1. Rationale
In order to locate the network services offered at a particular Internet domain one is faced with the choice of selecting from a growing number of centralized databases - typically Web or Usenet News "wanderers", or attempting to infer the existence of network services from whatever DNS information may be available. The former approach is not practical in some cases, notably when the entity seeking service information is a program.
Perhaps the most visible example of the latter approach at work is in the case of World-Wide Web HTTP servers. It is common practice to try prefixing the domain name of an organization with "http://www." in order to reach its World-Wide Web site, e.g. taking "hivnet.fr" and arriving at "http://www.hivnet.fr." Some popular World-Wide Web browsers have gone so far as to provide automatic support for this domain name expansion.
Ideally, the DNS or some complementary directory service would provide a means for programs to determine automatically the network services which are offered at a particular Internet domain, the protocols which are used to deliver them, and other technical information. Unfortunately, although much work has been done to develop said directory service technologies and to define new types of DNS resource record to provide this type of information, there is no widely agreed upon or widely deployed solution to the problem - except in a small number of cases.
The first case is where the DNS already provides a lookup capability for the type of information being sought after. For example: Mail Exchanger (MX) records specify how mail to a particular domain should be routed [RFC-974], the Start of Authority (SOA) records make it possible to determine who is responsible for a given domain, and Name Server (NS) records indicate which hosts provide DNS name service for a given domain.
The second case is where the DNS does not provide an appropriate lookup capability, but there is some widely accepted convention for finding this information. Some use has been made of Text (TXT) [RFC-1035] records in this scenario, but in the vast majority of cases a Canonical Name (CNAME) or Address (A) record pointer is used to indicate the host or hosts which provide the service. This document proposes a slight formalization of this well-known alias approach.
It should be noted that the DNS provides a Well Known Services (WKS) [RFC-1035] lookup capability, which makes it possible to determine the network services offered at a given domain name. In practice this is not widely used, perhaps because of the absence of a suitable programming interface. Use of WKS for mail routing was deprecated in the Host Requirements specification [RFC-1123] in favour of the MX record, and in the long term it is conceivable that SRV records will supersede both WKS and MX.