INTERNET-DRAFT Editor: Kurt D. Zeilenga
Intended Category: Standard Track OpenLDAP Foundation
Expires in six months 18 August 2002
Obsoletes: 2253
LDAP: String Representation of Distinguished Names
<draft-ietf-ldapbis-dn-08.txt>
Status of Memo
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.
This document is intended to be, after appropriate review and
revision, submitted to the RFC Editor as a Standard Track document
replacing RFC 2253. Distribution of this memo is unlimited.
Technical discussion of this document will take place on the IETF LDAP
Revision (LDAPbis) Working Group mailing list
<ietf-ldapbis@openldap.org>. Please send editorial comments directly
to the document editor <Kurt@OpenLDAP.org>.
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Copyright 2002, The Internet Society. All Rights Reserved.
Please see the Copyright section near the end of this document for
more information.
Abstract
The X.500 Directory uses distinguished names (DNs) as primary keys to
entries in the directory. This document defines the string
representation used in the Lightweight Directory Access Protocol
(LDAP) to transfer distinguished names. The string representation is
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designed to give a clean representation of commonly used distinguished
names, while being able to represent any distinguished name.
1. Background and Intended Usage
In X.500-based directory systems [X.500], including those accessed
using the Lightweight Directory Access Protocol (LDAP) [LDAPTS],
distinguished names (DNs) are used to unambiguously refer to a
directory entry [X.501][Models].
The structure of a DN [X.501] is described in terms of ASN.1 [X.680].
In the X.500 Directory Access Protocol [X.511] (and other ITU-defined
directory protocols), DNs are encoded using the Basic Encoding Rules
(BER) [X.690]. In LDAP, DNs are represented in string form.
It is important to have a common format to be able to unambiguously
represent a distinguished name. The primary goal of this
specification is ease of encoding and decoding. A secondary goal is
to have names that are human readable. It is not expected that LDAP
implementations with a human user interface would display these
strings directly to the user, but would most likely be performing
translations (such as expressing attribute type names in one of the
local national languages).
This document defines the string representation of Distinguished Names
used in LDAP [Protocol][Syntaxes]. Section 2 details how to convert a
DN from ASN.1 structured representation to a string. Section 3
details how to convert a DN from string to ASN.1 structured
representation.
This document does not define a canonical string representation for
DNs. Comparison of DNs for equality is to be performed in accordance
with the distinguishedNameMatch matching rule [Syntaxes].
This document is an integral part of the LDAP Technical Specification
[Roadmap].
This document obsoletes RFC 2253. Changes since RFC 2253 are
summarized in Appendix B.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14 [RFC2119].
This specification assumes familiarity with X.500 [X.500], and the
concept of Distinguished Name [X.501][Models].
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2. Converting DistinguishedName from ASN.1 to a String
In X.501 [X.501] the ASN.1 [X.680] structure of distinguished name is
defined as:
DistinguishedName ::= RDNSequence
RDNSequence ::= SEQUENCE OF RelativeDistinguishedName
RelativeDistinguishedName ::= SET SIZE (1..MAX) OF
AttributeTypeAndValue
AttributeTypeAndValue ::= SEQUENCE {
type AttributeType,
value AttributeValue }
This section defines the RECOMMENDED algorithm for converting a
distinguished name from an ASN.1 structured representation to an UTF-8
[RFC2279] encoded Universal Character Set (UCS) [ISO10646] character
string representation.
2.1. Converting the RDNSequence
If the RDNSequence is an empty sequence, the result is the empty or
zero length string.
Otherwise, the output consists of the string encodings of each
RelativeDistinguishedName in the RDNSequence (according to Section
2.2), starting with the last element of the sequence and moving
backwards toward the first.
The encodings of adjoining RelativeDistinguishedNames are separated by
a comma character ("," U+0002C).
2.2. Converting RelativeDistinguishedName
When converting from an ASN.1 RelativeDistinguishedName to a string,
the output consists of the string encodings of each
AttributeTypeAndValue (according to Section 2.3), in any order.
