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rfc:rfc1664

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
 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

Allocchio, Bonito, Cole, Giordano & Hagens [Page 2] RFC 1664 Internet DNS for Mail Mapping Tables August 1994

 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:
  1. It avoids fetching and storing of entire mapping tables by every

host that wishes to implement RFC1327 gateways and/or tools

  1. Modifications to the DNS based mapping information can be made

available in a more timely manner than with a table driven

     approach.
  1. It allows full authority delegation, in agreement with the

Internet regionalization process.

Allocchio, Bonito, Cole, Giordano & Hagens [Page 3] RFC 1664 Internet DNS for Mail Mapping Tables August 1994

  1. Table management is not necessarily required for DNS-based

RFC1327 gateways.

  1. 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.

Allocchio, Bonito, Cole, Giordano & Hagens [Page 4] RFC 1664 Internet DNS for Mail Mapping Tables August 1994

 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:

Allocchio, Bonito, Cole, Giordano & Hagens [Page 5] RFC 1664 Internet DNS for Mail Mapping Tables August 1994

                                  . (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

Allocchio, Bonito, Cole, Giordano & Hagens [Page 6] RFC 1664 Internet DNS for Mail Mapping Tables August 1994

 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:

Allocchio, Bonito, Cole, Giordano & Hagens [Page 7] RFC 1664 Internet DNS for Mail Mapping Tables August 1994

 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 <domain-name> element containing <rfc822-domain>, the
              RFC822 part of the RFC1327 mapping information;
 MAPX400      A <domain-name> element containing the value of
              <x400-in-domain-syntax> 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:
           <name> [<class>] [<TTL>] <type> <RDATA>
 When we store in DNS a 'table1' entry, then <name> 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, <name> 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 <name>, MAP822 and
 MAPX400, as <domain-name> 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 <name> 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.

Allocchio, Bonito, Cole, Giordano & Hagens [Page 8] RFC 1664 Internet DNS for Mail Mapping Tables August 1994

 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 <blank> (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=<blank> 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=<blank> 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 <name> 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

Allocchio, Bonito, Cole, Giordano & Hagens [Page 9] RFC 1664 Internet DNS for Mail Mapping Tables August 1994

 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 <domain-name> element, i.e.,
 <domain-name>    ::= <subdomain> | " "
 <subdomain>      ::= <label> | <label> "." <subdomain>
 <label>          ::= <alphanum>|
                      <alphanum> {<alphanumhyphen>} <alphanum>
 <alphanum>       ::= "0".."9" | "A".."Z" | "a".."z"
 <alphanumhyphen> ::= "0".."9" | "A".."Z" | "a".."z" | "-"
 (see RFC1035, section 2.3.1, page 8).  The legal character set for
 <label> does not correspond to the IA5 Printablestring one used in
 RFC1327 to define mapping rules. However a very simple "escape
 mechanism" can be applied in order to bypass the problem. We can in
 fact simply describe the X.400 part of an RFC1327 mapping rule format
 as:
   <map-rule>   ::= <map-elem> | <map-elem> { "." <map-elem> }
   <map-elem>   ::= <attr-label> "$" <attr-value>
   <attr-label> ::= "C" | "ADMD" | "PRMD" | "O" | "OU"
   <attr-value> ::= " " | "@" | IA5-Printablestring
 As you can notice <domain-name> and <map-rule> look similar, and also
 <label> and <map-elem> look the same. If we define the correct method
 to transform a <map-elem> into a <label> and vice versa the problem
 to write an RFC1327 mapping rule in <domain-name> syntax is solved.
 The RFC822 domain part of any RFC1327 mapping rule is of course
 already in <domain-name> syntax, and thus remains unchanged.
 In particular, in a 'table1' mapping rule the 'keyword' value must be
 converted into <x400-in-domain-syntax> (X.400 mail DNS mail domain),
 while the 'translator' value is already a valid RFC822 domain.  Vice
 versa in a 'table2' or 'gate' mapping rule, the 'translator' must be
 converted into <x400-in-domain-syntax>, while the 'keyword' is
 already a valid RFC822 domain.

