INTERNET-DRAFT Editor: Kurt D. Zeilenga
Intended Category: Standard Track OpenLDAP Foundation
Expires: 29 September 2001 29 March 2001
Obsoletes: 2253
Lightweight Directory Access Protocol (v3):
UTF-8 String Representation of Distinguished Names
<draft-ietf-ldapbis-dn-02.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 Working Group (LDAPbis) mailing list
<ietf-ldapbis@openldap.org>. Please send editorial comments directly
to the document editor <Kurt@OpenLDAP.org>.
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Copyright 2001, 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 as the primary keys to
entries in the directory. Distinguished Names are encoded in ASN.1 in
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the X.500 Directory protocols. In the Lightweight Directory Access
Protocol, a string representation of distinguished names is
transferred. This specification defines the string format for
representing names, which is designed to give a clean representation
of commonly used distinguished names, while being able to represent
any distinguished name.
1. Background
This specification assumes familiarity with X.500 [X.500], and the
concept of Distinguished Name (DN). 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 clients 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 obsoletes RFC 2253.
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 RFC 2119 [RFC2119].
2. Converting DistinguishedName from ASN.1 to a String
In X.501 [X.501] the ASN.1 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 }
The following sections define the algorithm for converting from an
ASN.1 structured representation to a UTF-8 [RFC2279] string
representation.
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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 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 (',' ASCII 44).
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 2.3), in any order.
Where there is a multi-valued RDN, the outputs from adjoining
AttributeTypeAndValues are separated by a plus ('+' ASCII 43)
character.
2.3. Converting AttributeTypeAndValue
The AttributeTypeAndValue is encoded as the string representation of
the AttributeType, followed by an equals character ('=' ASCII 61),
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 [RFC2252bis], then the type name string from that
table is used, otherwise it is encoded as the dotted-decimal encoding
of the AttributeType's OBJECT IDENTIFIER. The dotted-decimal notation
is described in [RFC2251bis].
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)
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STREET streetAddress (2.5.4.9)
DC domainComponent (0.9.2342.19200300.100.1.25)
UID userId (0.9.2342.19200300.100.1.1)
2.4. Converting an AttributeValue from ASN.1 to a String
If the AttributeValue is of a type which does not have a string
representation defined for it, then it is simply encoded as an
octothorpe character ('#' ASCII 35) followed by the hexadecimal
representation of each of the octets of the BER encoding of the X.500
AttributeValue. This form SHOULD be used if the AttributeType is of
the dotted-decimal form.
Otherwise, if the AttributeValue is of a type which has a string
representation, the value is converted first to a UTF-8 string
according to its syntax specification (see for example section 6 of
[RFC2252bis]).
If the UTF-8 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 (' ' ASCII 32) or octothorpe ('#' ASCII 35) occurring at the
beginning of the string
- a space (' ' ASCII 32) character occurring at the end of the string
- one of the characters ",", "+", """, "\", "<", ">" or ";" (ASCII 44,
43, 34, 92, 60, 62, or 59, respectively)
Implementations MAY escape other characters.
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 " "
(ASCII 44, 43, 34, 92, 60, 62, 59, 35, or 32, respectively)
it may be prefixed by a backslash ('\' ASCII 92).
Examples of the escaping mechanism are shown in section 4.
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3. Parsing a String back to a Distinguished Name
The structure of the UTF-8 [RFC2279] string is specified using the
following Augmented BNF [RFC2234] grammar.
distinguishedName = [name]
; may be empty
name = name-component *(COMMA name-component)
name-component = attributeTypeAndValue *(PLUS attributeTypeAndValue)
attributeTypeAndValue
= attributeType EQUALS attributeValue
attributeType = keyword / oid
keyword = ALPHA 1*keychar
keychar = ALPHA / DIGIT / MINUS
oid = 1*DIGIT *(DOT 1*DIGIT)
attributeValue = string / hexstring
string = *( stringchar / pair )
; the string MUST NOT start with SHARP or SP
; and MUST NOT end with SP
stringchar = <any UTF-8 character (can be multiple octets)
excepting escaped or ESC>
pair = ESC ( ESC / special / hexpair )
special = escaped / SHARP / EQUALS / SP
escaped = COMMA / PLUS / %x22 / %x3C / %x3E / %3B
; "," / "+" / """ / "<" / ">" / ";"
hexstring = SHARP 1*hexpair
hexpair = HEX HEX
HEX = DIGIT / %x41-46 / %x61-66
; 0-9 / A-F / a-f
ALPHA = %x41-5A / %x61-7A
; A-Z / a-z
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DIGIT = %x30-39
; 0-9
SP = %x20 ; space (" ")
SHARP = %x23 ; sharp sign ("#")
PLUS = %x2B ; plus sign ("+")
COMMA = %x2C ; comma (",")
MINUS = %x2D ; minus sign ("-")
DOT = %x2E ; period (".")
EQUALS = %x3D ; equals sign ("=")
ESC = %x5C ; backslash ("\")
Implementations MUST recognize AttributeType string type names
(keywords) listed in the Section 2 table, but MAY recognize other
names (keywords). Implementations MAY recognize other DN string
representations (such as that described in RFC 1779) but SHALL 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 quoting 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
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Finally, an example of an RDN commonName value consisting of 5
letters:
Unicode Letter Description 10646 code UTF-8 Quoted
=============================== ========== ====== =======
LATIN CAPITAL LETTER L U0000004C 0x4C L
LATIN SMALL LETTER U U00000075 0x75 u
LATIN SMALL LETTER C WITH CARON U0000010D 0xC48D \C4\8D
LATIN SMALL LETTER I U00000069 0x69 i
LATIN SMALL LETTER C WITH ACUTE U00000107 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
[RFC2251bis] and its normative references.
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,
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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 octothorpe ('#')
as described in the first paragraph of section 2.4.
6. References
[X.500] "The Directory -- overview of concepts, models and
services," ITU-T Rec. X.500(1993).
[X.501] "The Directory -- Models," ITU-T Rec. X.501(1993).
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119.
[RFC2234] Crocker, D., and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 2279, January 1998.
[RFC2251bis] LDAPbis WG, "Lightweight Directory Access Protocol (v3)",
a work in progress.
[RFC2252bis] LDAPbis WG, "LDAPv3: Attribute Syntax Definitions", a
work in progress.
[RFC2256bis] LDAPbis WG, "LDAPv3: User Schema", a work in progress.
7. 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.
8. Document Editor's Address
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Kurt D. Zeilenga
OpenLDAP Foundation
<Kurt@OpenLDAP.org>
Appendix A. Changes made since RFC 2253
This appendix is provided for informational purposes and is not a
normative part of this specification.
The following substantive changes were made to RFC 2253:
- Removed IESG Note. The IESG Note is addressed by RFC 2829.
- Replaced specification of additional requirements for LDAPv2
implementations which also support LDAPv3 (Section 4) with a
statement (in Section 3) allowing recongization of alternative
string representations.
- Updated 2.3 to clarify which table is the published table of names
which may be appear in DNs. Remove "as an example" language.
Added statement (in Section 3) allowing recongization of
additional names. - Updated 2.3 to indicate attribute type name
strings are not case sensitive.
- Updated 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 LDAP-specific
security considerations.
In addition, numerous editorial changes were made.
Copyright 2001, The Internet Society. 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.
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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 AUTHORS, 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
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