, though again, most name resolution requests are of the “standard” variety, making it the primary focus in our discussion.

DNS Resolution Concepts and Resolver Operations 1 2 DNS Basic Name Resolution Techniques: Iterative and Recursive Resolution