4.2.1 IA5-Printablestring to <alphanumhyphen> mappings

 The problem of unmatching IA5-Printablestring and <label> character
 set definition is solved by a simple character mapping rule: whenever
 an IA5 character does not belong to <alphanumhyphen>, then it is
 mapped using its 3 digit decimal ASCII code, enclosed in hyphens. A
 small set of special rules is also defined for the most frequent
 cases. Moreover some frequent characters combinations used in RFC1327

Allocchio, Bonito, Cole, Giordano & Hagens [Page 10] RFC 1664 Internet DNS for Mail Mapping Tables August 1994

 rules are also mapped as special cases.
 Let's then define the following simple rules:
  RFC1327 rule          DNS store translation    conditions
  -----------------------------------------------------------------
  <attr-label>$@        <attr-label>             missing attribute
  <attr-label>$<blank>  <attr-label>"b"          blank attribute
  <attr-label>$xxx      <attr-label>-xxx         elsewhere
 Non <alphanumhyphen> characters in <attr-value>:
  RFC1327 rule          DNS store translation    conditions
  -----------------------------------------------------------------
  -                     -h-                      hyphen
  \.                    -d-                      quoted dot
  <blank>               -b-                      blank
  <non A/N character>   -<3digit-decimal>-       elsewhere
 If the DNS store translation of <attr-value> happens to end with an
 hyphen, then this last hyphen is omitted.
 Let's now have some examples:
  RFC1327 rule          DNS store translation    conditions
  -----------------------------------------------------------------
  PRMD$@                PRMD                     missing attribute
  ADMD$<blank>          ADMDb                    blank attribute
  ADMD$400-net          ADMD-400-h-net           hyphen mapping
  PRMD$UK\.BD           PRMD-UK-d-BD             quoted dot mapping
  O$ACME Inc\.          O-ACME-b-Inc-d           blank & final hyphen
  PRMD$main-400-a       PRMD-main-h-400-h-a      hyphen mapping
  O$-123-b              O--h-123-h-b             hyphen mapping
  OU$123-x              OU-123-h-x               hyphen mapping
  PRMD$Adis+co          PRMD-Adis-043-co         3digit mapping
 Thus, an X.400 part from an RFC1327 mapping rule like
   OU$uuu.O$@.PRMD$ppp\.rrr.ADMD$aaa ddd-mmm.C$cc
 translates to
   OU-uuu.O.PRMD-ppp-d-rrr.ADMD-aaa-b-ddd-h-mmm.C-cc

Another example:

   OU$sales dept\..O$@.PRMD$ACME.ADMD$ .C$GB

Allocchio, Bonito, Cole, Giordano & Hagens [Page 11] RFC 1664 Internet DNS for Mail Mapping Tables August 1994

 translates to
   OU-sales-b-dept-d.O.PRMD-ACME.ADMDb.C-GB

4.2.2 Flow chart

 In order to achieve the proper DNS store translations of the X.400
 part of an RFC1327 mapping rules or any other X.400 O/R name, some
 software tools will be used. It is in fact evident that the above
 rules for converting mapping table from RFC1327 to DNS format (and
 vice versa) are not user friendly enough to think of a human made
 conversion.
 To help in designing such tools, we describe hereunder a small flow
 chart. The fundamental rule to be applied during translation is,
 however, the following:
    "A string must be parsed from left to right, moving appropriately
    the pointer in order not to consider again the already translated
    left section of the string in subsequent analysis."
 Flow chart 1 - Translation from RFC1327 to DNS format:
               parse  single attribute
            (enclosed in "." separators)
                         |
          (yes)  ---  <label>$@ ?  ---  (no)
            |                             |
      map to <label>        (no)  <label>$<blank> ?  (yes)
            |                 |                        |
            |           map to <label>-        map to <label>"b"
            |                 |                        |
            |           map "\." to -d-                |
            |                 |                        |
            |           map "-" to -h-                 |
            |                 |                        |
            |    map non A/N char to -<3digit>-        |
restart     |                 |                        |
   ^        |      remove (if any) last "-"            |
   |        |                 |                        |
   |        \------->     add a  "."    <--------------/
   |                          |
   \----------  take  next  attribute  (if  any)

