RFC 1664
Network Working Group C. Allocchio
Request for Comments: 1664 A. Bonito
Category: Experimental GARR-Italy
B. Cole
Cisco Systems Inc.
S. Giordano
Centro Svizzero Calcolo Scientifico
R. Hagens
Advanced Network & Services
August 1994
Using the Internet DNS to Distribute
RFC1327 Mail Address Mapping Tables
Status of this Memo
This memo defines an Experimental Protocol for the Internet
community. This memo does not specify an Internet standard of any
kind. Distribution of this memo is unlimited.
Abstract
This memo defines how to store in the Internet Domain Name System the
mapping information needed by e-mail gateways and other tools to map
RFC822 domain names into X.400 O/R names and vice versa. Mapping
information can be managed in a distributed rather than a centralised
way. Gateways located on Internet hosts can retrieve the mapping
information querying the DNS instead of having fixed tables which
need to be centrally updated and distributed. This memo is a joint
effort of X400 operation working group (x400ops) and RARE Mail and
Messaging working group (WG-MSG).
1. Introduction
The connectivity between the Internet SMTP mail and other mail
services, including the Internet X.400 mail and the commercial X.400
service providers, is assured by the Mail eXchanger (MX) record
information distributed via the Internet Domain Name System (DNS). A
number of documents then specify in details how to convert or encode
addresses from/to RFC822 style to the other mail system syntax.
However, only conversion methods provide, via some algorithm or a set
of mapping rules, a smooth translation, resulting in addresses
indistinguishable from the native ones in both RFC822 and foreign
world.
RFC1327 describes a set of mappings which will enable interworking
between systems operating the CCITT X.400 (1984/88) Recommendations
Allocchio, Bonito, Cole, Giordano & Hagens [Page 1]
RFC 1664 Internet DNS for Mail Mapping Tables August 1994
and systems using the RFC822 mail protocol, or protocols derived from
RFC822. That document addresses conversion of services, addresses,
message envelopes, and message bodies between the two mail systems.
This document is concerned with one aspect of RFC1327: the mechanism
for mapping between X.400 O/R addresses and RFC822 domain names. As
described in Appendix F of RFC1327, implementation of the mappings
requires a database which maps between X.400 O/R addresses and domain
names, and this database is statically defined.
This approach requires many efforts to maintain the correct mapping:
all the gateways need to get coherent tables to apply the same
mappings, the conversion tables must be distributed among all the
operational gateways, and also every update needs to be distributed.
This static mechanism requires quite a long time to be spent
modifying and distributing the information, putting heavy constraints
on the time schedule of every update. In fact it does not appear
efficient compared to the Internet Domain Name Service (DNS). More
over it does not look feasible to distribute the database to a large
number of other useful applications, like local address converters,
e-mail User Agents or any other tool requiring the mapping rules to
produce correct results.
A first proposal to use the Internet DNS to store, retrieve and
maintain those mappings was introduced by two of the authors (B. Cole
and R. Hagens) adopting two new DNS resource record (RR) types: TO-
X400 and TO-822. This new proposal adopts a more complete strategy,
and requires one new RR only. The distribution of the RFC1327 mapping
rules via DNS is in fact an important service for the whole Internet
community: it completes the information given by MX resource record
and it allows to produce clean addresses when messages are exchanged
among the Internet RFC822 world and the X.400 one (both Internet and
Public X.400 service providers).
A first experiment in using the DNS without expanding the current set
of RR and using available ones was in the mean time deployed by some
of the authors. The existing PTR resource records were used to store
the mapping rules, and a new DNS tree was created under the ".it" top
level domain. The result of the experiment was positive, and a few
test applications ran under this provisional set up. This test was
also very useful in order to define a possible migration strategy
during the deployment of the new DNS containing the new RR. The
Internet DNS nameservers wishing to provide this mapping information
need in fact to be modified to support the new RR type, and in the
real Internet, due to the large number of different implementations,
this takes some time.
The basic idea is to adopt a new DNS RR to store the mapping
information. The RFC822 to X.400 mapping rules (including the so
Allocchio, Bonito, Cole, Giordano & Hagens [Page 2]
RFC 1664 Internet DNS for Mail Mapping Tables August 1994
called 'gate' rules) will be stored in the ordinary DNS tree, while
the definition of a new branch of the name space defined under each
national top level domain is envisaged in order to contain the X.400
to RFC822 mappings. A "two-way" mapping resolution schema is thus
fully implemented.
