RFC 1664 (rfc1664) - Page 2 of 23

Using the Internet DNS to Distribute RFC1327 Mail Address Mapping Tables

RFC 1664          Internet DNS for Mail Mapping Tables       August 1994


   and systems using the RFC 822 mail protocol, or protocols derived from
   RFC 822. 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 RFC 1327: the mechanism
   for mapping between X.400 O/R addresses and RFC 822 domain names. As
   described in Appendix F of RFC 1327, 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 RFC 1327 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 RFC 822 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
   test applications ran under this provisional set up. This test was
   also very useful in order to define a possible migration strategy
   during the deployment of the new DNS containing the new RR. The
   Internet DNS nameservers wishing to provide this mapping information
   need in fact to be modified to support the new RR type, and in the
   real Internet, due to the large number of different implementations,
   this takes some time.

   The basic idea is to adopt a new DNS RR to store the mapping
   information. The RFC 822 to X.400 mapping rules (including the so



Allocchio, Bonito, Cole, Giordano & Hagens