Allocchio, Bonito, Cole, Giordano & Hagens [Page 12] RFC 1664 Internet DNS for Mail Mapping Tables August 1994

 Flow chart 2 - Translation from DNS to RFC1327 format:
              parse single attribute
          (enclosed in "." separators)
                        |
          (yes) ---- <label> ? ---- (no)
            |                          |
    map to <label>$@        (no) <label>"b" ? (yes)
            |                 |                 |
            |           map to <label>$    map to <label>$<blank>
            |                 |                 |
            |           map -d- to "\."         |
            |                 |                 |
            |           map -h- to "-"          |
            |                 |                 |
            |           map -b- to " "          |
restart     |                 |                 |
   ^        |   map -<3digit>- to non A/N char  |
   |        |                 |                 |
   |        \-------->   add a "."   <----------/
   |                         |
   \------------- take next attribute (if any)
 Note that the above flow charts deal with the translation of the
 attributes syntax, only.

4.2.3 The Country Code convention in the <name> value.

 The RFC822 domain space and the X.400 O/R address space, as said in
 section 3, have one specific common feature: the X.400 ISO country
 codes are the same as the RFC822 ISO top level domains for countries.
 In the previous sections we have also defined a method to write in
 <domain-name> syntax any X.400 domain, while in section 3 we
 described the new name space starting at each country top level
 domain under the X42D.cc (where 'cc' is then two letter ISO country
 code).
 The <name> value for a 'table1' entry in DNS should thus be derived
 from the X.400 domain value, translated to <domain-name> syntax,
 adding the 'X42D.cc.' post-fix to it, i.e.,
    ADMD$acme.C$fr
 produces in <domain-name> syntax the key:
    ADMD-acme.C-fr

Allocchio, Bonito, Cole, Giordano & Hagens [Page 13] RFC 1664 Internet DNS for Mail Mapping Tables August 1994

 which is post-fixed by 'X42D.fr.' resulting in:
    ADMD-acme.C-fr.X42D.fr.
 However, due to the identical encoding for X.400 country codes and
 RFC822 country top level domains, the string 'C-fr.X42D.fr.' is
 clearly redundant.
 We thus define the 'Country Code convention' for the <name> key,
 i.e.,
    "The C-cc section of an X.400 domain in <domain-name> syntax must
    be omitted when creating a <name> key, as it is identical to the
    top level country code used to identify the DNS zone where the
    information is stored".
 Thus we obtain the following <name> key examples:
 X.400 domain                       DNS <name> key
 --------------------------------------------------------------------
 ADMD$acme.C$fr                     ADMD-acme.X42D.fr.
 PRMD$ux\.av.ADMD$ .C$gb            PRMD-ux-d-av.ADMDb.X42D.gb.
 PRMD$ppb.ADMD$Dat 400.C$de         PRMD-ppb.ADMD-Dat-b-400.X42D.de.

4.3 Creating the appropriate DNS files

 Using RFC1327's assumption of an asymmetric mapping between X.400 and
 RFC822 addresses, two separate relations are required to store the
 mapping database: RFC1327 'table1' and RFC1327 'table2'; thus also in
 DNS we will maintain the two different sections, even if they will
 both use the PX resource record. More over RFC1327 also specify a
 third table: RFC1327 'gate' Table. This additional table, however,
 has the same syntax rules than RFC1327 'table2' and thus the same
 translation procedure as 'table2' will be applied; some details about
 the RFC1327 'gate' table are discussed in section 4.4.
 Let's now check how to create, from an RFC1327 mapping rule entry,
 the appropriate DNS entry in a DNS data file. We can again define an
 RFC1327 mapping rule entry as defined in appendix F of that document
 as:
   <x400-domain>#<rfc822-domain>#  (case A: 'table1' entry)
 and
   <rfc822-domain>#<x400-domain>#  (case B: 'table2' and 'gate' entry)
 The two cases must be considered separately. Let's consider case A.

