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Network Working Group                                       C. AllocchioRequest for Comments: 2163                                    GARR-ItalyObsoletes:1664                                             January 1998Category: Standards TrackUsing the Internet DNS to DistributeMIXER Conformant Global Address Mapping (MCGAM)Status of this Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (1998).  All Rights Reserved.Abstract   This memo is the complete technical specification to store in the   Internet Domain Name System (DNS) the mapping information (MCGAM)   needed by MIXER conformant e-mail gateways and other tools to mapRFC822 domain names into X.400 O/R names and vice versa.  Mapping   information can be managed in a distributed rather than a centralised   way. Organizations can publish their MIXER mapping or preferred   gateway routing information using just local resources (their local   DNS server), avoiding the need for a strong coordination with any   centralised organization. MIXER conformant gateways and tools 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 obsoletesRFC1664. It includes the changes introduced by   MIXER specification with respect toRFC1327: the new 'gate1' (O/R   addresses to domain) table is fully supported. Full backward   compatibility withRFC1664 specification is mantained, too.RFC1664 was a joint effort of IETF X400 operation working group   (x400ops) and TERENA (formely named "RARE") Mail and Messaging   working group (WG-MSG). This update was performed by the IETF MIXER   working group.Allocchio                   Standards Track                     [Page 1]

RFC 2163                      MIXER MCGAM                   January 19981. 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/toRFC822 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 bothRFC822 and foreign   world.   MIXER describes a set of mappings (MIXER Conformant Global Address   Mapping - MCGAM) which will enable interworking between systems   operating the CCITT X.400 (1984/88/92) Recommendations and systems   using using theRFC822 mail protocol, or protocols derived fromRFC822. 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 MIXER: the mechanism   for mapping between X.400 O/R addresses andRFC822 domain names. As   described inAppendix F of MIXER, implementation of the mappings   requires a database which maps between X.400 O/R addresses and domain   names; inRFC1327 this database was statically defined.   The original approach inRFC1327 required many efforts to maintain   the correct mapping: all the gateways needed to get coherent tables   to apply the same mappings, the conversion tables had to be   distributed among all the operational gateways, and also every update   needed to be distributed.   The concept of mapping rules distribution and use has been revised in   the new MIXER specification, introducing the concept of MIXER   Conformant Global Address Mapping (MCGAM). A MCGAM does not need to   be globally installed by any MIXER conformant gateway in the world   any more. However MIXER requires now efficient methods to publish its   MCGAM.   Static tables are one of the possible methods to publish MCGAM.   However 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.Allocchio                   Standards Track                     [Page 2]

RFC 2163                      MIXER MCGAM                   January 1998   Two much more efficient methods are proposed by MIXER for publication   of MCGAM: the Internet DNS and X.500. This memo is the complete   technical specification for publishing MCGAM via Internet DNS.   A first proposal to use the Internet DNS to store, retrieve and   maintain those mappings was introduced by two of the authors ofRFC1664 (B. Cole and R. Hagens) adopting two new DNS resource record   (RR)  types: TO-X400 and TO-822. This proposal now adopts a more   complete strategy, and requires one new RR only. The distribution of   MCGAMs 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 InternetRFC822 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 deployed by some of the authors ofRFC1664 at the time of its development. 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. TheRFC822 to X.400 mapping rules (including the so   called 'gate2' 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   toRFC822 mappings ('table1' and 'gate1'). 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 MCGAM syntax, showing how it can be   stored into the Internet DNS.Allocchio                   Standards Track                     [Page 3]

RFC 2163                      MIXER MCGAM                   January 19981.1 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 MIXER gateways require that a database store   address mapping information for X.400 andRFC822. This information   must be made available (published) to all MIXER 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 MIXER 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.     - Table management is not necessarily required for DNS-based       MIXER 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 MIXER 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 anRFC822 e-mail address) and their associated information, i.e., host   IP addresses, mail exchanger names, etc., are stored in the DNS as aAllocchio                   Standards Track                     [Page 4]

RFC 2163                      MIXER MCGAM                   January 1998   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. This memo defines in the   above structure the appropriate way to locate the X.400 O/R name   space, thus enabling to store in DNS the MIXER mappings (MCGAMs).   The MIXER mapping information is composed by four tables:    - 'table1' and 'gate1' gives the translation from X.400 toRFC822;    - 'table2' and 'gate2' tables mapRFC822 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 'gate2' table entry is a validRFC822   domain; thus the usual domain name space can be used without problems   to store these entries.   On the other hand, the keyword of a 'table1' and 'gate1' 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 authorityAllocchio                   Standards Track                     [Page 5]

