RFC2247 - Using Domains in LDAP/X.500 Distinguished Names

AT&T
January 1998

Using Domains in LDAP/X.500 Distinguished Names

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.

1. Abstract

The Lightweight Directory Access Protocol (LDAP) uses X.500-
compatible distinguished names [3] for providing unique
identification of entries.

This document defines an algorithm by which a name registered with
the Internet Domain Name Service [2] can be represented as an LDAP
distinguished name.

2. Background

The Domain (Nameserver) System (DNS) provides a hierarchical resource
labeling system. A name is made up of an ordered set of components,
each of which are short strings. An example domain name with two
components would be "CRITICAL-ANGLE.COM".

LDAP-based directories provide a more general hierarchical naming
framework. A primary difference in specification of distinguished
names from domain names is that each component of an distinguished
name has an explicit attribute type indication.

X.500 does not mandate any particular naming structure. It does
contain suggested naming structures which are based on geographic and
national regions, however there is not currently an established
registration infrastructure in many regions which would be able to
assign or ensure uniqueness of names.

The mechanism described in this document automatically provides an
enterprise a distinguished name for each domain name it has obtained
for use in the Internet. These distinguished names may be used to
identify objects in an LDAP directory.

An example distinguished name represented in the LDAP string format
[3] is "DC=CRITICAL-ANGLE,DC=COM". As with a domain name, the most
significant component, closest to the root of the namespace, is
written last.

This document does not define how to represent objects which do not
have domain names. Nor does this document define the procedure to
locate an enterprise's LDAP directory server, given their domain
name. Such procedures may be defined in future RFCs.

3. Mapping Domain Names into Distinguished Names

This section defines a subset of the possible distinguished name
structures for use in representing names allocated in the Internet
Domain Name System. It is possible to algorithmically transform any
Internet domain name into a distinguished name, and to convert these
distinguished names back into the original domain names.

The algorithm for transforming a domain name is to begin with an
empty distinguished name (DN) and then attach Relative Distinguished
Names (RDNs) for each component of the domain, most significant (e.g.
rightmost) first. Each of these RDNs is a single
AttributeTypeAndValue, where the type is the attribute "DC" and the
value is an IA5 string containing the domain name component.

Thus the domain name "CS.UCL.AC.UK" can be transformed into

DC=CS,DC=UCL,DC=AC,DC=UK

Distinguished names in which there are one or more RDNs, all
containing only the attribute type DC, can be mapped back into domain
names. Note that this document does not define a domain name
equivalence for any other distinguished names.

4. Attribute Type Definition

The DC (short for domainComponent) attribute type is defined as
follows:

( 0.9.2342.19200300.100.1.25 NAME 'dc' EQUALITY caseIgnoreIA5Match
SUBSTR caseIgnoreIA5SubstringsMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.26 SINGLE-VALUE )

The value of this attribute is a string holding one component of a
domain name. The encoding of IA5String for use in LDAP is simply the
characters of the string itself. The equality matching rule is case
insensitive, as is today's DNS.

5. Object Class Definitions

An object with a name derived from its domain name using the
algorithm of section 3 is represented as an entry in the directory.
The "DC" attribute is present in the entry and used as the RDN.

An attribute can only be present in an entry held by an LDAP server
when that attribute is permitted by the entry's object class.

This section defines two object classes. The first, dcObject, is
intended to be used in entries for which there is an appropriate
structural object class. For example, if the domain represents a
particular organization, the entry would have as its structural
object class 'organization', and the 'dcObject' class would be an
auxiliary class. The second, domain, is a structural object class
used for entries in which no other information is being stored. The
domain object class is typically used for entries that are
placeholders or whose domains do not correspond to real-world
entities.

5.1. The dcObject object class

The dcObject object class permits the dc attribute to be present in
an entry. This object class is defined as auxiliary, as it would
typically be used in conjunction with an existing structural object
class, such as organization, organizationalUnit or locality.

The following object class, along with the dc attribute, can be added
to any entry.

( 1.3.6.1.4.1.1466.344 NAME 'dcObject' SUP top AUXILIARY MUST dc )

An example entry would be:

dn: dc=critical-angle,dc=com
objectClass: top
objectClass: organization
objectClass: dcObject
dc: critical-angle
o: Critical Angle Inc.

5.2. The domain object class

If the entry does not correspond to an organization, organizational
unit or other type of object for which an object class has been
defined, then the "domain" object class can be used. The "domain"
object class requires that the "DC" attribute be present, and permits
several other attributes to be present in the entry.

The entry will have as its structural object class the "domain"
object class.

( 0.9.2342.19200300.100.4.13 NAME 'domain' SUP top STRUCTURAL
MUST dc
MAY ( userPassword $ searchGuide $ seeAlso $ businessCategory $
x121Address $ registeredAddress $ destinationIndicator $
preferredDeliveryMethod $ telexNumber $ teletexTerminalIdentifier $
telephoneNumber $ internationaliSDNNumber $ facsimileTelephoneNumber $
street $ postOfficeBox $ postalCode $ postalAddress $
physicalDeliveryOfficeName $ st $ l $ description $ o $
associatedName ) )

The optional attributes of the domain class are used for describing
the object represented by this domain, and may also be useful when
searching. These attributes are already defined for use with LDAP
[4].

An example entry would be:

dn: dc=tcp,dc=critical-angle,dc=com
objectClass: top
objectClass: domain
dc: tcp
description: a placeholder entry used with SRV records

The DC attribute is used for naming entries of the domain class, and
this can be represented in X.500 servers by the following name form
rule.

( 1.3.6.1.4.1.1466.345 NAME 'domainNameForm' OC domain MUST ( dc ) )

6. References

[1] The Directory: Selected Attribute Types. ITU-T Recommendation
X.520, 1993.

[2] Mockapetris, P., " Domain Names - Concepts and Facilities,"
STD 13, RFC1034, November 1987.

[3] Kille, S., and M. Wahl, " Lightweight Directory Access Protocol
(v3): UTF-8 String Representation of Distinguished Names", RFC
2253, December 1997.

[4] Wahl, M., "A Summary of the X.500(96) User Schema for use with
LDAP", RFC2256, December 1997.

7. Security Considerations

This memo describes how attributes of objects may be discovered and
retrieved. Servers should ensure that an appropriate security policy
is maintained.

An enterprise is not restricted in the information which it may store
in DNS or LDAP servers. A client which contacts an untrusted server
may have incorrect or misleading information returned (e.g. an
organization's server may claim to hold naming contexts representing
domain names which have not been delegated to that organization).

8. Authors' Addresses

Steve Kille
Isode Ltd.
The Dome
The Square
Richmond, Surrey
TW9 1DT
England

Phone: +44-181-332-9091
EMail: [email protected]

Mark Wahl
Critical Angle Inc.
4815 W. Braker Lane #502-385
Austin, TX 78759
USA

Phone: (1) 512 372 3160
EMail: [email protected]

Al Grimstad
AT&T
Room 1C-429, 101 Crawfords Corner Road
Holmdel, NJ 07733-3030
USA

EMail: [email protected]

Rick Huber
AT&T
Room 1B-433, 101 Crawfords Corner Road
Holmdel, NJ 07733-3030
USA

EMail: [email protected]

Sri Sataluri
AT&T
Room 4G-202, 101 Crawfords Corner Road
Holmdel, NJ 07733-3030
USA

EMail: [email protected]

9. Full Copyright Statement

Copyright (C) The Internet Society (1998). All Rights Reserved.

This document and translations of it may be copied and furnished to
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