Allocchio, Bonito, Cole, Giordano & Hagens [Page 14] RFC 1664 Internet DNS for Mail Mapping Tables August 1994

  1. take <x400-domain> and translate it into <domain-name> syntax,

obtaining <x400-in-domain-syntax>;

  1. create the <name> key from <x400-in-domain-syntax> i.e., apply

the Country Code convention described in sect. 4.2.3;

  1. construct the DNS PX record as:
  • .<name> IN PX 50 <rfc822-domain> <x400-in-domain-syntax>
 Please note that within PX RDATA the <rfc822-domain> precedes the
 <x400-in-domain-syntax> also for a 'table1' entry.
 an example: from the rule
   PRMD$ab.ADMD$ac.C$fr#ab.fr#
 we obtain
  • .PRMD-ab.ADMD-ac.X42D.fr. IN PX 50 ab.fr. PRMD-ab.ADMD-ac.C-fr.
 Note that <name>, <rfc822-domain> and <x400-in-domain-syntax> are
 fully qualified <domain-name> elements, thus ending with a ".".
 Let's now consider case B.
  1. take <rfc822-domain> as <name> key;
  2. translate <x400-domain> into <x400-in-domain-syntax>;
  3. construct the DNS PX record as:
  • .<name> IN PX 50 <rfc822-domain> <x400-in-domain-syntax>
 an example: from the rule
   ab.fr#PRMD$ab.ADMD$ac.C$fr#
 we obtain
  • .ab.fr. IN PX 50 ab.fr. PRMD-ab.ADMD-ac.C-fr.
 Again note the fully qualified <domain-name> elements.

Allocchio, Bonito, Cole, Giordano & Hagens [Page 15] RFC 1664 Internet DNS for Mail Mapping Tables August 1994

 A file containing the RFC1327 mapping rules and RFC1327 'gate' table
 written in DNS format will look like the following fictious example:
   !
   ! RFC1327 table 1: X.400 --> RFC822
   !
   *.ADMD-acme.X42D.it.               IN  PX  50  it. ADMD-acme.C-it.
   *.PRMD-accred.ADMD-tx400.X42D.it.  IN  PX  50   \
                              accred.it. PRMD-accred.ADMD-tx400.C-it.
   *.O-u-h-newcity.PRMD-x4net.ADMDb.X42D.it.  IN  PX  50   \
                     cs.ncty.it. O-u-h-newcity.PRMD-x4net.ADMDb.C-it.
   !
   ! RFC1327 table 2: RFC822 --> X.400
   !
   *.nrc.it.    IN  PX  50   nrc.it. PRMD-nrc.ADMD-acme.C-it.
   *.ninp.it.   IN  PX  50   ninp.it. O.PRMD-ninp.ADMD-acme.C-it.
   *.bd.it.     IN  PX  50   bd.it. PRMD-uk-d-bd.ADMDb.C-it.
   !
   ! RFC1327 Gate Table
   !
   my.it.  IN PX 50  my.it. OU-int-h-gw.O.PRMD-ninp.ADMD-acme.C-it.G.
   co.it.  IN PX 50  co.it. O-mhs-h-relay.PRMD-x4net.ADMDb.C-it.G.
 (here the "\" indicates continuation on the same line, as wrapping is
 done only due to typographical reasons).
 Note the special suffix ".G." on the right side of the 'gate' Table
 section whose aim is described in section 4.4. The corresponding
 RFC1327 tables are:
   #
   # RFC1327 table 1: X.400 --> RFC822
   #
   ADMD$acme.C$it#it#
   PRMD$accred.ADMD$tx400.C$it#accred.it#
   O$u-newcity.PRMD$x4net.ADMD$ .C$it#cs.ncty.it#
   #
   # RFC1327 table 2: RFC822 --> X.400
   #
   nrc.it#PRMD$nrc.ADMD$acme.C$it#
   ninp.it#O.PRMD$ninp.ADMD$acme.C$it#
   bd.it#PRMD$uk\.bd.ADMD$ .C$it#
   #
   # RFC1327 Gate Table
   #
   my.it#OU$int-gw.O$@.PRMD$ninp.ADMD$acme.C$it#
   co.it#O$mhs-relay.PRMD$x4net.ADMD$ .C$t#