The creation of the new domain name space representing the X.400 O/R
names structure also provides the chance to use the DNS to distribute
dynamically other X.400 related information, thus solving other
efficiency problems currently affecting the X.400 MHS service.
In this paper we will adopt the RFC1327 mapping rules syntax, showing
how it can be stored into the Internet DNS.
1.1 Definitions syntax
The definitions in this document is given in BNF-like syntax, using
the following conventions:
| means choice
\ is used for continuation of a definition over several lines
[] means optional
{} means repeated one or more times
The definitions, however, are detailed only until a certain level,
and below it self-explaining character text strings will be used.
2. Motivation
Implementations of RFC1327 gateways require that a database store
address mapping information for X.400 and RFC822. This information
must be disseminated to all RFC1327 gateways. In the Internet
community, the DNS has proven to be a practical mean for providing a
distributed name service. Advantages of using a DNS based system over
a table based approach for mapping between O/R addresses and domain
names are:
- It avoids fetching and storing of entire mapping tables by every
host that wishes to implement RFC1327 gateways and/or tools
- Modifications to the DNS based mapping information can be made
available in a more timely manner than with a table driven
approach.
- It allows full authority delegation, in agreement with the
Internet regionalization process.
Allocchio, Bonito, Cole, Giordano & Hagens [Page 3]
RFC 1664 Internet DNS for Mail Mapping Tables August 1994
- Table management is not necessarily required for DNS-based
RFC1327 gateways.
- One can determine the mappings in use by a remote gateway by
querying the DNS (remote debugging).
Also many other tools, like address converters and User Agents can
take advantage of the real-time availability of RFC1327 tables,
allowing a much easier maintenance of the information.
3. The domain space for X.400 O/R name addresses
Usual domain names (the ones normally used as the global part of an
RFC822 e-mail address) and their associated information, i.e., host
IP addresses, mail exchanger names, etc., are stored in the DNS as a
distributed database under a number of top-level domains. Some top-
level domains are used for traditional categories or international
organisations (EDU, COM, NET, ORG, INT, MIL...). On the other hand
any country has its own two letter ISO country code as top-level
domain (FR, DE, GB, IT, RU, ...), including "US" for USA. The
special top-level/second-level couple IN-ADDR.ARPA is used to store
the IP address to domain name relationship. Our proposal defines in
the above structure the appropriate way to locate the X.400 O/R name
space, thus enabling us to store in DNS the RFC1327 mapping data.
The RFC1327 mapping information is composed by three tables: 'table1'
gives the translation from X.400 to RFC822 while 'table2' and 'gate'
tables map RFC822 into X.400. Each mapping table is composed by
mapping rules, and a single mapping rule is composed by a keyword
(the argument of the mapping function derived from the address to be
translated) and a translator (the mapping function parameter):
keyword#translator#
the '#' sign is a delimiter enclosing the translator. An example:
foo.bar.us#PRMD$foo\.bar.ADMD$intx.C$us#
Local mappings are not intended for use outside their restricted
environment, thus they should not be included in DNS. If local
mappings are used, they should be stored using static local tables,
exactly as local static host tables can be used with DNS.
The keyword of a 'table2' and 'gate' table entry is a valid RFC822
domain; thus the usual domain name space can be used without problems
to store these entries.
Allocchio, Bonito, Cole, Giordano & Hagens [Page 4]
RFC 1664 Internet DNS for Mail Mapping Tables August 1994
On the other hand, the keyword of a 'table1' entry belongs to the
X.400 O/R name space. The X.400 O/R name space does not usually fit
into the usual domain name space, although there are a number of
similarities; a new name structure is thus needed to represent it.
This new name structure contains the X.400 mail domains.
To ensure the correct functioning of the DNS system, the new X.400
name structure must be hooked to the existing domain name space in a
way which respects the existing name hierarchy.
A possible solution was to create another special branch, starting
from the root of the DNS tree, somehow similar to the in-addr.arpa
tree. This idea would have required to establish a central authority
to coordinate at international level the management of each national
X.400 name tree, including the X.400 public service providers. This
coordination problem is a heavy burden if approached globally. More
over the X.400 name structure is very 'country oriented': thus while
it requires a coordination at national level, it does not have
concepts like the international root. In fact the X.400 international
service is based on a large number of bilateral agreements, and only
within some communities an international coordination service exists.