Where there is a multi-valued RDN, the outputs from adjoining
AttributeTypeAndValues are separated by a plus sign ("+" U+0002B)
character.
2.3. Converting AttributeTypeAndValue
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The AttributeTypeAndValue is encoded as the string representation of
the AttributeType, followed by an equals character ("=" U+0003D),
followed by the string representation of the AttributeValue. The
encoding of the AttributeValue is given in Section 2.4.
If the AttributeType is in the following table of attribute types
associated with LDAP [Schema], then the type name string, a <descr>,
from that table is used, otherwise it is encoded as the dotted-decimal
encoding, a <numericoid>, of the AttributeType's OBJECT IDENTIFIER.
The <descr> and <numericoid> is defined in [Models].
The type name string is not case sensitive.
String X.500 AttributeType
------ --------------------------------------------
CN commonName (2.5.4.3)
L localityName (2.5.4.7)
ST stateOrProvinceName (2.5.4.8)
O organizationName (2.5.4.10)
OU organizationalUnitName (2.5.4.11)
C countryName (2.5.4.6)
STREET streetAddress (2.5.4.9)
DC domainComponent (0.9.2342.19200300.100.1.25)
UID userId (0.9.2342.19200300.100.1.1)
Note: This table lists the complete set of type name strings which
all implementations MUST recognize in DN string representation.
As no extension could reasonable require all existing
implementations be updated to recognize additional type name
strings, this table is not extensible.
2.4. Converting an AttributeValue from ASN.1 to a String
If the AttributeType is of the dotted-decimal form, the AttributeValue
is represented by an number sign character ("#" U+00023) followed by
the hexadecimal encoding of each of the octets of the BER encoding of
the X.500 AttributeValue. This form is also used when the syntax of
the AttributeValue does not have a native string encoding defined for
it or the native string encoding is not restricted to UTF-8 encoded
UCS (or a subset of UCS) characters. This form may also be used in
other cases, such as when a reversible string representation is
desired (see Section 5.2).
Otherwise, if the AttributeValue is of a syntax which has a native
string encoding, the value is converted first to a UTF-8 encoded UCS
string according to its syntax specification (see for example Section
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6 of [Syntaxes]). If that UTF-8 encoded UCS string does not have any
of the following characters which need escaping, then that string can
be used as the string representation of the value.
- a space (" " U+00020) or number sign ("#" U+00023) occurring at
the beginning of the string;
- a space (" " U+00020) character occurring at the end of the
string;
- one of the characters """, "+", ",", ";", "<", ">", or "\"
(U+00022, U+0002B, U+0002C, U+0003B, U+0003C, U+0003E, or
U+0005C respectively);
- the null (U+00000) character.
Other characters may be escaped.
Each octet of the character to be escaped is replaced by a backslash
and two hex digits, which form a single octet in the code of the
character. Alternatively, if and only if the character to be escaped
is one of
" ", """, "#", "+", ",", ";", "<", "=", ">", or "\"
(U+00020, U+00022, U+00023, U+0002B, U+0002C, U+0003B,
U+0003C, U+0003D, U+0003E, U+0005C respectively)
it can be prefixed by a backslash ("\" U+00005C).
Examples of the escaping mechanism are shown in Section 4.
3. Parsing a String back to a Distinguished Name
The string representation of Distinguished Names is restricted to
UTF-8 [RFC2279] encoded characters from the Universal Character Set
(UCS) [ISO10646]. The structure of this string representation is
specified using the following Augmented BNF [RFC2234] grammar using
the common productions defined in [Models].
distinguishedName = [ relativeDistinguishedName
*( COMMA relativeDistinguishedName ) ]
relativeDistinguishedName = attributeTypeAndValue
*( PLUS attributeTypeAndValue )
attributeTypeAndValue = attributeType EQUALS attributeValue
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attributeType = descr / numericoid
attributeValue = string / hexstring
; The UTF-8 string shall not contain NULL, ESC, or
; one of escaped, shall not start with SHARP or SPACE,
; and shall must not end with SPACE.