Allocchio, Bonito, Cole, Giordano & Hagens [Page 16] RFC 1664 Internet DNS for Mail Mapping Tables August 1994

4.4 Storing the RFC1327 Gate table

 Section 4.3.4 of RFC1327 also specify how an address should be
 converted between RFC822 and X.400 in case a complete mapping is
 impossible. To allow the use of DDAs for non mappable domains, the
 RFC1327 'gate' table is thus introduced. DNS must store and
 distribute also these data.
 One of the major features of the DNS is the ability to distribute the
 authority: a certain site runs the "primary" nameserver for one
 determined sub-tree and thus it is also the only place allowed to
 update information regarding that sub-tree. This fact allows, in our
 case, a further additional feature to the table based approach. In
 fact we can avoid one possible ambiguity about the use of the 'gate'
 table (and thus of DDAs encoding).
 The authority maintaining a DNS entry in the usual RFC822 domain
 space is the only one allowed to decide if its domain should be
 mapped using Standard Attributes (SA) syntax or Domain Defined
 Attributes (DDA) one. If the authority decides that its RFC822 domain
 should be mapped using SA, then the PX RDATA will be a 'table2'
 entry, otherwise it will be a 'gate' table entry. Thus for an RFC822
 domain we cannot have any more two possible entries, one from 'table2
 and another one from 'gate' table, and the action for a gateway
 results clearly stated.
 The RFC1327 'gate' table syntax is actually identical to RFC1327
 'table2'. Thus the same syntax translation rules from RFC1327 to DNS
 format can be applied. However a gateway or any other application
 must know if the answer it got from DNS contains some 'table2' or
 some 'gate' table information. This is easily obtained flagging with
 an additional ".G." post-fix the PX RDATA value when it contains a
 'gate' table entry. The example in section 4.3 shows clearly the
 result. As any X.400 O/R domain must end with a country code ("C-xx"
 in our DNS syntax) the additional ".G." creates no conflicts or
 ambiguities at all. This postfix must obviously be removed before
 using the RFC1327 'gate' table data.

5. Finding RFC1327 mapping information from DNS

 The RFC1327 mapping information is stored in DNS both in the normal
 RFC822 domain name space, and in the newly defined X.400 name space.
 The information, stored in PX resource records, does not represent a
 full RFC822 or X.400 O/R address: it is a template which specifies
 the fields of the domain that are used by the mapping algorithm.
 When mapping information is stored in the DNS, queries to the DNS are
 issued whenever an iterative search through the mapping table would

Allocchio, Bonito, Cole, Giordano & Hagens [Page 17] RFC 1664 Internet DNS for Mail Mapping Tables August 1994

 be performed (RFC1327: section 4.3.4, State I; section 4.3.5, mapping
 B). Due to the DNS search mechanism, DNS by itself returns the
 longest possible match in the stored mapping rule with a single
 query, thus no iteration and/or multiple queries are needed. As
 specified in RFC1327, a search of the mapping table will result in
 either success (mapping found) or failure (query failed, mapping not
 found).
 When a DNS query is issued, a third possible result is timeout. If
 the result is timeout, the gateway operation is delayed and then
 retried at a later time. A result of success or failure is processed
 according to the algorithms specified in RFC1327. If a DNS error code
 is returned, an error message should be logged and the gateway
 operation is delayed as for timeout. These pathological situations,
 however, should be avoided with a careful duplication and chaching
 mechanism which DNS itself provides.
 Searching the nameserver which can authoritatively solve the query is
 automatically performed by the DNS distributed name service.

