Using the Internet DNS to Distribute RFC1327 Mail Address Mapping Tables
RFC 1664

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Network Working Group                                       C. Allocchio
Request for Comments: 1664                                     A. Bonito
Category: Experimental                                        GARR-Italy
                                                                 B. Cole
                                                      Cisco Systems Inc.
                                                             S. Giordano
                                     Centro Svizzero Calcolo Scientifico
                                                               R. Hagens
                                             Advanced Network & Services
                                                             August 1994

                 Using the Internet DNS to Distribute
                  RFC1327 Mail Address Mapping Tables

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  This memo does not specify an Internet standard of any
   kind.  Distribution of this memo is unlimited.

Abstract

   This memo defines how to store in the Internet Domain Name System the
   mapping information needed by e-mail gateways and other tools to map
   RFC822 domain names into X.400 O/R names and vice versa.  Mapping
   information can be managed in a distributed rather than a centralised
   way. Gateways located on Internet hosts can retrieve the mapping
   information querying the DNS instead of having fixed tables which
   need to be centrally updated and distributed.  This memo is a joint
   effort of X400 operation working group (x400ops) and RARE Mail and
   Messaging working group (WG-MSG).

1. Introduction

   The connectivity between the Internet SMTP mail and other mail
   services, including the Internet X.400 mail and the commercial X.400
   service providers, is assured by the Mail eXchanger (MX) record
   information distributed via the Internet Domain Name System (DNS). A
   number of documents then specify in details how to convert or encode
   addresses from/to RFC822 style to the other mail system syntax.
   However, only conversion methods provide, via some algorithm or a set
   of mapping rules, a smooth translation, resulting in addresses
   indistinguishable from the native ones in both RFC822 and foreign
   world.

   RFC1327 describes a set of mappings which will enable interworking
   between systems operating the CCITT X.400 (1984/88) Recommendations

Allocchio, Bonito, Cole, Giordano & Hagens                      [Page 1]
RFC 1664          Internet DNS for Mail Mapping Tables       August 1994

   and systems using the RFC822 mail protocol, or protocols derived from
   RFC822. That document addresses conversion of services, addresses,
   message envelopes, and message bodies between the two mail systems.
   This document is concerned with one aspect of RFC1327: the mechanism
   for mapping between X.400 O/R addresses and RFC822 domain names. As
   described in Appendix F of RFC1327, implementation of the mappings
   requires a database which maps between X.400 O/R addresses and domain
   names, and this database is statically defined.

   This approach requires many efforts to maintain the correct mapping:
   all the gateways need to get coherent tables to apply the same
   mappings, the conversion tables must be distributed among all the
   operational gateways, and also every update needs to be distributed.
   This static mechanism requires quite a long time to be spent
   modifying and distributing the information, putting heavy constraints
   on the time schedule of every update.  In fact it does not appear
   efficient compared to the Internet Domain Name Service (DNS).  More
   over it does not look feasible to distribute the database to a large
   number of other useful applications, like local address converters,
   e-mail User Agents or any other tool requiring the mapping rules to
   produce correct results.

   A first proposal to use the Internet DNS to store, retrieve and
   maintain those mappings was introduced by two of the authors (B. Cole
   and R. Hagens) adopting two new DNS resource record (RR)  types: TO-
   X400 and TO-822. This new proposal adopts a more complete strategy,
   and requires one new RR only. The distribution of the RFC1327 mapping
   rules via DNS is in fact an important service for the whole Internet
   community: it completes the information given by MX resource record
   and it allows to produce clean addresses when messages are exchanged
   among the Internet RFC822 world and the X.400 one (both Internet and
   Public X.400 service providers).

   A first experiment in using the DNS without expanding the current set
   of RR and using available ones was in the mean time deployed by some
   of the authors. The existing PTR resource records were used to store
   the mapping rules, and a new DNS tree was created under the ".it" top
   level domain. The result of the experiment was positive, and a few