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



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   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
   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 RFC822 to X.400 mapping rules (including the so



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   called 'gate' rules) will be stored in the ordinary DNS tree, while
   the definition of a new branch of the name space defined under each
   national top level domain is envisaged in order to contain the X.400
   to RFC822 mappings. A "two-way" mapping resolution schema is thus
   fully implemented.

   The creation of the new domain name space representing the X.400 O/R
   names structure also provides the chance to use the DNS to distribute
   dynamically other X.400 related information, thus solving other
   efficiency problems currently affecting the X.400 MHS service.

   In this paper we will adopt the RFC1327 mapping rules syntax, showing
   how it can be stored into the Internet DNS.

1.1 Definitions syntax

   The definitions in this document is given in BNF-like syntax, using
   the following conventions:

      |   means choice
      \   is used for continuation of a definition over several lines
      []  means optional
      {}  means repeated one or more times

   The definitions, however, are detailed only until a certain level,
   and below it self-explaining character text strings will be used.

2. Motivation

   Implementations of RFC1327 gateways require that a database store
   address mapping information for X.400 and RFC822. This information
   must be disseminated to all RFC1327 gateways. In the Internet
   community, the DNS has proven to be a practical mean for providing a
   distributed name service. Advantages of using a DNS based system over
   a table based approach for mapping between O/R addresses and domain
   names are:

     - It avoids fetching and storing of entire mapping tables by every
       host that wishes to implement RFC1327 gateways and/or tools

     - Modifications to the DNS based mapping information can be made
       available in a more timely manner than with a table driven
       approach.

     - It allows full authority delegation, in agreement with the
       Internet regionalization process.





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     - Table management is not necessarily required for DNS-based
       RFC1327 gateways.

     - One can determine the mappings in use by a remote gateway by
       querying the DNS (remote debugging).

   Also many other tools, like address converters and User Agents can
   take advantage of the real-time availability of RFC1327 tables,
   allowing a much easier maintenance of the information.

3. The domain space for X.400 O/R name addresses

   Usual domain names (the ones normally used as the global part of an
   RFC822 e-mail address) and their associated information, i.e., host
   IP addresses, mail exchanger names, etc., are stored in the DNS as a
   distributed database under a number of top-level domains. Some top-
   level domains are used for traditional categories or international
   organisations (EDU, COM, NET, ORG, INT, MIL...). On the other hand
   any country has its own two letter ISO country code as top-level
   domain (FR, DE, GB, IT, RU, ...), including "US" for USA.  The
   special top-level/second-level couple IN-ADDR.ARPA is used to store
   the IP address to domain name relationship. Our proposal defines in
   the above structure the appropriate way to locate the X.400 O/R name
   space, thus enabling us to store in DNS the RFC1327 mapping data.

   The RFC1327 mapping information is composed by three tables: 'table1'
   gives the translation from X.400 to RFC822 while 'table2' and 'gate'
   tables map RFC822 into X.400. Each mapping table is composed by
   mapping rules, and a single mapping rule is composed by a keyword
   (the argument of the mapping function derived from the address to be
   translated) and a translator (the mapping function parameter):

                          keyword#translator#

   the '#' sign is a delimiter enclosing the translator. An example:

                foo.bar.us#PRMD$foo\.bar.ADMD$intx.C$us#

   Local mappings are not intended for use outside their restricted
   environment, thus they should not be included in DNS. If local
   mappings are used, they should be stored using static local tables,
   exactly as local static host tables can be used with DNS.

   The keyword of a 'table2' and 'gate' table entry is a valid RFC822
   domain; thus the usual domain name space can be used without problems
   to store these entries.





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   On the other hand, the keyword of a 'table1' entry belongs to the
   X.400 O/R name space. The X.400 O/R name space does not usually fit
   into the usual domain name space, although there are a number of
   similarities; a new name structure is thus needed to represent it.
   This new name structure contains the X.400 mail domains.

   To ensure the correct functioning of the DNS system, the new X.400
   name structure must be hooked to the existing domain name space in a
   way which respects the existing name hierarchy.

   A possible solution was to create another special branch, starting
   from the root of the DNS tree, somehow similar to the in-addr.arpa
   tree. This idea would have required to establish a central authority
   to coordinate at international level the management of each national
   X.400 name tree, including the X.400 public service providers. This
   coordination problem is a heavy burden if approached globally. More
   over the X.400 name structure is very 'country oriented': thus while
   it requires a coordination at national level, it does not have
   concepts like the international root. In fact the X.400 international
   service is based  on a large number of bilateral agreements, and only
   within some communities an international coordination service exists.

