INTERNET-DRAFT                           Editor: Kurt D. Zeilenga
Intended Category: Standard Track                OpenLDAP Foundation
Expires in six months                            4 May                            30 June 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) (LDAPBIS) Working Group mailing list
  <>.  Please send editorial comments directly
  to the document editor <>.

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  Copyright 2003, (C) The Internet Society. Society (2003).  All Rights Reserved.

  Please see the Full Copyright section near the end of this document
  for more information.


  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.


  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
  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
  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 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). 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 an integral part of the LDAP Technical Specification

  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 ::= 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
  [UTF-8] 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
  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)

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] [REGISTRY][BCP64bis] as identifying the
  AttributeType, that short name, a <descr>, is used.  Otherwise the
  AttributeType is encoded as the dotted-decimal encoding, a
  <numericoid>, of its OBJECT IDENTIFIER.  The <descr> and <numericoid>
  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

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.

      - 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

      - one of the characters """, "+", ",", ";", "<", ">",  or "\"
        (U+0022, U+002B, U+002C, U+003B, U+003C, U+003E, or U+005C

      - 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] [UTF-8] 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
      ; 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 <descr>, <numericoid>, <COMMA>, <DQUOTE>,
  <SPACE>, <SHARP>, <UTFMB> are defined in [Models].

  Each <attributeType>, either a <descr> or a <numericoid>, refers to an
  attribute type of an attribute value assertion (AVA).  The
  <attributeType> is followed by a <EQUALS> and an <attributeValue>.
  The <attributeValue> is either in <string> or <hexstring> form.

  If in <string> form, a LDAP string representation asserted value can
  be obtained by replacing (left-to-right, non-recursively) each <pair>
  appearing in the <string> as follows:
      replace <ESC><ESC> with <ESC>;
      replace <ESC><special> with <special>;
      replace <ESC><hexpair> with the octet indicated by the <hexpair>.

  If in <hexstring> form, a BER representation can be obtained from
  converting each <hexpair> of the <hexstring> to the octet indicated by
  the <hexpair>.

  One or more attribute values assertions, separated by <PLUS>, for a
  relative distinguished name.

  Zero or more relative distinguished names, separated by <COMMA>, 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 (
      L       localityName (
      ST      stateOrProvinceName (
      O       organizationName (
      OU      organizationalUnitName (
      C       countryName (
      STREET  streetAddress (
      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):


      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

          CN=John Smith\, III,DC=example,DC=net

      An example name in which a value contains a carriage return


      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.


      Finally, an example of an RDN commonName value consisting of 5

      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):


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.

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]       International Telecommunication Union -
                Telecommunication Standardization Sector, "The Directory
                -- Models," ITU-T Rec. X.501(1993). X.501(1993) (also ISO/IEC 9594-2:1994).

  [X.680]      ITU-T,       International Telecommunication Union -
                Telecommunication Standardization Sector, "Abstract
                Syntax Notation One (ASN.1) - Specification of Basic
                Notation", X.680, 1994. 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). 2119), March 1997.

  [RFC2234]     Crocker, D., D. and P. Overell, "Augmented BNF for Syntax
                Specifications: ABNF", RFC 2234, November 1997.


                [UTF-8]       Yergeau, F., "UTF-8, a transformation
                format of ISO 10646", RFC 2279, January 1998. draft-yergeau-rfc2279bis, a work
                in progress.

  [Models]      Zeilenga, K. Zeilenga (editor), "LDAP: Directory Information
                Models", draft-ietf-ldapbis-models-xx.txt, a work in

  [Roadmap]    K.     Zeilenga, K. (editor), "LDAP: Technical Specification
                Road Map", draft-ietf-ldapbis-roadmap-xx.txt, a work in

  [Protocol]    Sermersheim, J. Sermersheim (editor), "LDAP: The Protocol",
                draft-ietf-ldapbis-protocol-xx.txt, a work in progress.

  [Syntaxes]    Legg, S. Legg (editor), "LDAP: Syntaxes", Syntaxes and Matching Rules",
                draft-ietf-ldapbis-syntaxes-xx.txt, a work in progress.

  [Schema]      Dally, K. Dally (editor), "LDAP: User Schema",
                draft-ietf-ldapbis-user-schema-xx.txt, a work in

  [ISO10646]   Universal    International Organization for Standardization,
                "Universal Multiple-Octet Coded Character Set (UCS) -
                Architecture and Basic Multilingual Plane, Plane", ISO/IEC
                10646-1 : 1993.

                [REGISTRY]    IANA, Object Identifier Descriptors
                Registry, <>.

9. Informative References

  [X.500]       International Telecommunication Union -
                Telecommunication Standardization Sector, "The Directory
                -- overview Overview of concepts, models and services,"  ITU-T Rec. X.500(1993).
                X.500(1993) (also ISO/IEC 9594-1:1994).

  [X.690]      ITU-T,       International Telecommunication Union -
                Telecommunication Standardization Sector, "Specification
                of ASN.1 encoding rules:  Basic,
               Canonical, Basic Encoding Rules (BER),
                Canonical Encoding Rules (CER), and Distinguished
                Encoding Rules", X.690,

  [RFC3383]    K. Zeilenga, "IANA Considerations for LDAP", BCP 64 Rules (DER)", X.690(1997) (also
               RFC 3383), September 2002. ISO/IEC

  [RFC2849]    G.     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.

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 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, <CN=Sam\ > 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.

  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), 1) that this document does not define a
      canonical string representation.
    - 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.
    - 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
    - Rewrote examples.
    - Added reference to documentations containing general LDAP security
    - Added discussion of presentation issues (Appendix A).
    - Added this appendix.

  In addition, numerous editorial changes were made.

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