Many of the contentious issues that arise in the context of the DNS concern domain names themselves—in particular, the definition of permissible names and the rights to their use. Some of those issues are introduced and discussed in Chapter 2.


Many things happen when a query to the DNS is initiated. If the DNS were a centralized database, such as HOSTS.TXT,2 every query would go directly to a central file where the answer would be found (or its absence noted). However, because the DNS is a hierarchical, distributed database, a search in response to a query generally requires several steps. If necessary, it can begin at the root and traverse a course through the tree of files to the one in which the sought-for answer resides. However, frequently the search can begin further down the tree because previous answers are stored and reused by the querying client. The design of the DNS ensures that the path down the tree will be followed without detours or false starts, leading directly to the desired file because the structure of the domain name spells out the route. This process may best be understood through an example, shown in Figure 3.1, which illustrates the use of the DNS to find the IP address corresponding to the hypothetical domain name

This is what would happen if, for example, the user wanted to access a Web site at that name, in which case the requesting application would be a browser. However, the same process would be followed for, say, an e-mail application or any other application supported by a host4 on the Internet.

Two versions of the process are described below: first, the version shown in Figure 3.1, which would be followed if this were a new query from a computer that was not on the same DNS subtree as the server; and then a version shortened by taking advantage of additional information from shared servers or previous queries.


This section elaborates on the high-level explanation in Chapter 2. It draws extensively on material in Paul Albitz and Cricket Liu, DNS and BIND, O’Reilly & Associates, Sebastopol, Calif., 2001.


HOSTS.TXT is the predecessor of the DNS and is described in Section 2.1.


The process shown in Figure 3.1 assumes that the querying client has stored no relevant previous answers.


A “host” is a network computer on which applications run providing services, such as computation or database access, to end users on the network.

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