RFC 2253
Lightweight Directory Access Protocol (v3): UTF-8 String Representation of Distinguished Names
Network Working Group M. Wahl Request for Comments: 2253 Critical Angle Inc. Obsoletes: 1779 S. Kille Category: Standards Track Isode Ltd. T. Howes Netscape Communications Corp. December 1997 Lightweight Directory Access Protocol (v3): UTF-8 String Representation of 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 (1997). All Rights Reserved. IESG Note This document describes a directory access protocol that provides both read and update access. Update access requires secure authentication, but this document does not mandate implementation of any satisfactory authentication mechanisms. In accordance with RFC 2026, section 4.4.1, this specification is being approved by IESG as a Proposed Standard despite this limitation, for the following reasons: a. to encourage implementation and interoperability testing of these protocols (with or without update access) before they are deployed, and b. to encourage deployment and use of these protocols in read-only applications. (e.g. applications where LDAPv3 is used as a query language for directories which are updated by some secure mechanism other than LDAP), and c. to avoid delaying the advancement and deployment of other Internet standards-track protocols which require the ability to query, but not update, LDAPv3 directory servers.
Readers are hereby warned that until mandatory authentication mechanisms are standardized, clients and servers written according to this specification which make use of update functionality are UNLIKELY TO INTEROPERATE, or MAY INTEROPERATE ONLY IF AUTHENTICATION IS REDUCED TO AN UNACCEPTABLY WEAK LEVEL. Implementors are hereby discouraged from deploying LDAPv3 clients or servers which implement the update functionality, until a Proposed Standard for mandatory authentication in LDAPv3 has been approved and published as an RFC. 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 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 [6]. 1. Background This specification assumes familiarity with X.500 [1], 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 [2] 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
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 [4], 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 [3]. 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
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
[4]).
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.
o a space or "#" character occurring at the beginning of the
string
o a space character occurring at the end of the string
o 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 in a BNF grammar, based on the grammar defined in RFC 822 [5]. Server implementations parsing a DN string generated by an LDAPv2 client MUST also accept (and ignore) the variants given in section 4 of this document. distinguishedName = [name] ; may be empty string name = name-component *("," name-component) name-component = attributeTypeAndValue *("+" attributeTypeAndValue) attributeTypeAndValue = attributeType "=" attributeValue attributeType = (ALPHA 1*keychar) / oid keychar = ALPHA / DIGIT / "-" oid = 1*DIGIT *("." 1*DIGIT) attributeValue = string string = *( stringchar / pair ) / "#" hexstring / QUOTATION *( quotechar / pair ) QUOTATION ; only from v2 quotechar = <any character except "\" or QUOTATION > special = "," / "=" / "+" / "<" / ">" / "#" / ";" pair = "\" ( special / "\" / QUOTATION / hexpair ) stringchar = <any character except one of special, "\" or QUOTATION > hexstring = 1*hexpair hexpair = hexchar hexchar hexchar = 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) QUOTATION = <the ASCII double quotation mark character '"' decimal 34>
4. Relationship with RFC 1779 and LDAPv2 The syntax given in this document is more restrictive than the syntax in RFC 1779. Implementations parsing a string generated by an LDAPv2 client MUST accept the syntax of RFC 1779. Implementations MUST NOT, however, generate any of the RFC 1779 encodings which are not described above in section 2. Implementations MUST allow a semicolon character to be used instead of a comma to separate RDNs in a distinguished name, and MUST also allow whitespace characters to be present on either side of the comma or semicolon. The whitespace characters are ignored, and the semicolon replaced with a comma. Implementations MUST allow an oid in the attribute type to be prefixed by one of the character strings "oid." or "OID.". Implementations MUST allow for space (' ' ASCII 32) characters to be present between name-component and ',', between attributeTypeAndValue and '+', between attributeType and '=', and between '=' and attributeValue. These space characters are ignored when parsing. Implementations MUST allow a value to be surrounded by quote ('"' ASCII 34) characters, which are not part of the value. Inside the quoted value, the following characters can occur without any escaping: ",", "=", "+", "<", ">", "#" and ";" 5. 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): CN=Steve Kille,O=Isode Limited,C=GB Here is an example name containing three RDNs, in which the first RDN is multi-valued: OU=Sales+CN=J. Smith,O=Widget Inc.,C=US 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,O=Test,C=GB 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 6. References [1] The Directory -- overview of concepts, models and services. ITU-T Rec. X.500(1993). [2] The Directory -- Models. ITU-T Rec. X.501(1993). [3] Wahl, M., Howes, T., and S. Kille, "Lightweight Directory Access Protocol (v3)", RFC 2251, December 1997. [4] Wahl, M., Coulbeck, A., Howes, T. and S. Kille, "Lightweight Directory Access Protocol (v3): Attribute Syntax Definitions", RFC 2252, December 1997. [5] Crocker, D., "Standard of the Format of ARPA-Internet Text Messages", STD 11, RFC 822, August 1982. [6] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119.
7. Security Considerations 7.1. 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. 7.2. 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. 8. Authors' Addresses Mark Wahl Critical Angle Inc. 4815 W. Braker Lane #502-385 Austin, TX 78759 USA EMail: M.Wahl@critical-angle.com
Steve Kille Isode Ltd. The Dome The Square Richmond, Surrey TW9 1DT England Phone: +44-181-332-9091 EMail: S.Kille@ISODE.COM Tim Howes Netscape Communications Corp. 501 E. Middlefield Rd, MS MV068 Mountain View, CA 94043 USA Phone: +1 650 937-3419 EMail: howes@netscape.com
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