   The X.400 two letter ISO country codes, however, are the same used
   for the RFC822 country top-level domains and this gives us an
   appropriate hook to insert the new branches. Our proposal is, in
   fact, to create under each national top level ISO country code a new
   branch in the name space. This branch represents exactly the X.400
   O/R name structure as defined in each single country, following the
   ADMD, PRMD, O, OU hierarchy. A unique reserved label 'X42D' is placed
   under each country top-level domain, and hence the national X.400
   name space derives its own structure:




















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                                    . (root)
                                    |
      +-----------------+-----------+--------+-----------------+...
      |                 |                    |                 |
     edu                it                   us                fr
      |                 |                    |                 |
  +---+---+...    +-----+-----+...     +-----+-----+...     +--+---+...
  |       |       |     |     |        |     |     |        |      |
 ...     ...     cnr   X42D  infn      va    ca   X42D     X42D  inria
                        |                    |     |        |
           +------------+------------+...   ...   ...  +----+-------+...
           |            |            |                 |            |
    ADMD-PtPostel  ADMD-garr  ADMD-Master400        ADMD-atlas  ADMD-red
                        |            |                 |            |
             +----------+----+...   ...        +-------+------+... ...
             |               |                 |              |
         PRMD-infn       PRMD-STET        PRMD-Telecom   PRMD-Renault
             |               |                 |              |
            ...             ...               ...            ...


   The creation of the X.400 new name tree at national level solves the
   problem of the international coordination. Actually the coordination
   problem is just moved at national level, but it thus becomes easier
   to solve. The coordination at national level between the X.400
   communities and the Internet world is already a requirement for the
   creation of the national static RFC1327 mapping tables; the use of
   the Internet DNS gives further motivations for this coordination.

   The coordination at national level also fits in the ongoing proposal
   intended to define exactly the RFC1327 Mapping Authorities. The DNS
   in fact allows a step by step authority distribution, up to a final
   complete delegation, which can be easily controlled at national level
   accordingly with national needs and situations. A further advantage
   of the national based solution is to allow each country to set up its
   own X.400 name structure in DNS and to deploy its own authority
   delegation according to its local time scale and requirements, with
   no loss of global service in the mean time. And last, placing the new
   X.400 name tree and coordination process at national level fits into
   the Internet regionalization and internationalisation process, as it
   requires local bodies to take care of local coordination problems.

   The DNS name space thus contains completely the information required
   by an e-mail gateway or tool to perform the X.400-RFC822 mapping: a
   simple query to the nearest nameserver provides it. Moreover there is
   no more any need to store, maintain and distribute manually any
   mapping table. The new X.400 name space can also contain further
   information about the X.400 community, as DNS allows for it a



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   complete set of resource records, and thus it allows further
   developments. This set of RRs in the new X.400 name space must be
   considered 'reserved' and thus not used until further specifications.

   The construction of the new domain space trees will follow the same
   procedures used when organising at first the already existing DNS
   space: at first the information will be stored in a quite centralised
   way, and distribution of authority will be gradually achieved. A
   separate document will describe the implementation phase and the
   methods to assure a smooth introduction of the new service.

4. The new DNS resource record for RFC1327 mapping rules: PX

   The specification of the Internet DNS (RFC1035) provides a number of
   specific resource records (RRs) to contain specific pieces of
   information. In particular they contain the Mail eXchanger (MX) RR
   and the host Address (A) records which are used by the Internet SMTP
   mailers. As we will store the RFC822 to X.400 mapping information in
   the already existing DNS name tree, we need to define a new DNS RR in
   order to avoid any possible clash or misuse of already existing data
   structures. The same new RR will also be used to store the mappings
   from X.400 to RFC822. More over the mapping information, i.e., the
   RFC1327 mapping rules, has a specific format and syntax which require
   an appropriate data structure and processing. A further advantage of
   defining a new RR is the ability to include flexibility for some
   eventual future development.