RFC 2163                      MIXER MCGAM                   January 1998   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 theRFC822 country top-level domains and this gives us an   appropriate hook to insert the new branches. The 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:                                    . (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 MIXER mapping tables; the use of the   Internet DNS gives further motivations for this coordination.Allocchio                   Standards Track                     [Page 6]

RFC 2163                      MIXER MCGAM                   January 1998   The coordination at national level also fits in the new concept of   MCGAM pubblication. The DNS in fact allows a step by step authority   distribution, up to a final complete delegation: thus organizations   whishing to publish their MCGAM just need to receive delegation also   for their branch of the new X.400 name space. 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   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 MIXER 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 theRFC822 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 toRFC822. More over the mapping information, i.e., the   MCGAMs, 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.Allocchio                   Standards Track                     [Page 7]

RFC 2163                      MIXER MCGAM                   January 1998   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:   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>, theRFC822 part of the MCGAM;   MAPX400      A <domain-name> element containing the value of                <x400-in-domain-syntax> derived from the X.400 part of                the MCGAM (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' or a 'gate1' 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 'gate2' table entry, <name> will be anRFC822   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.Allocchio                   Standards Track                     [Page 8]

RFC 2163                      MIXER MCGAM                   January 19984.1 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 MIXER   specification for mapping rules. In fact, any MCGAM entry is an   implicit wildcard entry, i.e., the rule      net2.it#PRMD$net2.ADMD$p400.C$it#   covers anyRFC822 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 a MCGAM 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 MIXER tables. However an   entry like      ab.net2.it.  IN  PX  10   ab.net2.it.  O-ab.PRMD-net2.ADMDb.C-it.   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   MIXER original syntax.   Another extension to the MIXER 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 fromRFC822 to X.400 and vice versa), giving as the preference   order. In MIXER static tables, however, you cannot specify more than   one mapping per eachRFC822 domain, and the same restriction apply   for any X.400 domain mapping to anRFC822 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 theirAllocchio                   Standards Track                     [Page 9]

RFC 2163                      MIXER MCGAM                   January 1998   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 MIXER 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 MCGAM rules is defined inappendix F of   that document. However that syntax is not very human oriented and   contains a number of characters which have a special 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   MCGAM 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" | "-"   (seeRFC1035, section 2.3.1, page 8).  The legal character set for   <label> does not correspond to the IA5 Printablestring one used in   MIXER to define MCGAM 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 a MCGAM 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-PrintablestringAllocchio                   Standards Track                    [Page 10]

RFC 2163                      MIXER MCGAM                   January 1998   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 a MCGAM rule in <domain-name> syntax is solved.   TheRFC822 domain part of any MCGAM rule is of course already in   <domain-name> syntax, and thus remains unchanged.   In particular, in a 'table1' or 'gate1' 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 validRFC822   domain.  Vice versa in a 'table2' or 'gate2' mapping rule, the   'translator' must be converted into <x400-in-domain-syntax>, while   the 'keyword' is already a validRFC822 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 MIXER   rules are also mapped as special cases.   Let's then define the following simple rules:    MCGAM 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>:    MCGAM 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:Allocchio                   Standards Track                    [Page 11]

RFC 2163                      MIXER MCGAM                   January 1998    MCGAM 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 a MCGAM 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   translates to     OU-sales-b-dept-d.O.PRMD-ACME.ADMDb.C-GB4.2.2 Flow chart   In order to achieve the proper DNS store translations of the X.400   part of a MCGAM 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 MIXER 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."Allocchio                   Standards Track                    [Page 12]

RFC 2163                      MIXER MCGAM                   January 1998   Flow chart 1 - Translation from MIXER 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)   Flow chart 2 - Translation from DNS to MIXER 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)Allocchio                   Standards Track                    [Page 13]

RFC 2163                      MIXER MCGAM                   January 1998   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.   TheRFC822 domain space and the X.400 O/R address space, as said insection 3, have one specific common feature: the X.400 ISO country   codes are the same as theRFC822 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 insection 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' or 'gate1' 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   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 andRFC822 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.Allocchio                   Standards Track                    [Page 14]