The X.400 two letter ISO country codes, however, are the same used
for the RFC822 country top-level domains and this gives us an
appropriate hook to insert the new branches. Our proposal is, in
fact, to create under each national top level ISO country code a new
branch in the name space. This branch represents exactly the X.400
O/R name structure as defined in each single country, following the
ADMD, PRMD, O, OU hierarchy. A unique reserved label 'X42D' is placed
under each country top-level domain, and hence the national X.400
name space derives its own structure:
Allocchio, Bonito, Cole, Giordano & Hagens [Page 5]
RFC 1664 Internet DNS for Mail Mapping Tables August 1994
. (root)
|
+-----------------+-----------+--------+-----------------+...
| | | |
edu it us fr
| | | |
+---+---+... +-----+-----+... +-----+-----+... +--+---+...
| | | | | | | | | |
... ... cnr X42D infn va ca X42D X42D inria
| | | |
+------------+------------+... ... ... +----+-------+...
| | | | |
ADMD-PtPostel ADMD-garr ADMD-Master400 ADMD-atlas ADMD-red
| | | |
+----------+----+... ... +-------+------+... ...
| | | |
PRMD-infn PRMD-STET PRMD-Telecom PRMD-Renault
| | | |
... ... ... ...
The creation of the X.400 new name tree at national level solves the
problem of the international coordination. Actually the coordination
problem is just moved at national level, but it thus becomes easier
to solve. The coordination at national level between the X.400
communities and the Internet world is already a requirement for the
creation of the national static RFC1327 mapping tables; the use of
the Internet DNS gives further motivations for this coordination.
The coordination at national level also fits in the ongoing proposal
intended to define exactly the RFC1327 Mapping Authorities. The DNS
in fact allows a step by step authority distribution, up to a final
complete delegation, which can be easily controlled at national level
accordingly with national needs and situations. A further advantage
of the national based solution is to allow each country to set up its
own X.400 name structure in DNS and to deploy its own authority
delegation according to its local time scale and requirements, with
no loss of global service in the mean time. And last, placing the new
X.400 name tree and coordination process at national level fits into
the Internet regionalization and internationalisation process, as it
requires local bodies to take care of local coordination problems.
The DNS name space thus contains completely the information required
by an e-mail gateway or tool to perform the X.400-RFC822 mapping: a
simple query to the nearest nameserver provides it. Moreover there is
no more any need to store, maintain and distribute manually any
mapping table. The new X.400 name space can also contain further
information about the X.400 community, as DNS allows for it a
Allocchio, Bonito, Cole, Giordano & Hagens [Page 6]
RFC 1664 Internet DNS for Mail Mapping Tables August 1994
complete set of resource records, and thus it allows further
developments. This set of RRs in the new X.400 name space must be
considered 'reserved' and thus not used until further specifications.
The construction of the new domain space trees will follow the same
procedures used when organising at first the already existing DNS
space: at first the information will be stored in a quite centralised
way, and distribution of authority will be gradually achieved. A
separate document will describe the implementation phase and the
methods to assure a smooth introduction of the new service.
4. The new DNS resource record for RFC1327 mapping rules: PX
The specification of the Internet DNS (RFC1035) provides a number of
specific resource records (RRs) to contain specific pieces of
information. In particular they contain the Mail eXchanger (MX) RR
and the host Address (A) records which are used by the Internet SMTP
mailers. As we will store the RFC822 to X.400 mapping information in
the already existing DNS name tree, we need to define a new DNS RR in
order to avoid any possible clash or misuse of already existing data
structures. The same new RR will also be used to store the mappings
from X.400 to RFC822. More over the mapping information, i.e., the
RFC1327 mapping rules, has a specific format and syntax which require
an appropriate data structure and processing. A further advantage of
defining a new RR is the ability to include flexibility for some
eventual future development.
