INTERNET-DRAFT Editor: Kurt D. Zeilenga Intended Category: Standard Track OpenLDAP Foundation Expires in six months 4 May 2003 Obsoletes: 2253 LDAP: String Representation of Distinguished Names 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 . Please send editorial comments directly to the document editor . Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as ``work in progress.'' The list of current Internet-Drafts can be accessed at . The list of Internet-Draft Shadow Directories can be accessed at . Copyright 2003, 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 Zeilenga LDAP: Distinguished Names [Page 1] INTERNET-DRAFT draft-ietf-ldapbis-dn-10.txt 4 May 2003 designed to give a clean representation of commonly used distinguished 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 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 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]. Zeilenga LDAP: Distinguished Names [Page 2] INTERNET-DRAFT draft-ietf-ldapbis-dn-10.txt 4 May 2003 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 [RFC2279] 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 Zeilenga LDAP: Distinguished Names [Page 3] INTERNET-DRAFT draft-ietf-ldapbis-dn-10.txt 4 May 2003 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] as identifying the AttributeType, that short name, a , is used. Otherwise the AttributeType is encoded as the dotted-decimal encoding, a , of its OBJECT IDENTIFIER. The and is defined in [Models]. Implementations are not expected 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. Zeilenga LDAP: Distinguished Names [Page 4] INTERNET-DRAFT draft-ietf-ldapbis-dn-10.txt 4 May 2003 - 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 [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: 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 Zeilenga LDAP: Distinguished Names [Page 5] INTERNET-DRAFT draft-ietf-ldapbis-dn-10.txt 4 May 2003 ; 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 , , , , , , , , , , , , , , are defined in [Models]. Each , either a or a , refers to an attribute type of an attribute value assertion (AVA). The is followed by a and an . The is either in or form. If in form, a LDAP string representation asserted value can be obtained by replacing (left-to-right, non-recursively) each appearing in the as follows: replace with ; replace with ; replace with the octet indicated by the . If in form, a BER representation can be obtained from converting each of the to the octet indicated by the . One or more attribute values assertions, separated by , for a Zeilenga LDAP: Distinguished Names [Page 6] INTERNET-DRAFT draft-ietf-ldapbis-dn-10.txt 4 May 2003 relative distinguished name. Zero or more relative distinguished names, separated by , 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 Zeilenga LDAP: Distinguished Names [Page 7] INTERNET-DRAFT draft-ietf-ldapbis-dn-10.txt 4 May 2003 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. Zeilenga LDAP: Distinguished Names [Page 8] INTERNET-DRAFT draft-ietf-ldapbis-dn-10.txt 4 May 2003 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 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 Zeilenga LDAP: Distinguished Names [Page 9] INTERNET-DRAFT draft-ietf-ldapbis-dn-10.txt 4 May 2003 Specifications: ABNF", RFC 2234, November 1997. [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. [RFC3383] K. Zeilenga, "IANA Considerations for LDAP", BCP 64 (also RFC 3383), September 2002. [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. Zeilenga LDAP: Distinguished Names [Page 10] INTERNET-DRAFT draft-ietf-ldapbis-dn-10.txt 4 May 2003 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, 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. 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. Zeilenga LDAP: Distinguished Names [Page 11] INTERNET-DRAFT draft-ietf-ldapbis-dn-10.txt 4 May 2003 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. - 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. - 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. Zeilenga LDAP: Distinguished Names [Page 12] INTERNET-DRAFT draft-ietf-ldapbis-dn-10.txt 4 May 2003 In addition, numerous editorial changes were made. Copyright 2003, The Internet Society. All Rights Reserved. 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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 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Zeilenga LDAP: Distinguished Names [Page 13]