INTERNET-DRAFT Editor: Kurt D. Zeilenga Intended Category: Standard Track OpenLDAP Foundation Expires in six months 10 February 2005 Obsoletes: RFC 2253 LDAP: String Representation of Distinguished Names Status of Memo 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 . By submitting this Internet-Draft, I accept the provisions of Section 4 of RFC 3667. By submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed, or will be disclosed, and any of which I become aware will be disclosed, in accordance with RFC 3668. 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 http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Copyright (C) The Internet Society (2005). All Rights Reserved. Please see the Full Copyright section near the end of this document for more information. Zeilenga LDAP: Distinguished Names [Page 1] INTERNET-DRAFT draft-ietf-ldapbis-dn-16.txt 10 February 2005 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 names, while being able to represent any distinguished name. 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 the string form described in this document. 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 a integral part of the LDAP technical specification [Roadmap] which obsoletes the previously defined LDAP technical Zeilenga LDAP: Distinguished Names [Page 2] INTERNET-DRAFT draft-ietf-ldapbis-dn-16.txt 10 February 2005 specification, RFC 3377, in its entirety. 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]. 1.1. 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]. Character names in this document use the notation for code points and names from the Unicode Standard [Unicode]. For example, the letter "a" may be represented as either or . Note: a glossary of terms used in Unicode can be found in [Glossary]. Information on the Unicode character encoding model can be found in [CharModel]. 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 [RFC3629] encoded Unicode [Unicode] 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 SHALL be produced by LDAP implementations. 2.1. Converting the RDNSequence Zeilenga LDAP: Distinguished Names [Page 3] INTERNET-DRAFT draft-ietf-ldapbis-dn-16.txt 10 February 2005 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 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 sign ('=' 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 (descriptor) [Models] and that short name is known to be registered [REGISTRY][BCP64bis] 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 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 Zeilenga LDAP: Distinguished Names [Page 4] INTERNET-DRAFT draft-ietf-ldapbis-dn-16.txt 10 February 2005 the X.500 AttributeValue. This form is also used when the syntax of the AttributeValue does not have a LDAP-specific [Syntaxes, Section 3.1] string encoding defined for it or the LDAP-specific string encoding is not restricted to UTF-8 encoded Unicode 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 LDAP-specific string encoding, the value is converted first to a UTF-8 encoded Unicode string according to its syntax specification (see [Syntaxes, Section 3.3] for examples). If that UTF-8 encoded Unicode 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 (' ' 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+005C). 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 [RFC3629] encoded Unicode [Unicode] characters. The structure of this string representation is specified using the following Zeilenga LDAP: Distinguished Names [Page 5] INTERNET-DRAFT draft-ietf-ldapbis-dn-16.txt 10 February 2005 Augmented BNF [RFC2234] grammar: distinguishedName = [ relativeDistinguishedName *( COMMA relativeDistinguishedName ) ] relativeDistinguishedName = attributeTypeAndValue *( PLUS attributeTypeAndValue ) attributeTypeAndValue = attributeType EQUALS attributeValue attributeType = descr / numericoid attributeValue = string / hexstring ; The following characters are to be escaped when they appear ; in the value to be encoded: ESC, one of , leading ; SHARP or SPACE, trailing SPACE, and NULL. 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 ; Zeilenga LDAP: Distinguished Names [Page 6] INTERNET-DRAFT draft-ietf-ldapbis-dn-16.txt 10 February 2005 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 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 Zeilenga LDAP: Distinguished Names [Page 7] INTERNET-DRAFT draft-ietf-ldapbis-dn-16.txt 10 February 2005 This example shows the method of escaping of a special characters appearing in a common name: CN=James \"Jim\" Smith\, III,DC=example,DC=net The following shows the method for encoding a value which contains a carriage return character: CN=Before\0dAfter,DC=example,DC=net In this RDN example, the type in the RDN is unrecognized, and 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 Finally, this example shows an RDN whose commonName value consisting of 5 letters: Unicode Character 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 encoded in printable ASCII (useful for debugging purposes) as: 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: Zeilenga LDAP: Distinguished Names [Page 8] INTERNET-DRAFT