The definition of the new 'PX' DNS resource record is:
class: IN (Internet)
name: PX (pointer to X.400/RFC822 mapping information)
value: 26
The PX RDATA format is:
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| PREFERENCE |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/ MAP822 /
/ /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/ MAPX400 /
/ /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
Allocchio, Bonito, Cole, Giordano & Hagens [Page 7]
RFC 1664 Internet DNS for Mail Mapping Tables August 1994
PREFERENCE A 16 bit integer which specifies the preference given to
this RR among others at the same owner. Lower values
are preferred;
MAP822 A element containing , the
RFC822 part of the RFC1327 mapping information;
MAPX400 A element containing the value of
derived from the X.400 part of
the RFC1327 mapping information (see sect. 4.2);
PX records cause no additional section processing. The PX RR format
is the usual one:
[] []
When we store in DNS a 'table1' entry, then will be an X.400
mail domain name in DNS syntax (see sect. 4.2). When we store a
'table2' or a 'gate' table entry, will be an RFC822 mail
domain name, including both fully qualified DNS domains and mail only
domains (MX-only domains). All normal DNS conventions, like default
values, wildcards, abbreviations and message compression, apply also
for all the components of the PX RR. In particular , MAP822 and
MAPX400, as elements, must have the final "." (root)
when they are fully qualified.
4.1 Additional features of the PX resource record
The definition of the RDATA for the PX resource record, and the fact
that DNS allows a distinction between an exact value and a wildcard
match for the parameter, represent an extension of the RFC1327
specification for mapping rules. In fact, any RFC1327 mapping table
entry is an implicit wildcard entry, i.e., the rule
net2.it#PRMD$net2.ADMD$p400.C$it#
covers any RFC822 domain ending with 'net2.it', unless more detailed
rules for some subdomain in 'net2.it' are present. Thus there is no
possibility to specify explicitly an RFC1327 entry as an exact match
only rule. In DNS an entry like
*.net2.it. IN PX 10 net2.it. PRMD-net2.ADMD-p400.C-it.
specify the usual wildcard match as for RFC1327 tables. However an
entry like
ab.net2.it. IN PX 10 ab.net2.it. O-ab.PRMD-net2.ADMDb.C-it.
Allocchio, Bonito, Cole, Giordano & Hagens [Page 8]
RFC 1664 Internet DNS for Mail Mapping Tables August 1994
is valid only for an exact match of 'ab.net2.it' RFC822 domain.
Note also that in DNS syntax there is no '#' delimiter around MAP822
and MAPX400 fields: the syntax defined in sect. 4.2 in fact does not
allow the (ASCII decimal 32) character within these fields,
making unneeded the use of an explicit delimiter as required in the
RFC1327 original syntax.
Another extension to the RFC1327 specifications is the PREFERENCE
value defined as part of the PX RDATA section. This numeric value has
exactly the same meaning than the similar one used for the MX RR. It
is thus possible to specify more than one single mapping for a domain
(both from RFC822 to X.400 and vice versa), giving as the preference
order. In RFC1327 static tables, however, you cannot specify more
than one mapping per each RFC822 domain, and the same restriction
apply for any X.400 domain mapping to an RFC822 one.
More over, in the X.400 recommendations a note suggests than an
ADMD= should be reserved for some special cases. Various
national functional profile specifications for an X.400 MHS states
that if an X.400 PRMD is reachable via any of its national ADMDs,
independently of its actual single or multiple connectivity with
them, it should use ADMD= to advertise this fact. Again, if a
PRMD has no connections to any ADMD it should use ADMD=0 to notify
its status, etc. However, in most of the current real situations, the
ADMD service providers do not accept messages coming from their
subscribers if they have a blank ADMD, forcing them to have their own
ADMD value. In such a situation there are problems in indicating
properly the actually working mappings for domains with multiple
connectivity. The PX RDATA 'PREFERENCE' extension was introduced to
take in consideration these problems.
However, as these extensions are not available with RFC1327 static
tables, it is strongly discouraged to use them when interworking with
any table based gateway or application. The extensions were in fact
introduced just to add more flexibility, like the PREFERENCE value,
or they were already implicit in the DNS mechanism, like the wildcard
specification. They should be used very carefully or just considered
'reserved for future use'. In particular, for current use, the
PREFERENCE value in the PX record specification should be fixed to a
value of 50, and only wildcard specifications should be used when
specifying values.
4.2 The DNS syntax for an X.400 'domain'
The syntax definition of the RFC1327 mapping rules is defined in
appendix F of that document. However that syntax is not very human
oriented and contains a number of characters which have a special
Allocchio, Bonito, Cole, Giordano & Hagens [Page 9]
RFC 1664 Internet DNS for Mail Mapping Tables August 1994
meaning in other fields of the Internet DNS. Thus in order to avoid
any possible problem, especially due to some old DNS implementations
still being used in the Internet, we define a syntax for the X.400
part of any RFC1327 mapping rules (and hence for any X.400 O/R name)
which makes it compatible with a element, i.e.,
::= | " "
::=