Mail routing and the domain system (RFC0974)
Original Publication Date: 1986-Jan-01
Included in the Prior Art Database: 2019-Feb-14
Internet Society Requests For Comment (RFCs)
This RFC presents a description of how mail systems on the Internet are expected to route messages based on information from the domain system. This involves a discussion of how mailers interpret MX RRs, which are used for message routing.
Network Working Group Craig Partridge Request for Comments: 974 CSNET CIC BBN Laboratories Inc January 1986
MAIL ROUTING AND THE DOMAIN SYSTEM
Status of this Memo
This RFC presents a description of how mail systems on the Internet are expected to route messages based on information from the domain system described in RFCs 882, 883 and 973. Distribution of this memo is unlimited.
The purpose of this memo is to explain how mailers are to decide how to route a message addressed to a given Internet domain name. This involves a discussion of how mailers interpret MX RRs, which are used for message routing. Note that this memo makes no statement about how mailers are to deal with MB and MG RRs, which are used for interpreting mailbox names.
Under RFC-882 and RFC-883 certain assumptions about mail addresses have been changed. Up to now, one could usually assume that if a message was addressed to a mailbox, for example, at LOKI.BBN.COM, that one could just open an SMTP connection to LOKI.BBN.COM and pass the message along. This system broke down in certain situations, such as for certain UUCP and CSNET hosts which were not directly attached to the Internet, but these hosts could be handled as special cases in configuration files (for example, most mailers were set up to automatically forward mail addressed to a CSNET host to CSNET-RELAY.ARPA).
Under domains, one cannot simply open a connection to LOKI.BBN.COM, but must instead ask the domain system where messages to LOKI.BBN.COM are to be delivered. And the domain system may direct a mailer to deliver messages to an entirely different host, such as SH.CS.NET. Or, in a more complicated case, the mailer may learn that it has a choice of routes to LOKI.BBN.COM. This memo is essentially a set of guidelines on how mailers should behave in this more complex world.
Readers are expected to be familiar with RFCs 882, 883, and the updates to them (e.g., RFC-973).
Partridge [Page 1]
RFC 974 January 1986 Mail Routing and the Domain System
What the Domain Servers Know
The domain servers store information as a series of resource records (RRs), each of which contains a particular piece of information about a given domain name (which is usually, but not always, a host). The simplest way to think of a RR is as a typed pair of datum, a domain name matched with relevant data, and stored with some additional type information to help systems determine when the RR is relevant. For the purposes of message routing, the system stores RRs known as MX RRs. Each MX matches a domain name with two pieces of data, a preference value (an unsigned 16-bit integer), and the name of a host. The preference number is used to indicate in what order the mailer should attempt deliver to the MX hosts, with the lowest numbered MX being the one to try first. Multiple MXs with the same preference are permitted and have the same priority.
In addition to mail information, the servers store certain other types of RR’s which mailers ma...