string = [ (leadchar / pair)
[ *( stringchar / pair ) ( trailchar / pair ) ] ]
leadchar = LUTF1 / UTFMB
LUTF1 = %x01-1F / %x21 / %x24-2A / %x2D-3A /
%x3D / %x3F-5B / %x5D-7F
trailchar = TUTF1 / UTFMB
TUTF1 = %x01-1F / %x21 / %x23-2A / %x2D-3A /
%x3D / %x3F-5B / %x5D-7F
stringchar = SUTF1 / UTFMB
SUTF1 = %x01-21 / %x23-2A / %x2D-3A /
%x3D / %x3F-5B / %x5D-7F
pair = ESC ( ESC / special / hexpair )
special = escaped / SPACE / SHARP / EQUALS
escaped = DQUOTE / PLUS / COMMA / SEMI / LANGLE / RANGLE
hexstring = SHARP 1*hexpair
hexpair = HEX HEX
where the productions <descr>, <numericoid>, <COMMA>, <DQUOTE>,
<EQUALS>, <ESC>, <HEX>, <LANGLE>, <NULL>, <PLUS>, <RANGLE>, <SEMI>,
<SPACE>, <SHARP>, <UTFMB> are defined in [Models].
Implementations MUST recognize AttributeType name strings
(descriptors) listed in the Section 2.3 table, but MAY recognize other
name strings. Implementations MAY recognize other DN string
representations (such as that described in RFC 1779). However, as
there is no requirement for other names or alternative DN string
representations to be recognized (and, if so, how), implementations
SHOULD only generate DN strings in accordance with Section 2 of this
document.
4. Examples
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This notation is designed to be convenient for common forms of name.
This section gives a few examples of distinguished names written using
this notation. First is a name containing three relative
distinguished names (RDNs):
UID=jsmith,DC=example,DC=net
Here is an example name containing three RDNs, in which the first RDN
is multi-valued:
OU=Sales+CN=J. Smith,DC=example,DC=net
This example shows the method of escaping of a comma in a common name:
CN=John Smith\, III,DC=example,DC=net
An example name in which a value contains a carriage return character:
CN=Before\0dAfter,DC=example,DC=net
An example name in which an RDN was of an unrecognized type. The
value is the BER encoding of an OCTET STRING containing two octets
0x48 and 0x69.
1.3.6.1.4.1.1466.0=#04024869,DC=example,DC=com
Finally, an example of an RDN commonName value consisting of 5
letters:
Unicode Letter Description UCS code UTF-8 Quoted
------------------------------- -------- ------ --------
LATIN CAPITAL LETTER L U+0004C 0x4C L
LATIN SMALL LETTER U U+00075 0x75 u
LATIN SMALL LETTER C WITH CARON U+0010D 0xC48D \C4\8D
LATIN SMALL LETTER I U+00069 0x69 i
LATIN SMALL LETTER C WITH ACUTE U+00107 0xC487 \C4\87
could be written in printable ASCII (useful for debugging purposes):
CN=Lu\C4\8Di\C4\87
5. Security Considerations
The following security considerations are specific to the handling of
distinguished names. LDAP security considerations are discussed in
[Protocol] and other documents comprising the LDAP Technical
Specification [Roadmap].
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5.1. Disclosure
Distinguished Names typically consist of descriptive information about
the entries they name, which can be people, organizations, devices or
other real-world objects. This frequently includes some of the
following kinds of information:
- the common name of the object (i.e. a person's full name)
- an email or TCP/IP address
- its physical location (country, locality, city, street address)
- organizational attributes (such as department name or affiliation)
Most countries have privacy laws regarding the publication of
information about people.
5.2. Use of Distinguished Names in Security Applications
The transformations of an AttributeValue value from its X.501 form to
an LDAP string representation are not always reversible back to the
same BER or DER form. An example of a situation which requires the
DER form of a distinguished name is the verification of an X.509
certificate.
For example, a distinguished name consisting of one RDN with one AVA,
in which the type is commonName and the value is of the TeletexString
choice with the letters 'Sam' would be represented in LDAP as the
string CN=Sam. Another distinguished name in which the value is still
'Sam' but of the PrintableString choice would have the same
representation CN=Sam.