RFC 2163                      MIXER MCGAM                   January 19984.3 Creating the appropriate DNS files   Using MIXER's assumption of an asymmetric mapping between X.400 andRFC822 addresses, two separate relations are required to store the   mapping database: MIXER 'table1' and MIXER 'table2'; thus also in DNS   we will maintain the two different sections, even if they will both   use the PX resource record. More over MIXER also specify two   additional tables: MIXER 'gate1' and 'gate2' tables. These additional   tables, however, have the same syntax rules than MIXER 'table1' and   'table2' respectively, and thus the same translation procedure as   'table1' and 'table2' will be applied; some details about the MIXER   'gate1' and 'gate2' tables are discussed insection 4.4.   Let's now check how to create, from an MCGAM entry, the appropriate   DNS entry in a DNS data file. We can again define an MCGAM entry as   defined inappendix F of that document as:     <x400-domain>#<rfc822-domain>#  (case A: 'table1' and 'gate1'     entry)   and     <rfc822-domain>#<x400-domain>#  (case B: 'table2' and 'gate2'     entry)   The two cases must be considered separately. Let's consider case A.    - take <x400-domain> and translate it into <domain-name> syntax,     obtaining <x400-in-domain-syntax>;    - create the <name> key from <x400-in-domain-syntax> i.e., apply     the Country Code convention described in sect. 4.2.3;    - 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' and 'gate1' entry.   an example: from the 'table1' 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 ".".Allocchio                   Standards Track                    [Page 15]

RFC 2163                      MIXER MCGAM                   January 1998   Let's now consider case B.    - take <rfc822-domain> as <name> key;    - translate <x400-domain> into <x400-in-domain-syntax>;    - construct the DNS PX record as:     *.<name>  IN  PX  50  <rfc822-domain>  <x400-in-domain-syntax>   an example: from the 'table2' 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.   A file containing the MIXER mapping rules and MIXER 'gate1' and   'gate2' table written in DNS format will look like the following   fictious example:     !     ! MIXER 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.     !     ! MIXER 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.     !     ! MIXER Gate 1 Table     !     *.ADMD-XKW-h-Mail.X42D.it.         IN  PX  50   \                            XKW-gateway.it. ADMD-XKW-h-Mail.C-it.G.     *.PRMD-Super-b-Inc.ADMDb.X42D.it.  IN  PX  50   \                            GlobalGw.it. PRMD-Super-b-Inc.ADMDb.C-it.G.     !     ! MIXER Gate 2 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.Allocchio                   Standards Track                    [Page 16]

RFC 2163                      MIXER MCGAM                   January 1998   (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 'gate1' and   'gate2' Tables section whose aim is described insection 4.4. The   corresponding MIXER tables are:     #     # MIXER 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$it#cs.ncty.it#     #     # MIXER table 2:RFC822 --> X.400     #     nrc.it#PRMD$nrc.ADMD$acme.C$it#     ninp.it#O.PRMD$ninp.ADMD$acme.C$it#     bd.it#PRMD$uk\.bd.ADMD$ .C$it#     #     # MIXER Gate 1 Table     #     ADMD$XKW-Mail.C$it#XKW-gateway.it#     PRMD$Super Inc.ADMD$ .C$it#GlobalGw.it#     #     # MIXER Gate 2 Table     #     my.it#OU$int-gw.O$@.PRMD$ninp.ADMD$acme.C$it#     co.it#O$mhs-relay.PRMD$x4net.ADMD$ .C$t#4.4 Storing the MIXER 'gate1' and 'gate2' tablesSection 4.3.4 of MIXER also specify how an address should be   converted betweenRFC822 and X.400 in case a complete mapping is   impossible. To allow the use of DDAs for non mappable domains, the   MIXER 'gate2' table is thus introduced.   In a totally similar way, when an X.400 address cannot be completely   converted inRFC822, section 4.3.5 of MIXER specifies how to encode   (LHS encoding) the address itself, pointing then to the appropriate   MIXER conformant gateway, indicated in the MIXER 'gate1' table.   DNS must store and distribute also these 'gate1' and 'gate2' 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 ourAllocchio                   Standards Track                    [Page 17]

