Postfix: The Definitive Guide

DNS Overview

At one time, hostname to IP address mapping was handled by one large, centrally managed text file that contained an entry for every host accessible on the Internet. Each site downloaded a copy of the file periodically to get the latest hostname information. That scheme quickly became unwieldy, and the DNS service was conceived. It was defined in RFC 882 in 1983, and introduced two key ideas: the data is distributed and the naming of hosts is hierarchical. Making the data distributed means that every site updates its own information, and the updates become available almost immediately. Hierarchical naming prevents hostname conflicts and gives us the current domain-naming system that we are all very familiar with today. Each site obtains at least one domain name, and all of the hosts at that site are named by prefixing the simple hostname to the site’s domain name. For example, a site that controls the domain name example.com might have any number of hosts with names like server1.example.com, hp4100.example.com, or www.example.com.

Each domain has at least two domain nameservers that are considered authoritative for the domain. Authoritative nameservers should have direct access to the database that contains all the information about a domain.

The data is comprised of different types of records called resource records . Different resource records provide different kinds of information, such as IP addresses, nameservers, hostname aliases, and mail routing. The resource ...