5.1 A DNS query example

 An RFC1327 mail-gateway located in the Internet, when translating
 addresses from RFC822 to X.400, can get information about the RFC1327
 mapping rule asking the DNS. As an example, when translating the
 address SUN.CCE.NRC.IT, the gateway will just query DNS for the
 associated PX resource record. The DNS should contain a PX record
 like this:
  • .cce.nrc.it. IN PX 50 cce.nrc.it. O-cce.PRMD-nrc.ADMD-acme.C-it.
 The first query will return immediately the appropriate mapping rule
 in DNS store format.
 There is no ".G." at the end of the obtained PX RDATA value, thus
 applying the syntax translation specified in paragraph 4.2 the
 RFC1327 Table 2 mapping rule will be obtained.
 Let's now take another example where a 'gate' table rule is returned.
 If we are looking for an RFC822 domain ending with top level domain
 "MW", and the DNS contains a PX record like this,
  • .mw. IN PX 50 mw. O-cce.PRMD-nrc.ADMD-acme.C-it.G.
 DNS will return 'mw.' and 'O-cce.PRMD-nrc.ADMD-acme.C-it.G.', i.e., a
 'gate' table entry in DNS store format. Dropping the final ".G." and
 applying the syntax translation specified in paragraph 4.2 the
 original rule will be available. More over, the ".G." flag also tells

Allocchio, Bonito, Cole, Giordano & Hagens [Page 18] RFC 1664 Internet DNS for Mail Mapping Tables August 1994

 the gateway to use DDA encoding for the inquired RFC822 domain.
 On the other hand, translating from X.400 to RFC822 the address
    C=de; ADMD=pkz; PRMD=nfc; O=top;
 the mail gateway should convert the syntax according to paragraph
 4.2, apply the 'Country code convention' described in 4.2.3 to derive
 the appropriate DNS translation of the X.400 O/R name and then query
 DNS for the corresponding PX resource record. The obtained record for
 which the PX record must be queried is thus:
    O-top.PRMD-nfc.ADMD-pkz.X42D.de.
 The DNS could contain:
  • .ADMD-pkz.X42D.de. IN PX 50 pkz.de. ADMD-pkz.C-de.
 Assuming that there are not more specific records in DNS, the
 wildcard mechanism will return the RFC1327 'table1' rule in encoded
 format.

6. Administration of mapping information

 The DNS, using the PX RR, will be able to distribute the mapping
 information to all RFC1327 gateways located on the Internet. However,
 not all RFC1327 gateways will be able to use the Internet DNS. It is
 expected that some gateways in a particular management domain will
 conform to one of the following models:
    (a) Table-based, (b) DNS-based, (c) X.500-based
 Table-based management domains will continue to submit and retrieve
 their mapping tables from the International Mapping Table coordinator
 manually or via some automated procedures. Their mapping information
 should be made available in DNS by the appropriate DNS authority
 using the same mechanism already in place for MX records: if a branch
 has not yet in place its own DNS server, some higher authority in the
 DNS tree will provide the service for it. A transition procedure
 similar to the one used to migrate from the 'hosts.txt' tables to DNS
 can be applied also to the deployment phase of this proposal. An
 informational document describing the implementation phase and the
 detailed coordination procedures is expected. The deployment phase
 must also follow the directives produced by the current work on
 RFC1327 mapping authorities, in order to insure consistency in the
 mapping information itself.

Allocchio, Bonito, Cole, Giordano & Hagens [Page 19] RFC 1664 Internet DNS for Mail Mapping Tables August 1994

 Another distributed directory service which can distribute the
 RFC1327 mapping information is X.500. The coordination, alignment and
 uniqueness of mapping information between DNS and X.500 is an
 essential fact if it happens to have both systems in place. The ideal
 solution is a dynamic alignment mechanism which transparently makes
 the DNS mapping information available in X.500 and vice versa. Some
 work in this specific field is already being done [see Costa] which
 can result in a global transparent directory service, where the
 information is stored in DNS or in X.500, but is visible completely
 by any of the two systems.

7. Conclusion

 The introduction of the new PX resource record and the definition of
 the X.400 O/R name space in the DNS structure provide a good
 repository for mapping information. The mapping information is stored
 in the DNS tree structure so that it can be easily obtained using the
 DNS distributed name service. At the same time the definition of the
 appropriate DNS space for X.400 O/R names provide a repository where
 to store and distribute some other X.400 MHS information. The use of
 the DNS has many known advantages in storing, managing and updating
 the information. A successful number of tests have been performed
 under the provisional top level domain "X400.IT", and their results
 confirmed the advantages of the method.
 Software to query the DNS and then to convert between the textual
 representation of DNS resource records and the address format defined
 in RFC1327 needs to be developed. This software must also allow a
 smooth implementation and deployment period, eventually taking care
 of the transition phase. A further informational document describing
 operational and implementation of the service is expected.

