Domain Name Server

DNS: The Domain Name System

The Domain Name System, or DNS, is a distributed database that is used by TCP/IP applications to map between hostnames and IP addresses, and to provide electronic mail routing information. We use the term distributed because no single site on the Internet knows all the information. Each site (university department, campus, company, or department within a company, for example) maintains its own database of information and runs a server program that other systems across the Internet (clients) can query. The DNS provides the protocol that allows clients and servers to communicate with each other.

From an application's point of view, access to the DNS is through a resolver. On Unix hosts the resolver is accessed primarily through two library functions, gethostbyname(3) and gethostbyaddr(3), which are linked with the application when the application is built. The first takes a hostname and returns an IP address, and the second takes an IP address and looks up a hostname. The resolver contacts one or more name servers to do the mapping.

14.2 DNS Basics

Every node (circles in Figure 14.1) has a label of up to 63 characters. The root of the tree is a special node with a null label. The domain name of any node in the tree is the list of labels, starting at that node, working up to the root, using a period ("dot") to separate the labels. Every node in the tree must have a unique domain name, but the same label can be used at different points in the tree.

A domain name that ends with a period is called an absolute domain name or a fully qualified domain name (FQDN). An example is sun.tuc.noao.edu.. If the domain name does not end with a period, it is assumed that the name needs to be completed. How the name is completed depends on the DNS software being used. If the uncompleted name consists of two or more labels, it might be considered to be complete; otherwise a local addition might be added to the right of the name. For example, the...