INTERNET-DRAFT Editor: Kurt D. Zeilenga
Intended Category: Standard Track OpenLDAP Foundation
Expires in six months 30 June 2003
Obsoletes: 2253
LDAP: String Representation of Distinguished Names
<draft-ietf-ldapbis-dn-11.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 (C) The Internet Society (2003). All Rights Reserved.
Please see the Full 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
designed to give a clean representation of commonly used distinguished
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names, while being able to represent any distinguished name.
Conventions
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].
1. Background and Intended Usage
In X.500-based directory systems [X.500], including those accessed
using the Lightweight Directory Access Protocol (LDAP) [Roadmap],
distinguished names (DNs) are used to unambiguously refer to directory
entries [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 the local
national language).
This document defines the string representation of Distinguished Names
used in LDAP [Protocol][Syntaxes]. Section 2 details the RECOMMENDED
algorithm for converting a DN from its ASN.1 structured representation
to a string. Section 3 details how to convert a DN from a string to a
ASN.1 structured representation.
While other documents may define other algorithms for converting a DN
from its ASN.1 structured representation to a string, all algorithms
MUST produce strings which adhere to the requirements of Section 3.
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].
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This document obsoletes RFC 2253. Changes since RFC 2253 are
summarized in Appendix B.
This specification assumes familiarity with X.500 [X.500], and the
concept of Distinguished Name [X.501][Models].
2. Converting DistinguishedName from ASN.1 to a String
X.501 [X.501] defines the ASN.1 [X.680] structure of distinguished
name. The following is a variant provided for discussion purposes.
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
[UTF-8] encoded Universal Character Set (UCS) [ISO10646] character
string representation. Other documents may describe other algorithms
for converting a distinguished name to a string, but only strings
which conform to the grammar defined in Section 3 MUST be produced by
LDAP implementations.
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 ("," U+002C) character.
2.2. Converting RelativeDistinguishedName
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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+002B)
character.
2.3. Converting AttributeTypeAndValue
The AttributeTypeAndValue is encoded as the string representation of
the AttributeType, followed by an equals ("=" U+003D) character,
followed by the string representation of the AttributeValue. The
encoding of the AttributeValue is given in Section 2.4.
If the AttributeType is defined to have a short name and that short
name is known to be registered [REGISTRY][BCP64bis] as identifying the
AttributeType, that short name, a <descr>, is used. Otherwise the
AttributeType is encoded as the dotted-decimal encoding, a
<numericoid>, of its OBJECT IDENTIFIER. The <descr> and <numericoid>
is defined in [Models].
Implementations are not expected to dynamically update their knowledge
of registered short names. However, implementations SHOULD provide a
mechanism to allow its knowledge of registered short names to be
updated.
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 ("#" U+0023) character 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
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.
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- a space (" " U+0020) or number sign ("#" U+0023) occurring at
the beginning of the string;
- a space (" " U+0020) character occurring at the end of the
string;
- one of the characters """, "+", ",", ";", "<", ">", or "\"
(U+0022, U+002B, U+002C, U+003B, U+003C, U+003E, or U+005C
respectively);
- the null (U+0000) 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+0020, U+0022, U+0023, U+002B, U+002C, U+003B,
U+003C, U+003D, U+003E, U+005C respectively)
it can be prefixed by a backslash ("\" U+0005C).
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 [UTF-8] encoded characters from the Universal Character Set
(UCS) [ISO10646]. The structure of this string representation is
specified using the following Augmented BNF [RFC2234] grammar:
distinguishedName = [ relativeDistinguishedName
*( COMMA relativeDistinguishedName ) ]
relativeDistinguishedName = attributeTypeAndValue
*( PLUS attributeTypeAndValue )
attributeTypeAndValue = attributeType EQUALS attributeValue
attributeType = descr / numericoid
attributeValue = string / hexstring
; The UTF-8 string shall not contain NULL, ESC, or
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; 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].
Each <attributeType>, either a <descr> or a <numericoid>, refers to an
attribute type of an attribute value assertion (AVA). The
<attributeType> is followed by a <EQUALS> and an <attributeValue>.
The <attributeValue> is either in <string> or <hexstring> form.
If in <string> form, a LDAP string representation asserted value can
be obtained by replacing (left-to-right, non-recursively) each <pair>
appearing in the <string> as follows:
replace <ESC><ESC> with <ESC>;
replace <ESC><special> with <special>;
replace <ESC><hexpair> with the octet indicated by the <hexpair>.
If in <hexstring> form, a BER representation can be obtained from
converting each <hexpair> of the <hexstring> to the octet indicated by
the <hexpair>.
One or more attribute values assertions, separated by <PLUS>, for a
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relative distinguished name.
