INTERNET-DRAFT                          Editor:  Kurt D. Zeilenga
Intended Category: Standard Track                OpenLDAP Foundation
Expires in six months                            1 March 2002
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 Working Group mailing list <>.
  Please send editorial comments directly to the document editor

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  Copyright 2002, The Internet Society.  All Rights Reserved.

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


  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

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  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.

1.  Background

  This specification assumes familiarity with X.500 [X.500], and the
  concept of Distinguished Name (DN).  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).

  This document is an integral part of the LDAP Technical Specification

  This document obsoletes RFC 2253.  Changes since RFC 2253 are
  summarized in Appendix A.

  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
  document are to be interpreted as described in BCP 14 [RFC2119].

2.  Converting DistinguishedName from ASN.1 to a String

  In X.501 [X.501] the ASN.1 structure of distinguished name is defined

      DistinguishedName ::= RDNSequence

      RDNSequence ::= SEQUENCE OF RelativeDistinguishedName

      RelativeDistinguishedName ::= SET SIZE (1..MAX) OF

      AttributeTypeAndValue ::= SEQUENCE {
          type  AttributeType,
          value AttributeValue }

  The following sections define the RECOMMENDED algorithm for converting
  from an ASN.1 structured representation to a UTF-8 [RFC2279] string

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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)

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 the following table of attribute types
  associated with LDAP [Schema], 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
  (numericoid) is described in [Models].

  The type name string is not case sensitive.

      String  X.500 AttributeType
      ------  --------------------------------------------
      CN      commonName (
      L       localityName (
      ST      stateOrProvinceName (
      O       organizationName (
      OU      organizationalUnitName (
      C       countryName (

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      STREET  streetAddress (
      DC      domainComponent (0.9.2342.19200300.100.1.25)
      UID     userId (0.9.2342.19200300.100.1.1)

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 octets of the BER encoding of the X.500
  AttributeValue.  This form is also 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

  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 (" " ASCII 32) or octothorpe ("#" ASCII 35) occurring at
        the beginning of the string

      - a space (" " ASCII 32) character occurring at the end of the

      - one of the characters ",", "+", """, "\", "<", ">" or ";" (ASCII
        44, 43, 34, 92, 60, 62, or 59, respectively)

      - the null (ASCII 0) character

  Implementations MAY escape other characters.

  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 " "
      (ASCII 44, 43, 34, 92, 60, 62, 59, 35, 61 or 32 respectively)

  it can be prefixed by a backslash ("\" ASCII 92).

  Examples of the escaping mechanism are shown in Section 4.

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3. Parsing a String back to a Distinguished Name

  The structure of the UTF-8 [RFC2279] string is specified using the
  following Augmented BNF [RFC2234] grammar.

      distinguishedName = [name]
                          ; may be empty

      name              = name-component *(COMMA name-component)

      name-component    = attributeTypeAndValue *(PLUS attributeTypeAndValue)

                        = attributeType EQUALS attributeValue

      attributeType     = keyword / oid

      keyword           = ALPHA 1*keychar

      keychar           = ALPHA / DIGIT / MINUS

      oid               = number *(DOT number)

      number            = ( LDIGIT *DIGIT ) / DIGIT

      attributeValue    = string / hexstring

      string            = *( stringchar / pair )
                          ; the string MUST NOT start with SHARP or SP
                          ; and MUST NOT end with SP

      stringchar        = <any UTF-8 character (can be multiple octets)
                          except one of escaped or ESC or NULL>

      pair              = ESC ( ESC / special / hexpair )

      special           = escaped / SHARP / EQUALS / SP

      escaped           = COMMA / PLUS / %x22 / %x3C / %x3E / %x3B
                          ; "," / "+" / """ / "<" /  ">" / ";"

      hexstring         = SHARP 1*hexpair

      hexpair           = HEX HEX

      HEX               = DIGIT / %x41-46 / %x61-66
                           ; 0-9 / A-F / a-f

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      ALPHA             = %x41-5A / %x61-7A
                           ; A-Z / a-z

