INTERNET-DRAFT Editor: Kurt D. Zeilenga
Intended Category: Standard Track OpenLDAP Foundation
Expires: 24 April 2001 24 October 2000
Obsoletes: RFC 2253
Lightweight Directory Access Protocol (v3bis):
UTF-8 String Representation of Distinguished Names
<draft-zeilenga-ldapbis-rfc2253-01.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.
Distribution of this memo is unlimited. Technical discussion of this
document will take place on the IETF LDAP Revision (Proposed) Working
Group (LDAPbis) mailing list <ietf-ldapbis@openldap.org>. Please send
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Copyright 2000, 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
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
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representing names, which is designed to give a clean representation
of commonly used distinguished names, while being able to represent
any distinguished name.
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].
1. Background
This specification assumes familiarity with X.500 [X.500], and the
concept of Distinguished Name. 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).
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 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
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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 a published table of attribute types
associated with LDAP [RFC2252], 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 [RFC2251]. As an example, strings for a few of the
attribute types frequently seen in RDNs include:
String X.500 AttributeType
------------------------------
CN commonName
L localityName
ST stateOrProvinceName
O organizationName
OU organizationalUnitName
C countryName
STREET streetAddress
DC domainComponent
UID userid
2.4. Converting an AttributeValue from ASN.1 to a String
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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 bytes 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
[RFC2252]).
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 or "#" character occurring at the beginning of the string
- a space character occurring at the end of the string
- one of the characters ",", "+", """, "\", "<", ">" or ";"
Implementations MAY escape other characters.
If a character to be escaped is one of the list shown above, then it
is prefixed by a backslash ('\' ASCII 92).
Otherwise the character to be escaped is replaced by a backslash and
two hex digits, which form a single byte in the code of the character.
Examples of the escaping mechanism are shown in section 5.
3. Parsing a String back to a Distinguished Name
The structure of the string is specified using the following Augmented
BNF [RFC2234] grammar.
distinguishedName = [name]
; may be empty
name = name-component *("," name-component)
name-component = attributeTypeAndValue *("+" attributeTypeAndValue)
attributeTypeAndValue
= attributeType "=" attributeValue
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attributeType = (ALPHA 1*keychar) / oid
keychar = ALPHA / DIGIT / "-"
oid = 1*DIGIT *("." 1*DIGIT)
attributeValue = string | hexstring
string = *( stringchar | pair )
quotechar = <any character except ESC or QUOTE>
special = "," / "=" / "+" / "<" / ">" / "#" / ";"
pair = ESC ( ESC / special / QUOTE / hexpair )
stringchar = <any character except one of special,
ESC or QUOTE>
hexstring = "#" 1*hexpair
hexpair = HEX HEX
HEX = DIGIT
/ "A" / "B" / "C" / "D" / "E" / "F"
/ "a" / "b" / "c" / "d" / "e" / "f"
ALPHA = <any ASCII alphabetic character>
; decimal 65-90 and 97-122
DIGIT = <any ASCII decimal digit>
; decimal 48-57
QUOTE = <the ASCII double quotation mark character '"'>
; decimal 34
ESC = <the ASCII backslash character "\">
; decimal 92
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=com
Here is an example name containing three RDNs, in which the first
RDN is multi-valued:
OU=Sales+CN=J. Smith,O=Example Inc.,C=US
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This example shows the method of quoting of a comma in an
organization name:
CN=L. Eagle,O=Sue\, Grabbit and Runn,C=GB
An example name in which a value contains a carriage return
character:
CN=Before\0dAfter,O=Test,C=GB
An example name in which an RDN was of an unrecognized type. The
value is the BER encoding of an OCTET STRING containing two bytes
0x48 and 0x69.
1.3.6.1.4.1.1466.0=#04024869,DC=EXAMPLE,DC=COM
Finally, an example of an RDN surname 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):
SN=Lu\C4\8Di\C4\87
5. Security Considerations
The following security considerations are specific to the handling of
distinguished names. For security considerations specific to LDAP,
see RFC 2251, RFC 2829, and RFC 2830.
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
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- 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 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.
[RFC2251] Wahl, M., Howes, T., and S. Kille, "Lightweight Directory
Access Protocol (v3)", RFC 2251, December 1997.
[RFC2252] Wahl, M., Coulbeck, A., Howes, T. and S. Kille, "Lightweight
Directory Access Protocol (v3): Attribute Syntax
Definitions", RFC 2252, December 1997.
[RFC2234] Crocker, D., and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
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7. Acknowledgment
This document is an update to RFC 2251, by Mark Wahl, Tim Howes, and
Steve Kille. Design ideas included in this revised specification are
based upon those discussed in LDAP Revision (proposed) Working Group
(LDAPbis) and other IETF Working Groups. The contributions of
individuals in these working groups is gratefully acknowledged.
8. Additional Information
Discussions regarding these suggestions may directed to the editor:
Kurt D. Zeilenga
OpenLDAP Foundation
<Kurt@OpenLDAP.org>
or the LDAP Revision (proposed) Working Group (LDAPbis) mailing list:
<ietf-ldapbis@openldap.org>
Appendix A. Changes made to RFC 2253
The following substantive changes were made to RFC 2253:
- Removed IESG Note
- Removed Relationship with RFC 1779 and LDAPv2 Section.
- Removed all LDAPv2 requirements.
- Rewrote DN grammar using ABNF
- Rewrote examples
- Added reference to documentations containing LDAP-specific
security considerations
In addition, numerous editorial changes were made.
Copyright 2000, The Internet Society. All Rights Reserved.
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