Zero or more relative distinguished names, separated by <COMMA>, for a
distinguished name.
Implementations MUST recognize AttributeType name strings
(descriptors) listed in the following table, but MAY recognize other
name strings.
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)
Implementations MAY recognize other DN string representations
(such as that described in RFC 1779). However, as there is no
requirement that 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
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
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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 Escaped
------------------------------- -------- ------ --------
LATIN CAPITAL LETTER L U+004C 0x4C L
LATIN SMALL LETTER U U+0075 0x75 u
LATIN SMALL LETTER C WITH CARON U+010D 0xC48D \C4\8D
LATIN SMALL LETTER I U+0069 0x69 i
LATIN SMALL LETTER C WITH ACUTE U+0107 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].
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.
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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 (Basic Encoding Rules) or DER (Distinguished Encoding rules)
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.
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] International Telecommunication Union -
Telecommunication Standardization Sector, "The Directory
-- Models," X.501(1993) (also ISO/IEC 9594-2:1994).
[X.680] International Telecommunication Union -
Telecommunication Standardization Sector, "Abstract
Syntax Notation One (ASN.1) - Specification of Basic
Notation", X.680(1997) (also ISO/IEC 8824-1:1998).
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14 (also RFC 2119), March 1997.
[RFC2234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[UTF-8] Yergeau, F., "UTF-8, a transformation
format of ISO 10646", draft-yergeau-rfc2279bis, a work
in progress.
[Models] Zeilenga, K. (editor), "LDAP: Directory Information
Models", draft-ietf-ldapbis-models-xx.txt, a work in
progress.
[Roadmap] Zeilenga, K. (editor), "LDAP: Technical Specification
Road Map", draft-ietf-ldapbis-roadmap-xx.txt, a work in
progress.
[Protocol] Sermersheim, J. (editor), "LDAP: The Protocol",
draft-ietf-ldapbis-protocol-xx.txt, a work in progress.
[Syntaxes] Legg, S. (editor), "LDAP: Syntaxes and Matching Rules",
draft-ietf-ldapbis-syntaxes-xx.txt, a work in progress.
[Schema] Dally, K. (editor), "LDAP: User Schema",
draft-ietf-ldapbis-user-schema-xx.txt, a work in
progress.
[ISO10646] International Organization for Standardization,
"Universal Multiple-Octet Coded Character Set (UCS) -
Architecture and Basic Multilingual Plane", ISO/IEC
10646-1 : 1993.
[REGISTRY] IANA, Object Identifier Descriptors
Registry, <http://www.iana.org/...>.
9. Informative References
[X.500] International Telecommunication Union -
Telecommunication Standardization Sector, "The Directory
-- Overview of concepts, models and services,"
X.500(1993) (also ISO/IEC 9594-1:1994).
[X.690] International Telecommunication Union -
Telecommunication Standardization Sector, "Specification
of ASN.1 encoding rules: Basic Encoding Rules (BER),
Canonical Encoding Rules (CER), and Distinguished
Encoding Rules (DER)", X.690(1997) (also ISO/IEC
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8825-1:1998).
[RFC2849] Good, G., "The LDAP Data Interchange Format (LDIF) -
Technical Specification", RFC 2849, June 2000.
[BCP64bis] Zeilenga, K., "IANA Considerations for LDAP", draft-
ietf-ldapbis-bcp64-xx.txt, a work in progress.
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
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 appendix. Transcoding issues are also not discussed
in this appendix.
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 user. For example, an application 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 often 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 "<" (U+003C) and ">" (U+003E) 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
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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. 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 before the DN string is used in LDAP.
It is not advised to insert white space otherwise as it may not be
obvious to the user which white space is part of the DN string and
which white space was added for readability.
Another alternative is to use the LDAP Data Interchange Format (LDIF)
[RFC2849]. For example,
# This 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
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.
- Revised specification (in Section 2) to allow short names of any
registered attribute type to appear in string representations of
DNs instead of being restricted to a "published table". Remove
"as an example" language. Added statement (in Section 3) allowing
recognition of additional names but require recognization of those
names in the published table. The table is now published in
Section 3.
- Replaced specification of additional requirements for LDAPv2
implementations which also support LDAPv3 (RFC 2253, Section 4)
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with a statement (in Section 3) allowing recognition of
alternative string representations.
- 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.
- Updated Section 3 to describe how to parse elements of the
grammar.
- Rewrote examples.
- Added reference to documentations containing general LDAP security
considerations.
- Added discussion of presentation issues (Appendix A).
- Added this appendix.
In addition, numerous editorial changes were made.
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Full Copyright
Copyright (C) The Internet Society (2003). All Rights Reserved.
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