sendmail, 4th Edition

Overview

DNS stands for Domain Name System. A domain is any logical collection of related hostnames or site names. A naming system is best visualized as an inverted tree of information that corresponds to fully qualified domain names (see Figure 9-1) organized in a name space (as opposed to physical space). Local or regional knowledge about an individual part of that name space is call a DNS zone. We will expand on these concepts soon.

Figure 9-1. Domain names form a tree of information

The parts of a fully qualified name are separated from one another with dots. For example:

here.uofa.edu

This name describes the machine here that is part of the uofa subdomain of the edu top-level domain. In Figure 9-1, the dot at the top is the “root” of the tree. It is implied but not always^[147] included in fully qualified domain names:

here.uofa.edu.
             ↑
             implied

The root zone is served by machines running software enabling them to function as name servers.^[148] Each has knowledge of all the top-level domains (such as gov, com, biz, uk, au, etc.) and the server machines for those domains. Each of the top-level domain’s servers knows of one or more machines with knowledge of the next level below. For example, the server for edu “knows” about the subdomains uofa, uofb, and uofc but might not know about anything below those subdomains, nor about the other domains next to itself, such as com.^[149]

A knowledgeable ...