RFC 2163                      MIXER MCGAM                   January 1998   case, a further additional feature to the table based approach. In   fact we can avoid one possible ambiguity about the use of the 'gate1'   and 'gate2' tables (and thus of LHS and DDAs encoding).   The authority maintaining a DNS entry in the usualRFC822 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 itsRFC822 domain   should be mapped using SA, then the PX RDATA will be a 'table2'   entry, otherwise it will be a 'gate2' table entry. Thus for anRFC822   domain we cannot have any more two possible entries, one from 'table2   and another one from 'gate2' table, and the action for a gateway   results clearly stated.   Similarly, the authority mantaining a DNS entry in the new X.400 name   space is the only one allowed to decide if its X.400 domain should be   mapped using SA syntax or Left Hand Side (LHS) encoding. If the   authority decides that its X.400 domain should be mapped using SA,   then the PX RDATA will be a 'table1' entry, otherwise it will be a   'gate1' table entry. Thus also for an X.400 domain we cannot have any   more two possible entries, one from 'table1' and another one from   'gate1' table, and the action for a gateway results clearly stated.   The MIXER 'gate1' table syntax is actually identical to MIXER   'table1', and 'gate2' table syntax is identical to MIXER 'table2'.   Thus the same syntax translation rules from MIXER to DNS format can   be applied in both cases. However a gateway or any other application   must know if the answer it got from DNS contains some 'table1',   'table2' or some 'gate1', 'gate2' table information. This is easily   obtained flagging with an additional ".G." post-fix the PX RDATA   value when it contains a 'gate1' or 'gate2' table entry. The example   insection 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 MIXER 'gate1' or 'gate2' table   data.5. Finding MIXER mapping information from DNS   The MIXER mapping information is stored in DNS both in the normalRFC822 domain name space, and in the newly defined X.400 name space.   The information, stored in PX resource records, does not represent a   fullRFC822 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   be performed (MIXER:section 4.3.4, State I;section 4.3.5, mappingAllocchio                   Standards Track                    [Page 18]

RFC 2163                      MIXER MCGAM                   January 1998   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 MIXER, 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 MIXER. 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 MIXER mail-gateway located in the Internet, when translating   addresses fromRFC822 to X.400, can get information about the MCGAM   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 MIXER   Table 2 mapping rule will be obtained.   Let's now take another example where a 'gate2' table rule is   returned.  If we are looking for anRFC822 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   'gate2' 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   the gateway to use DDA encoding for the inquiredRFC822 domain.Allocchio                   Standards Track                    [Page 19]

RFC 2163                      MIXER MCGAM                   January 1998   On the other hand, translating from X.400 toRFC822 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 MIXER 'table1' rule in encoded   format.   Finally, an example where a 'gate1' rule is involved. If we are   looking for an X.400 domain ending with ADMD=PWT400; C=US; , and the   DNS contains a PX record like this,      *.ADMD-PWT400.X42D.us.  IN  PX  50  intGw.com. ADMD-PWT400.C-us.G.   DNS will return 'intGw.com.' and 'ADMD-PWT400.C-us.G.', i.e., a   'gate1' 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   the gateway to use LHS encoding for the inquired X.400 domain.6. Administration of mapping information   The DNS, using the PX RR, is able to distribute the MCGAM rules to   all MIXER gateways located on the Internet. However, not all MIXER   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 publish their MCGAM   rules and retrieve the mapping tables via the International Mapping   Table coordinator, manually or via some automated procedures. Their   MCGAM information can be made available also in DNS by the   appropriate DNS authorities, using the same mechanism already in   place for MX records: if a branch has not yet in place its own DNSAllocchio                   Standards Track                    [Page 20]

RFC 2163                      MIXER MCGAM                   January 1998   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 specification. An informational document   describing the implementation phase and the detailed coordination   procedures is expected.   Another distributed directory service which can distribute the MCGAM   information is X.500. Coordination with table-based domains can be   obtained in an identical way as for the DNS case.   Coordination of MCGAM information between DNS and X.500 is more   complex, as it requies some kind of uploading information between the   two systems. 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.   However we must remind that MIXER concept of MCGAM rules publication   is different from the oldRFC1327 concept of globally distributed,   coordinated and unique mapping rules. In fact MIXER does not requires   any more for any conformant gateway or tool to know the complete set   of MCGAM: it only requires to use some set (eventually empty) of   valid MCGAM rules, published either by Tables, DNS or X.500   mechanisms or any combination of these methods. More over MIXER   specifies that also incomplete sets of MCGAM can be used, and   supplementary local unpublished (but valid) MCGAM can also be used.   As a consequence, the problem of coordination between the three   systems proposed by MIXER for MCGAM publication is non essential, and   important only for efficient operational matters. It does not in fact   affect the correct behaviour of MIXER conformant gateways and tools.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 MCGAM 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 were been performed   under the provisional top level domain "X400.IT" whenRFC1664 was   developed, and their results confirmed the advantages of the method.   Operational exeprience for over 2 years withRFC1664 specificationAllocchio                   Standards Track                    [Page 21]