8. Acknowledgements

 We wish to thanks all those who contributed to the discussion and
 revision of this document: many of their ideas and suggestions
 constitute essential parts of this work. In particular thanks to Jon
 Postel, Paul Mockapetris, Rob Austin and the whole IETF x400ops, RARE
 wg-msg and IETF namedroppers groups. A special mention to Christian
 Huitema for his fundamental contribution to this work.

Allocchio, Bonito, Cole, Giordano & Hagens [Page 20] RFC 1664 Internet DNS for Mail Mapping Tables August 1994

9. References

 [CCITT] CCITT SG 5/VII, "Recommendation X.400, Message Handling
     Systems: System Model - Service Elements", October 1988.
 [RFC 1327] Kille, S., "Mapping between X.400(1988)/ISO 10021 and RFC
     822", RFC 1327, March 1992.
 [RFC 1034] Mockapetris, P., "Domain Names - Concepts and Facilities",
     STD 13, RFC 1034, USC/Information Sciences Institute, November
     1987.
 [RFC 1035] Mockapetris, P., "Domain names - Implementation and
     Specification", STD 13, RFC 1035, USC/Information Sciences
     Institute, November 1987.
 [RFC 1033] Lottor, M., "Domain Administrators Operation Guide", RFC
     1033, SRI International, November 1987.
 [Costa] Costa, A., Macedo, J., and V. Freitas, "Accessing and
     Managing DNS Information in the X.500 Directory", Proceeding of
     the 4th Joint European Networking Conference, Trondheim, NO, May
     1993.
 [Houttin] Houttin, J., Hansen, K., and S. Aumont, "Address Mapping
     Functions and Authorities", Internet-DRAFT, May 1993.

10. Security Considerations

 Security issues are not discussed in this memo.

Allocchio, Bonito, Cole, Giordano & Hagens [Page 21] RFC 1664 Internet DNS for Mail Mapping Tables August 1994

11. Authors' Addresses

 Claudio Allocchio
 Sincrotrone Trieste
 Padriciano 99
 I 34012 Trieste
 Italy
 RFC822: Claudio.Allocchio@elettra.trieste.it
 X.400:  C=it;A=garr;P=Trieste;O=Elettra;
 S=Allocchio;G=Claudio;
 Phone:  +39 40 3758523
 Fax:    +39 40 226338
 Antonio Blasco Bonito
 CNUCE - CNR
 Reparto infr. reti
 Viale S. Maria 36
 I 56126 Pisa
 Italy
 RFC822: bonito@cnuce.cnr.it
 X.400:  C=it;A=garr;P=cnr;O=cnuce;S=bonito;
 Phone:  +39 50 593246
 Fax:    +39 50 589354
 Bruce Cole
 Cisco Systems Inc.
 P.O. Box 3075
 1525 O'Brien Drive
 Menlo Park, CA 94026
 U.S.A.
 RFC822: bcole@cisco.com
 X.400:  C=us;A= ;P=Internet;
 DD.rfc-822=bcole(a)cisco.com;
 Phone:  +1 415 6888245
 Fax:    +1 415 6884575

Allocchio, Bonito, Cole, Giordano & Hagens [Page 22] RFC 1664 Internet DNS for Mail Mapping Tables August 1994

 Silvia Giordano
 Centro Svizzero di
 Calcolo Scientifico
 Via Cantonale
 CH 6928 Manno
 Switzerland
 RFC822: giordano@cscs.ch
 X.400:  C=ch;A=arcom;P=switch;O=cscs;
 S=giordano;
 Phone:  +41 91 508213
 Fax:    +41 91 506711
 Robert Hagens
 Advanced Network and Services
 1875 Campus Commons Drive
 Reston, VA 22091
 U.S.A.
 RFC822: hagens@ans.net
 X.400:  C=us;A= ;P=Internet;
 DD.rfc-822=hagens(a)ans.net;
 Phone:  +1 703 7587700

Allocchio, Bonito, Cole, Giordano & Hagens [Page 23]

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