Applications which require the reconstruction of the DER form of the
value SHOULD NOT use the string representation of attribute syntaxes
when converting a distinguished name to the LDAP format. Instead,
they SHOULD use the hexadecimal form prefixed by the number sign ('#')
as described in the first paragraph of Section 2.3.
5.3. Use of Other Names
Attribute type names are not unique. A string representation
generated with names other than those in the Section 2.3 table is
ambiguous. That is, two applications may recognize the string as
representing two different DNs possibly associated with two different
entries. This may lead to a wide range of unexpected behaviors which
can have both direct and indirect impacts upon security.
For example, a distinguished name consisting of one RDN with one AVA
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of the known locally attribute type FOO and the value "BAR" (an
octetString) could be represented in LDAP as the string FOO=BAR. As
the name FOO does not uniquely identify an attribute type, the DN
FOO=BAR is ambiguous. That is, FOO could be recognized as the
attribute type 1.1.1 by one application and 1.2.3.4 in another and not
recognized by another. This may lead to operations not behaving as
intended.
Applications desiring to generate an unambiguous string representation
of a DN SHOULD generate string representation per section 2, not use
names other than those in the Section 2.3 table, and while taking
Section 5.2 into consideration.
It is noted that while a registry for attribute type names
(descriptors) has been established [LDAPIANA], this registry does not
remove the ambiguity of attribute types names used in LDAP. It only
removes the ambiguity of attribute type names used in Standard Track
technical specifications.
6. Acknowledgment
This document is an update to RFC 2253, by Mark Wahl, Tim Howes, and
Steve Kille. RFC 2253 was a product of the IETF ASID Working Group.
This document is a product of the IETF LDAPbis Working Group.
7. Document Editor's Address
Kurt D. Zeilenga
OpenLDAP Foundation
<Kurt@OpenLDAP.org>
8. Normative References
[X.501] "The Directory -- Models," ITU-T Rec. X.501(1993).
[X.680] ITU-T, "Abstract Syntax Notation One (ASN.1) -
Specification of Basic Notation", X.680, 1994.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14 (also RFC 2119).
[RFC2234] Crocker, D., and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
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[RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 2279, January 1998.
[Models] K. Zeilenga (editor), "LDAP: Directory Information
Models", draft-ietf-ldapbis-models-xx.txt, a work in
progress.
[Roadmap] K. Zeilenga, "LDAP: Technical Specification Road Map",
draft-ietf-ldapbis-roadmap-xx.txt, a work in progress.
[Protocol] J. Sermersheim (editor), "LDAP: The Protocol",
draft-ietf-ldapbis-protocol-xx.txt, a work in progress.
[Syntaxes] S. Legg (editor), "LDAP: Syntaxes",
draft-ietf-ldapbis-syntaxes-xx.txt, a work in progress.
[Schema] K. Dally (editor), "LDAP: User Schema",
draft-ietf-ldapbis-user-schema-xx.txt, a work in
progress.
[ISO10646] Universal Multiple-Octet Coded Character Set (UCS) -
Architecture and Basic Multilingual Plane, ISO/IEC
10646-1 : 1993.
9. Informative References
[X.500] "The Directory -- overview of concepts, models and
services," ITU-T Rec. X.500(1993).
[X.690] ITU-T, "Specification of ASN.1 encoding rules: Basic,
Canonical, and Distinguished Encoding Rules", X.690,
1994.
[LDAPIANA] K. Zeilenga, "IANA Considerations for LDAP",
draft-ietf-ldapbis-xx.txt (a work in progress).
[RFC2849] G. Good, "The LDAP Data Interchange Format (LDIF) -
Technical Specification", RFC 2849, June 2000.
Appendix A. Presentation Issues
This appendix is provided for informational purposes only, it is not a
normative part of this specification.
