Network Working Group C. Allocchio
Request for Comments: 2163 GARR-Italy
Obsoletes: 1664 January 1998
Category: Standards Track
Using the Internet DNS to Distribute
MIXER 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 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. 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 obsoletes RFC1664. It includes the changes introduced by
MIXER specification with respect to RFC1327: the new 'gate1' (O/R
addresses to domain) table is fully supported. Full backward
compatibility with RFC1664 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.
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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.
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 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 MIXER: the mechanism
for mapping between X.400 O/R addresses and RFC822 domain names. As
described in Appendix F of MIXER, implementation of the mappings
requires a database which maps between X.400 O/R addresses and domain
names; in RFC1327 this database was statically defined.
The original approach in RFC1327 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.
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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 of
RFC1664 (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 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 deployed by some of the authors of
RFC1664 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. The RFC822 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
to RFC822 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.
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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 MIXER gateways require that a database store
address mapping information for X.400 and RFC822. 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 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
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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 to RFC822;
- 'table2' and 'gate2' 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:
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 valid RFC822
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 authority
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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. 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.
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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 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
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.
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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>, the
RFC822 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 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.
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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 MIXER
specification for mapping rules. In fact, any MCGAM entry is an
implicit wildcard entry, i.e., the rule
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 a MCGAM as an exact match only
rule. In DNS an entry like
specify the usual wildcard match as for MIXER tables. However an
entry like
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 from RFC822 to X.400 and vice versa), giving as the preference
order. In MIXER 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
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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 in appendix 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" | "-"
(see RFC1035, 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> "$" <attr-value>
<attr-label> ::= "C" | "ADMD" | "PRMD" | "O" | "OU"
<attr-value> ::= " " | "@" | IA5-Printablestring
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As you can notice <domain-name> and <map-rule> look similar, and also
<label> and <map-elem> look the same. If we define the correct method
to transform a <map-elem> into a <label> and vice versa the problem
to write a MCGAM rule in <domain-name> syntax is solved.
The RFC822 domain part of any MCGAM rule is of course already in
<domain-name> syntax, and thus remains unchanged.
In particular, in a 'table1' or 'gate1' mapping rule the 'keyword'
value must be converted into <x400-in-domain-syntax> (X.400 mail DNS
mail domain), while the 'translator' value is already a valid RFC822
domain. Vice versa in a 'table2' or 'gate2' mapping rule, the
'translator' must be converted into <x400-in-domain-syntax>, while
the 'keyword' is already a valid RFC822 domain.
4.2.1 IA5-Printablestring to <alphanumhyphen> mappings
The problem of unmatching IA5-Printablestring and <label> character
set definition is solved by a simple character mapping rule: whenever
an IA5 character does not belong to <alphanumhyphen>, then it is
mapped using its 3 digit decimal ASCII code, enclosed in hyphens. A
small set of special rules is also defined for the most frequent
cases. Moreover some frequent characters combinations used in MIXER
rules are also mapped as special cases.
Let's then define the following simple rules:
MCGAM rule DNS store translation conditions
-----------------------------------------------------------------
<attr-label>$@ <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:
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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
translates to
Another example:
translates to
4.2.2 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."
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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>$<blank>
| | |
| map -d- to "\." |
| | |
| map -h- to "-" |
| | |
| map -b- to " " |
restart | | |
^ | map -<3digit>- to non A/N char |
| | | |
| \--------> add a "." <----------/
| |
\------------- take next attribute (if any)
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Note that the above flow charts deal with the translation of the
attributes syntax, only.
4.2.3 The Country Code convention in the <name> value.
The RFC822 domain space and the X.400 O/R address space, as said in
section 3, have one specific common feature: the X.400 ISO country
codes are the same as the RFC822 ISO top level domains for countries.
In the previous sections we have also defined a method to write in
<domain-name> syntax any X.400 domain, while in section 3 we
described the new name space starting at each country top level
domain under the X42D.cc (where 'cc' is then two letter ISO country
code).
