INTERNET-DRAFT                           Editor: Kurt D. Zeilenga
Intended Category: Standard Track                OpenLDAP Foundation
Expires in six months                            1 March                            18 August 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 (LDAPbis) Working Group mailing list
  <>.  Please send editorial comments directly
  to the document editor <>.

  Internet-Drafts are working documents of the Internet Engineering Task
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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 (DNs) as the primary keys to
  entries in the directory.  Distinguished Names are encoded in ASN.1 in  This document defines the X.500 Directory protocols.  In string
  representation used in the Lightweight Directory Access
  Protocol, a string representation of Protocol
  (LDAP) to transfer distinguished names is
  transferred.  This specification defines the names.  The string format for
  representing names, which representation 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 Intended Usage

  In X.500-based directory systems [X.500], including those accessed
  using the
  concept Lightweight Directory Access Protocol (LDAP) [LDAPTS],
  distinguished names (DNs) are used to unambiguously refer to a
  directory entry [X.501][Models].

  The structure of Distinguished Name (DN). 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 clients
  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 how to convert a
  DN from ASN.1 structured representation to a string.  Section 3
  details how to convert a DN from string to ASN.1 structured

  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 A. B.

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

  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

  In X.501 [X.501] the ASN.1 [X.680] structure of distinguished name is
  defined as:

      DistinguishedName ::= RDNSequence

      RDNSequence ::= SEQUENCE OF RelativeDistinguishedName

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

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

  The following sections define

  This section defines the RECOMMENDED algorithm for converting a
  distinguished name from an ASN.1 structured representation to a an UTF-8
  [RFC2279] encoded Universal Character Set (UCS) [ISO10646] character
  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 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 character ("," ASCII 44). U+0002C).

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 ("+" ASCII 43) U+0002B)

2.3.  Converting AttributeTypeAndValue
  The AttributeTypeAndValue is encoded as the string representation of
  the AttributeType, followed by an equals character ("=" ASCII 61), U+0003D),
  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 string, a <descr>,
  from that table is used, otherwise it is encoded as the dotted-decimal encoding
  encoding, a <numericoid>, of the AttributeType's OBJECT IDENTIFIER.
  The dotted-decimal notation
  (numericoid) <descr> and <numericoid> is described defined 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 (
      STREET  streetAddress (
      DC      domainComponent (0.9.2342.19200300.100.1.25)
      UID     userId (0.9.2342.19200300.100.1.1)

      Note: This table lists the complete set of type name strings which
      all implementations MUST recognize in DN string representation.
      As no extension could reasonable require all existing
      implementations be updated to recognize additional type name
      strings, this table is not extensible.

2.4.  Converting an AttributeValue from ASN.1 to a String

  If the AttributeValue AttributeType is of a type which does not have a string
  representation defined for it, then it the dotted-decimal form, the AttributeValue
  is simply encoded as represented by an
  octothorpe number sign character ("#" ASCII 35) U+00023) followed by
  the hexadecimal
  representation encoding of each of the octets of the BER encoding of
  the X.500 AttributeValue.  This form is also be used if when the AttributeType is syntax of
  the dotted-decimal form. 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 type syntax which has a native
  representation, 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 the 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 (" " ASCII 32) U+00020) or octothorpe number sign ("#" ASCII 35) U+00023) occurring at
        the beginning of the string string;

      - a space (" " ASCII 32) U+00020) character occurring at the end of the

      - one of the characters ",", "+", """, "\", "+", ",", ";", "<", ">" ">",  or ";" (ASCII
        44, 43, 34, 92, 60, 62, "\"
        (U+00022, U+0002B, U+0002C, U+0003B, U+0003C, U+0003E, or 59, respectively)
        U+0005C respectively);

      - the null (ASCII 0) character

  Implementations MAY escape other characters. (U+00000) 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 " "
      (ASCII 44, 43, 34, 92, 60, 62, 59, 35, 61 or 32 "\"
      (U+00020, U+00022, U+00023, U+0002B, U+0002C, U+0003B,
       U+0003C, U+0003D, U+0003E, U+0005C respectively)

  it can be prefixed by a backslash ("\" ASCII 92). U+00005C).

  Examples of the escaping mechanism are shown in Section 4.

3. Parsing a String back to a Distinguished Name

  The structure string representation of the 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. grammar using
  the common productions defined in [Models].

      distinguishedName = [name]
                          ; may be empty

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

      name-component [ relativeDistinguishedName
          *( COMMA relativeDistinguishedName ) ]

      relativeDistinguishedName = attributeTypeAndValue *(PLUS attributeTypeAndValue)
          *( PLUS attributeTypeAndValue )

      attributeTypeAndValue = attributeType EQUALS attributeValue
      attributeType = keyword / oid

      keyword           = ALPHA 1*keychar

      keychar           = ALPHA descr / DIGIT / MINUS

      oid               = number *(DOT number)

      number            = ( LDIGIT *DIGIT ) / DIGIT numericoid

      attributeValue = string / hexstring

      string            = *( stringchar / pair )