draft-ietf-ldapbis-dn-16.txt 10 February 2005 - 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) In some cases, such information can be considered sensitive. In many countries, privacy laws exist which prohibit disclosure of certain kinds of descriptive information (e.g., email addresses). Hence, servers implementors are encouraged to support DIT structural rules and name forms [Models] as these provide a mechanism for administrators to select appropriate naming attributes for entries. Administrators are encouraged to use these mechanisms, access controls, and other administrative controls which may be available to restrict use of attributes containing sensitive information in naming of entries. Additionally, use of authentication and data security services in LDAP [AuthMeth][Protocol] should be considered. 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 . Another distinguished name in which the value is still 'Sam' but of the PrintableString choice would have the same representation . 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 ('#' U+0023) as described in the first paragraph of Section 2.4. 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. Zeilenga LDAP: Distinguished Names [Page 9] INTERNET-DRAFT draft-ietf-ldapbis-dn-16.txt 10 February 2005 7. Document Editor's Address Kurt D. Zeilenga OpenLDAP Foundation Email: Kurt@OpenLDAP.org 8. References [[Note to the RFC Editor: please replace the citation tags used in referencing Internet-Drafts with tags of the form RFCnnnn where possible.]] 8.1. 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). [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. [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC 3629 (also STD 63), November 2003. [Unicode] The Unicode Consortium, "The Unicode Standard, Version 3.2.0" is defined by "The Unicode Standard, Version 3.0" (Reading, MA, Addison-Wesley, 2000. ISBN 0-201-61633-5), as amended by the "Unicode Standard Annex #27: Unicode 3.1" (http://www.unicode.org/reports/tr27/) and by the "Unicode Standard Annex #28: Unicode 3.2" (http://www.unicode.org/reports/tr28/). [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 Zeilenga LDAP: Distinguished Names [Page 10] INTERNET-DRAFT draft-ietf-ldapbis-dn-16.txt 10 February 2005 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. [REGISTRY] IANA, Object Identifier Descriptors Registry, . 8.2. Informative References [ASCII] Coded Character Set--7-bit American Standard Code for Information Interchange, ANSI X3.4-1986. [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 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. [CharModel] Whistler, K. and M. Davis, "Unicode Technical Report #17, Character Encoding Model", UTR17, , August 2000. [Glossary] The Unicode Consortium, "Unicode Glossary", . Zeilenga LDAP: Distinguished Names [Page 11] INTERNET-DRAFT draft-ietf-ldapbis-dn-16.txt 10 February 2005 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 Unicode characters. Some Unicode 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: Zeilenga LDAP: Distinguished Names [Page 12] INTERNET-DRAFT draft-ietf-ldapbis-dn-16.txt 10 February 2005 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. - Replaced all references to ISO 10646-1 with [Unicode]. - Clarified (in Section 1) that this document does not define a canonical string representation. - Clarified that Section 2 describes the RECOMMENDED encoding algorithm and that alternative algorithms are allowed. Some encoding options described in RFC 2253 are now treated as alternative algorithms in this specification. - 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. - Removed specification of additional requirements for LDAPv2 implementations which also support LDAPv3 (RFC 2253, Section 4) as LDAPv2 is now Historic. - Allow recognition of alternative string representations. - Updated Section 2.4 to allow hex pair escaping of all characters and clarified escaping for when multiple octet UTF-8 encodings are present. Indicated that null (U+0000) character is to be escaped. Zeilenga LDAP: Distinguished Names [Page 13] INTERNET-DRAFT draft-ietf-ldapbis-dn-16.txt 10 February 2005 Indicated that equals sign ('=' U+003D) character may be escaped as '\='. - Rewrote Section 3 to use ABNF as defined in RFC 2234. - Updated the Section 3 ABNF. Changes include: + allow AttributeType short names of length 1 (e.g., 'L'), + use more restrictive production in AttributeTypes, + do not require escaping of equals sign ('=' U+003D) characters, + do not require escaping of non-leading number sign ('#' U+0023) characters, + allow space (' ' U+0020) to escaped as '\ ', + require hex escaping of null (U+0000) characters, and + removed LDAPv2-only constructs. - 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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The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Zeilenga LDAP: Distinguished Names [Page 14] INTERNET-DRAFT draft-ietf-ldapbis-dn-16.txt 10 February 2005 Full Copyright Copyright (C) The Internet Society (2005). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. 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