The <name> value for a 'table1' or 'gate1' entry in DNS should thus
be derived from the X.400 domain value, translated to <domain-name>
ADMD$acme.C$fr
produces in <domain-name> syntax the key:
which is post-fixed by 'X42D.fr.' resulting in:
However, due to the identical encoding for X.400 country codes and
RFC822 country top level domains, the string 'C-fr.X42D.fr.' is
clearly redundant.
We thus define the 'Country Code convention' for the <name> key,
i.e.,
"The C-cc section of an X.400 domain in <domain-name> syntax must
be omitted when creating a <name> key, as it is identical to the
top level country code used to identify the DNS zone where the
information is stored".
Thus we obtain the following <name> key examples:
X.400 domain DNS <name> key
--------------------------------------------------------------------
ADMD$acme.C$fr ADMD-acme.X42D.fr.
PRMD$ux\.av.ADMD$ .C$gb PRMD-ux-d-av.ADMDb.X42D.gb.
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4.3 Creating the appropriate DNS files
Using MIXER's assumption of an asymmetric mapping between X.400 and
RFC822 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 in section 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 in appendix 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
we obtain
Note that <name>, <rfc822-domain> and <x400-in-domain-syntax> are
fully qualified <domain-name> elements, thus ending with a ".".
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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
we obtain
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 \
*.O-u-h-newcity.PRMD-x4net.ADMDb.X42D.it. IN PX 50 \
!
! MIXER table 2: RFC822 --> X.400
!
!
! MIXER Gate 1 Table
!
*.ADMD-XKW-h-Mail.X42D.it. IN PX 50 \
*.PRMD-Super-b-Inc.ADMDb.X42D.it. IN PX 50 \
!
! MIXER Gate 2 Table
!
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(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 in section 4.4. The
corresponding MIXER tables are:
#
# MIXER table 1: X.400 --> RFC822
#
ADMD$acme.C$it#it#
#
# MIXER table 2: RFC822 --> X.400
#
#
# MIXER Gate 1 Table
#
ADMD$XKW-Mail.C$it#XKW-gateway.it#
PRMD$Super Inc.ADMD$ .C$it#GlobalGw.it#
#
# MIXER Gate 2 Table
#
4.4 Storing the MIXER 'gate1' and 'gate2' tables
Section 4.3.4 of MIXER 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
MIXER 'gate2' table is thus introduced.
In a totally similar way, when an X.400 address cannot be completely
converted in RFC822, 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 our
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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 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 'gate2' table entry. Thus for an RFC822
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
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 MIXER 'gate1' or 'gate2' table
data.
5. Finding MIXER mapping information from DNS
The MIXER 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
be performed (MIXER: section 4.3.4, State I; section 4.3.5, mapping
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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 from RFC822 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:
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 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
'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 inquired RFC822 domain.
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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:
The DNS could contain:
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,
'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 DNS
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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 old RFC1327 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" when RFC1664 was
developed, and their results confirmed the advantages of the method.
Operational exeprience for over 2 years with RFC1664 specification
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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 with RFC1664. 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
supporting RFC1664 also supports with no modification this memo new
specification.
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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 of RFC1664, edited by one of its authors
on behalf of the IETF MIXER working group. The current editor wishes
to thank here also the authors of RFC1664:
Antonio Blasco Bonito RFC822: 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 Cole RFC822: bcole@cisco.com
Cisco Systems Inc. X.400: C=us;A= ;P=Internet;
1525 O'Brien Drive
Menlo Park, CA 94026
Silvia Giordano RFC822: 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 Hagens RFC822: 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
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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.
[RFC 2156] Kille, S. E., " MIXER (Mime Internet X.400 Enhanced
Relay): Mapping between X.400 and RFC 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.
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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
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 3758565
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12. 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.
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