   The definition of the new 'PX' DNS resource record is:

      class:        IN   (Internet)

      name:         PX   (pointer to X.400/RFC822 mapping information)

      value:        26

   The PX RDATA format is:

          +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
          |                  PREFERENCE                   |
          +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
          /                    MAP822                     /
          /                                               /
          +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
          /                    MAPX400                    /
          /                                               /
          +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

   where:



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   PREFERENCE   A 16 bit integer which specifies the preference given to
                this RR among others at the same owner.  Lower values
                are preferred;

   MAP822       A  element containing , the
                RFC822 part of the RFC1327 mapping information;

   MAPX400      A  element containing the value of
                 derived from the X.400 part of
                the RFC1327 mapping information (see sect. 4.2);

   PX records cause no additional section processing. The PX RR format
   is the usual one:

              [] []  

   When we store in DNS a 'table1' entry, then  will be an X.400
   mail domain name in DNS syntax (see sect. 4.2). When we store a
   'table2' or a 'gate' table entry,  will be an RFC822 mail
   domain name, including both fully qualified DNS domains and mail only
   domains (MX-only domains). All normal DNS conventions, like default
   values, wildcards, abbreviations and message compression, apply also
   for all the components of the PX RR. In particular , MAP822 and
   MAPX400, as  elements, must have the final "." (root)
   when they are fully qualified.

4.1 Additional features of the PX resource record

   The definition of the RDATA for the PX resource record, and the fact
   that DNS allows a distinction between an exact value and a wildcard
   match for the  parameter, represent an extension of the RFC1327
   specification for mapping rules. In fact, any RFC1327 mapping table
   entry is an implicit wildcard entry, i.e., the rule

      net2.it#PRMD$net2.ADMD$p400.C$it#

   covers any RFC822 domain ending with 'net2.it', unless more detailed
   rules for some subdomain in 'net2.it' are present. Thus there is no
   possibility to specify explicitly an RFC1327 entry as an exact match
   only rule. In DNS an entry like

      *.net2.it.   IN  PX  10   net2.it.  PRMD-net2.ADMD-p400.C-it.

   specify the usual wildcard match as for RFC1327 tables. However an
   entry like

      ab.net2.it.  IN  PX  10   ab.net2.it.  O-ab.PRMD-net2.ADMDb.C-it.




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   is valid only for an exact match of 'ab.net2.it' RFC822 domain.

   Note also that in DNS syntax there is no '#' delimiter around MAP822
   and MAPX400 fields: the syntax defined in sect. 4.2 in fact does not
   allow the  (ASCII decimal 32) character within these fields,
   making unneeded the use of an explicit delimiter as required in the
   RFC1327 original syntax.

   Another extension to the RFC1327 specifications is the PREFERENCE
   value defined as part of the PX RDATA section. This numeric value has
   exactly the same meaning than the similar one used for the MX RR. It
   is thus possible to specify more than one single mapping for a domain
   (both from RFC822 to X.400 and vice versa), giving as the preference
   order. In RFC1327 static tables, however, you cannot specify more
   than one mapping per each RFC822 domain, and the same restriction
   apply for any X.400 domain mapping to an RFC822 one.

   More over, in the X.400 recommendations a note suggests than an
   ADMD= should be reserved for some special cases. Various
   national functional profile specifications for an X.400 MHS states
   that if an X.400 PRMD is reachable via any of its national ADMDs,
   independently of its actual single or multiple connectivity with
   them, it should use ADMD= to advertise this fact. Again, if a
   PRMD has no connections to any ADMD it should use ADMD=0 to notify
   its status, etc. However, in most of the current real situations, the
   ADMD service providers do not accept messages coming from their
   subscribers if they have a blank ADMD, forcing them to have their own
   ADMD value. In such a situation there are problems in indicating
   properly the actually working mappings for domains with multiple
   connectivity. The PX RDATA 'PREFERENCE' extension was introduced to
   take in consideration these problems.

   However, as these extensions are not available with RFC1327 static
   tables, it is strongly discouraged to use them when interworking with
   any table based gateway or application. The extensions were in fact
   introduced just to add more flexibility, like the PREFERENCE value,
   or they were already implicit in the DNS mechanism, like the wildcard
   specification. They should be used very carefully or just considered
   'reserved for future use'. In particular, for current use, the
   PREFERENCE value in the PX record specification should be fixed to a
   value of 50, and only wildcard specifications should be used when
   specifying  values.

4.2 The DNS syntax for an X.400 'domain'

   The syntax definition of the RFC1327 mapping rules is defined in
   appendix F of that document. However that syntax is not very human
   oriented and contains a number of characters which have a special



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   meaning in other fields of the Internet DNS. Thus in order to avoid
   any possible problem, especially due to some old DNS implementations
   still being used in the Internet, we define a syntax for the X.400
   part of any RFC1327 mapping rules (and hence for any X.400 O/R name)
   which makes it compatible with a  element, i.e.,

       ::=  | " "
         ::=