      ; the The UTF-8 string MUST NOT shall not contain NULL, ESC, or
      ; one of escaped, shall not start with SHARP or SP SPACE,
      ; and MUST NOT shall must not end with SP

      stringchar SPACE.
      string     = <any UTF-8 character (can be multiple octets)
                          except one of escaped or ESC or NULL> [ (leadchar / pair)
                     [ *( stringchar / pair              = ESC ) ( ESC / special trailchar / hexpair pair )

      special ] ]

      leadchar   = escaped LUTF1 / SHARP UTFMB
      LUTF1      = %x01-1F / EQUALS %x21 / SP

      escaped           = COMMA %x24-2A / PLUS %x2D-3A / %x22
                   %x3D / %x3C %x3F-5B / %x3E %x5D-7F

      trailchar  = TUTF1 / %x3B
                          ; "," UTFMB
      TUTF1      = %x01-1F / "+" %x21 / """ %x23-2A / "<" %x2D-3A /  ">"
                   %x3D / ";"

      hexstring         = SHARP 1*hexpair

      hexpair %x3F-5B / %x5D-7F

      stringchar = HEX HEX

      SUTF1      = DIGIT %x01-21 / %x41-46 %x23-2A / %x61-66
                           ; 0-9 %x2D-3A /
                   %x3D / A-F %x3F-5B / a-f
      ALPHA %x5D-7F

      pair       = %x41-5A ESC ( ESC / %x61-7A
                           ; A-Z special / a-z

      LDIGIT            = %x31-39
                           ; 1-9

      DIGIT hexpair )

      special    = %x30 escaped / SPACE / LDIGIT
                           ; 0-9

      SP                = %x20 ; space (" ") SHARP / EQUALS

      escaped    = %x23 ; octothorpe (or sharp sign) ("#") DQUOTE / PLUS              = %x2B ; plus sign ("+") / COMMA / SEMI / LANGLE / RANGLE

      hexstring  = %x2C ; comma (",")
      MINUS             = %x2D ; minus sign ("-")
      DOT SHARP 1*hexpair

      hexpair    = %x2E ; period (".")
      EQUALS            = %x3D ; equals sign ("=")
      ESC               = %x5C ; backslash ("\")
      NULL              = %x00 ; null (0) HEX HEX

  where the productions <descr>, <numericoid>, <COMMA>, <DQUOTE>,
  <SPACE>, <SHARP>, <UTFMB> are defined in [Models].

  Implementations MUST recognize AttributeType string type names
  (keywords) name strings
  (descriptors) listed in the Section 2.3 table, but MAY recognize other
  name strings.  Implementations MAY recognize other DN string
  representations (such as that described in RFC 1779). As  However, as
  there is no requirement for other names or alternative DN string
  representations to be recognized, recognized (and, if so, how), implementations
  SHOULD only generate DN strings in accordance with Section 2 of this

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:


  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       UCS code   UTF-8   Quoted
      =============================== ========== ====== =======
      -------------------------------  --------   ------  --------
      LATIN CAPITAL LETTER L          U+0000004C           U+0004C    0x4C    L
      LATIN SMALL LETTER U            U+00000075             U+00075    0x75    u
      LATIN SMALL LETTER C WITH CARON U+0000010D  U+0010D    0xC48D  \C4\8D
      LATIN SMALL LETTER I            U+00000069             U+00069    0x69    i
      LATIN SMALL LETTER C WITH ACUTE U+00000107  U+00107    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 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 number sign ('#')
  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
  Section 5.2 into consideration.

  It is noted that while a registry for attribute type names
  (descriptors) has been established [LDAPIANA], this registry does not
  remove the ambiguity of attribute types names used in LDAP.  It only
  removes the ambiguity of attribute type names used in Standard Track
  technical specifications.

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. 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), 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]   K. Dally   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

  [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,

  [LDAPIANA]   K. Zeilenga, "IANA Considerations for LDAP",
               draft-ietf-ldapbis-xx.txt (a work in progress).

  [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.

  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 document.  Transcoding issues are also not discussed
  in this document.

  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 human.  For example, an application only
  capable of displaying printable characters 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 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 "<" and ">" 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 in such presentations.
  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

      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 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 what white space is part of the DN string and what
  white space was added for readability.

  Another alternative is to use the LDAP Data Interchange Format (LDIF)
  [RFC2849].  For example,

      The following 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

  It is noted that that is often desirable to replace dotted-decimal
  OIDs appearing in DN strings with attribute type names.  Such
  replacement is viewed as a translation and, hence, not discussed here.

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 is addressed by RFC 2829. 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 (Section (RFC 2253, Section 4)
      with a statement (in Section 3) allowing recognition of
      alternative string representations.
    - Updated 2.3 to clarify which table is Clarified (in Section 2.3) that the published "published" table of names
      which may be appear in DNs. 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 consideration considerations
      (Section 5.3) regarding the use of other names.
    - Updated Section 2.3 to indicate attribute type name strings are not
      sensitive. 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.
    - Rewrote examples.
    - Added reference to documentations containing LDAP-specific general LDAP security
    - Added discussion of presentation issues (Appendix A).

    - Added this appendix.

  In addition, numerous editorial changes were made.

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