3.1. Sendmail and the Domain Name System

The Internet domain name system (DNS) is a distributed, coherent, reliable, autonomous database that maps domain names to Internet resources. A domain name is a dotted concatenation of tokens (words), such as nic.ddn.mil, uunet.uu.net, or 1.1.45.128.in-addr.arpa. An Internet resource could be a list of a host's network addresses, or a domain's list of mail exchanges or name servers, or a variety of other increasingly obscure things that don't concern the average Postmaster. We will present a brief overview of the DNS as it is viewed by Sendmail and its Postmasters.

The Internet DNS is described by Internet RFCs, a relevant subset of which are listed in Table 3-1. These RFCs are all of the readable variety; every Postmaster should have them on hand and should have at least glanced through them. Be aware that the use of the Well Known Service Resource Record (WKS RR) for detecting SMTP capable hosts, that was encouraged in RFC 974, was later deprecated in RFC 1123. RFC 974 is very short and very useful; section 5 of RFC 1123, dealing with e-mail and SMTP, is very interesting reading careful readers will note that several paragraphs of RFC 1123 appear to have been written with Sendmail in mind, and that's exactly what happened.