      LDIGIT            = %x31-39
                           ; 1-9

      DIGIT             = %x30 / LDIGIT
                           ; 0-9

      SP                = %x20 ; space (" ")
      SHARP             = %x23 ; octothorpe (or sharp sign) ("#")
      PLUS              = %x2B ; plus sign ("+")
      COMMA             = %x2C ; comma (",")
      MINUS             = %x2D ; minus sign ("-")
      DOT               = %x2E ; period (".")
      EQUALS            = %x3D ; equals sign ("=")
      ESC               = %x5C ; backslash ("\")
      NULL              = %x00 ; null (0)

  Implementations MUST recognize AttributeType string type names
  (keywords) listed in the Section 2.3 table, but MAY recognize other
  names.  Implementations MAY recognize other DN string representations
  (such as that described in RFC 1779). As there is no requirement for
  other names or alternative DN string representations to be recognized,
  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 name:

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

  An example name in which a value contains a carriage return character:

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  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.,DC=example,DC=com

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

      Unicode Letter Description      10646 code UTF-8  Quoted
      =============================== ========== ====== =======
      LATIN CAPITAL LETTER L          U+0000004C 0x4C   L
      LATIN SMALL LETTER U            U+00000075 0x75   u
      LATIN SMALL LETTER I            U+00000069 0x69   i
      LATIN SMALL LETTER C WITH ACUTE U+00000107 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.

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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

  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 2.3.

5.3. Use of Other Names

  Attribute type names are not unique.  A string representation
  generated with names other than those in the Section 2.3 table is
  ambiguous.  That is, two applications may recognize the string as
  representing two different DNs possibly associated with two different
  entries.  This may lead to a wide range of unexpected behaviors which
  can have both direct and indirect impacts upon security.

  For example, a distinguished name consisting of one RDN with one AVA
  of the known locally attribute type FOO and the value "BAR" (an
  octetString) could be represented in LDAP as the string FOO=BAR.  As
  the name FOO does not uniquely identify an attribute type, the DN
  FOO=BAR is ambiguous.  That is, FOO could be recognized as the
  attribute type 1.1.1 by one application and in another and not
  recognized by another.  This may lead to operations not behaving as

  Applications desiring to generate an unambiguous string representation
  of a DN SHOULD generate string representation per section 2, not use
  names other than those in the Section 2.3 table, and while taking 5.2
  into consideration.

6.  Acknowledgment

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  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).

  [RFC2119]    Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", RFC 2119.

  [RFC2234]    Crocker, D., and P. Overell, "Augmented BNF for Syntax
               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

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

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

  [Syntaxes]   K. Dally (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

9. Informative References

  [X.500]      "The Directory -- overview of concepts, models and

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               services,"  ITU-T Rec. X.500(1993).

Appendix A. 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 is addressed by RFC 2829.
    - Replaced specification of additional requirements for LDAPv2
      implementations which also support LDAPv3 (Section 4) with a
      statement (in Section 3) allowing recognition of alternative
      string representations.
    - Updated 2.3 to clarify which table is the published table of names
      which may be appear in DNs.  Remove "as an example" language.
      Added statement (in Section 3) allowing recognition of additional
      names.  Added security consideration (Section 5.3) regarding the
      use of other names.
    - Updated 2.3 to indicate attribute type name strings are not case
    - Updated 2.4 to allow hex pair escaping of all characters and
      clarified escaping for when multiple octet UTF-8 characters are
    - Rewrote Section 3 to use ABNF as defined in RFC 2234.
    - Rewrote Section 3 ABNF to be consistent with 2.4.
    - Rewrote examples.
    - Added reference to documentations containing LDAP-specific
      security considerations.

  In addition, numerous editorial changes were made.

Copyright 2002, The Internet Society.  All Rights Reserved.

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  or as required to translate it into languages other than English.

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  The limited permissions granted above are perpetual and will not be
  revoked by the Internet Society or its successors or assigns.

  This document and the information contained herein is provided on an

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