The string representation described in this document is not intended
to be presented to humans without translation. However, at times it
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may be desirable to present non-translated DN strings to users. This
section discusses presentation issues associated with non-translated
DN strings. Presentation of translated DN strings issues are not
discussed in this document. Transcoding issues are also not discussed
in this document.
This appendix provides guidance for applications presenting DN strings
to users. This section is not comprehensive, it does not discuss all
presentation issues which implementors may face.
Not all user interfaces are capable of displaying the full set of UCS
characters. Some UCS characters are not displayable.
It is recommended that human interfaces use the optional hex pair
escaping mechanism (Section 2.3) to produce a string representation
suitable for display to the human. For example, an application only
capable of displaying printable characters can generate a DN string
for display which escapes all non-printable characters appearing in
the AttributeValue's string representation (as demonstrated in the
final example of Section 4).
When a DN string is displayed in free form text, it is necessary to
distinguish the DN string from surrounding text. While this is often
done with white space (as demonstrated in Section 4), it is noted that
DN strings may end with white space. Careful readers of Section 3
will note that characters "<" and ">" may only appear in the DN string
if escaped. These characters are intended to be used in free form
text to distinguish a DN string from surrounding text. For example,
<CN=Sam\ > distinguished the string representation of the DN comprised
of one RDN consisting of the AVA: the commonName (CN) value "Sam "
from the surrounding text. It should be noted to the user that the
wrapping "<" and ">" characters are not part of the DN string.
DN strings can be quite long. It is often desirable to line-wrap
overly long DN strings in presentations. Line wrapping should be done
by inserting white space after the RDN separator character or, if
necessary, after the AVA separator character in such presentations.
It should be noted to the user that the inserted white space is not
part of the DN string and is to be removed before use in LDAP. For
example,
The following DN string is long:
CN=Kurt D. Zeilenga,OU=Engineering,L=Redwood Shores,
O=OpenLDAP Foundation,ST=California,C=US
so it has been line-wrapped for readability. The extra white
space is to be removed the DN string is used in LDAP.
It is not advised to insert white space otherwise as it may not be
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obvious to the user what white space is part of the DN string and what
white space was added for readability.
Another alternative is to use the LDAP Data Interchange Format (LDIF)
[RFC2849]. For example,
The following entry has a long DN:
dn: CN=Kurt D. Zeilenga,OU=Engineering,L=Redwood Shores,
O=OpenLDAP Foundation,ST=California,C=US
CN: Kurt D. Zeilenga
SN: Zeilenga
objectClass: person
It is noted that that is often desirable to replace dotted-decimal
OIDs appearing in DN strings with attribute type names. Such
replacement is viewed as a translation and, hence, not discussed here.
Appendix B. Changes made since RFC 2253
This appendix is provided for informational purposes only, it is not a
normative part of this specification.
The following substantive changes were made to RFC 2253:
- Removed IESG Note. The IESG Note has been addressed.
- Clarified (in Section 1), that this document does not define a
canonical string representation.
- Replaced specification of additional requirements for LDAPv2
implementations which also support LDAPv3 (RFC 2253, Section 4)
with a statement (in Section 3) allowing recognition of
alternative string representations.
- Clarified (in Section 2.3) that the "published" table of names
which may be appear in DNs is the table which Section 2.3
provides. Remove "as an example" language. Noted this table is
not extensible. Added statement (in Section 3) allowing
recognition of additional names. Added security considerations
(Section 5.3) regarding the use of other names.
- Updated Section 2.3 to indicate attribute type name strings are
case insensitive.
- Updated Section 2.4 to allow hex pair escaping of all characters
and clarified escaping for when multiple octet UTF-8 characters
are present.
- Rewrote Section 3 to use ABNF as defined in RFC 2234.
- Rewrote Section 3 ABNF to be consistent with 2.4.
- Rewrote examples.
- Added reference to documentations containing general LDAP security
considerations.
- Added discussion of presentation issues (Appendix A).
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- Added this appendix.
In addition, numerous editorial changes were made.
Copyright 2002, The Internet Society. All Rights Reserved.
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or assist in its implementation may be prepared, copied, published and
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The limited permissions granted above are perpetual and will not be
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