RFC 2163                      MIXER MCGAM                   January 1998   confirmed the feasibility of the method, and helped identifying some   operational procedures to deploy the insertion of MCGAM into DNS.   Software to query the DNS and then to convert between the textual   representation of DNS resource records and the address format defined   in MIXER was developed withRFC1664. This software also allows a   smooth implementation and deployment period, eventually taking care   of the transition phase. This software can be easily used (with   little or null modification) also for this updated specification,   supporting the new 'gate1' MIXER table. DNS software implementations   supportingRFC1664 also supports with no modification this memo new   specification.Allocchio                   Standards Track                    [Page 22]

RFC 2163                      MIXER MCGAM                   January 1998   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,   TERENA wg-msg and IETF namedroppers groups. A special mention to   Christian Huitema for his fundamental contribution to this work.   This document is a revision ofRFC1664, edited by one of its authors   on behalf of the IETF MIXER working group. The current editor wishes   to thank here also the authors ofRFC1664:     Antonio Blasco BonitoRFC822: bonito@cnuce.cnr.it     CNUCE - CNR               X.400:  C=it;A=garr;P=cnr;     Reparto infr. reti                O=cnuce;S=bonito;     Viale S. Maria 36     I 56126 Pisa     Italy     Bruce ColeRFC822: bcole@cisco.com     Cisco Systems Inc.        X.400:  C=us;A= ;P=Internet;     P.O. Box 3075                     DD.rfc-822=bcole(a)cisco.com;     1525 O'Brien Drive     Menlo Park, CA 94026     U.S.A.     Silvia GiordanoRFC822: giordano@cscs.ch     Centro Svizzero di        X.400:  C=ch;A=arcom;P=switch;O=cscs;     Calcolo Scientifico               S=giordano;     Via Cantonale     CH 6928 Manno     Switzerland     Robert HagensRFC822: hagens@ans.net     Advanced Network and Services   X.400:  C=us;A= ;P=Internet;     1875 Campus Commons Drive               DD.rfc-822=hagens(a)ans.net;     Reston, VA 22091     U.S.A.Allocchio                   Standards Track                    [Page 23]

RFC 2163                      MIXER MCGAM                   January 19989. 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 andRFC822",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",RFC1033, SRI International, November 1987.   [RFC 2156] Kille, S. E., " MIXER (Mime Internet X.400 Enhanced       Relay): Mapping between X.400 andRFC 822/MIME",RFC 2156,       January 1998.   [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       1993.10. Security Considerations   This document specifies a means by which DNS "PX" records can direct   the translation between X.400 and Internet mail addresses.   This can indirectly affect the routing of mail across an gateway   between X.400 and Internet Mail.  A succesful attack on this service   could cause incorrect translation of an originator address (thus   "forging" the originator address), or incorrect translation of a   recipient address (thus directing the mail to an unauthorized   recipient, or making it appear to an authorized recipient, that the   message was intended for recipients other than those chosen by the   originator) or could force the mail path via some particular gateway   or message transfer agent where mail security can be affected by   compromised software.Allocchio                   Standards Track                    [Page 24]

RFC 2163                      MIXER MCGAM                   January 1998   There are several means by which an attacker might be able to deliver   incorrect PX records to a client.  These include: (a) compromise of a   DNS server,  (b) generating a counterfeit response to a client's DNS   query, (c) returning incorrect "additional information" in response   to an unrelated query.   Clients using PX records SHOULD ensure that routing and address   translations are based only on authoritative answers.  Once DNS   Security mechanisms [RFC 2065] become more widely deployed, clients   SHOULD employ those mechanisms to verify the authenticity and   integrity of PX records.11. Author's Address   Claudio Allocchio   Sincrotrone Trieste   SS 14 Km 163.5 Basovizza   I 34012 Trieste   ItalyRFC822: 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 3758565Allocchio                   Standards Track                    [Page 25]

RFC 2163                      MIXER MCGAM                   January 199812.  Full Copyright Statement   Copyright (C) The Internet Society (1998).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Allocchio                   Standards Track                    [Page 26]