Network Working Group                                          T. Howes
Request for Comments: 2254                Netscape Communications Corp.
Category: Standards Track                                 December 1997

            The String Representation of LDAP Search Filters

1. Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1997).  All Rights Reserved.


   This document describes a directory access protocol that provides
   both read and update access.  Update access requires secure
   authentication, but this document does not mandate implementation of
   any satisfactory authentication mechanisms.

   In accordance with RFC 2026, section 4.4.1, this specification is
   being approved by IESG as a Proposed Standard despite this
   limitation, for the following reasons:

   a. to encourage implementation and interoperability testing of
      these protocols (with or without update access) before they
      are deployed, and

   b. to encourage deployment and use of these protocols in read-only
      applications.  (e.g. applications where LDAPv3 is used as
      a query language for directories which are updated by some
      secure mechanism other than LDAP), and

   c. to avoid delaying the advancement and deployment of other Internet
      standards-track protocols which require the ability to query, but
      not update, LDAPv3 directory servers.

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   Readers are hereby warned that until mandatory authentication
   mechanisms are standardized, clients and servers written according to
   this specification which make use of update functionality are

   Implementors are hereby discouraged from deploying LDAPv3 clients or
   servers which implement the update functionality, until a Proposed
   Standard for mandatory authentication in LDAPv3 has been approved and
   published as an RFC.

2. Abstract

   The Lightweight Directory Access Protocol (LDAP) [1] defines a
   network representation of a search filter transmitted to an LDAP
   server.  Some applications may find it useful to have a common way of
   representing these search filters in a human-readable form.  This
   document defines a human-readable string format for representing LDAP
   search filters.

   This document replaces RFC 1960, extending the string LDAP filter
   definition to include support for LDAP version 3 extended match
   filters, and including support for representing the full range of
   possible LDAP search filters.

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3. LDAP Search Filter Definition

   An LDAPv3 search filter is defined in Section 4.5.1 of [1] as

        Filter ::= CHOICE {
                and                [0] SET OF Filter,
                or                 [1] SET OF Filter,
                not                [2] Filter,
                equalityMatch      [3] AttributeValueAssertion,
                substrings         [4] SubstringFilter,
                greaterOrEqual     [5] AttributeValueAssertion,
                lessOrEqual        [6] AttributeValueAssertion,
                present            [7] AttributeDescription,
                approxMatch        [8] AttributeValueAssertion,
                extensibleMatch    [9] MatchingRuleAssertion

        SubstringFilter ::= SEQUENCE {
                type    AttributeDescription,
                SEQUENCE OF CHOICE {
                        initial        [0] LDAPString,
                        any            [1] LDAPString,
                        final          [2] LDAPString

        AttributeValueAssertion ::= SEQUENCE {
                attributeDesc   AttributeDescription,
                attributeValue  AttributeValue

        MatchingRuleAssertion ::= SEQUENCE {
                matchingRule    [1] MatchingRuleID OPTIONAL,
                type            [2] AttributeDescription OPTIONAL,
                matchValue      [3] AssertionValue,
                dnAttributes    [4] BOOLEAN DEFAULT FALSE

        AttributeDescription ::= LDAPString

        AttributeValue ::= OCTET STRING

        MatchingRuleID ::= LDAPString

        AssertionValue ::= OCTET STRING

        LDAPString ::= OCTET STRING

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   where the LDAPString above is limited to the UTF-8 encoding of the
   ISO 10646 character set [4].  The AttributeDescription is a string
   representation of the attribute description and is defined in [1].
   The AttributeValue and AssertionValue OCTET STRING have the form
   defined in [2].  The Filter is encoded for transmission over a
   network using the Basic Encoding Rules defined in [3], with
   simplifications described in [1].

4. String Search Filter Definition

   The string representation of an LDAP search filter is defined by the
   following grammar, following the ABNF notation defined in [5].  The
   filter format uses a prefix notation.

        filter     = "(" filtercomp ")"
        filtercomp = and / or / not / item
        and        = "&" filterlist
        or         = "|" filterlist
        not        = "!" filter
        filterlist = 1*filter
        item       = simple / present / substring / extensible
        simple     = attr filtertype value
        filtertype = equal / approx / greater / less
        equal      = "="
        approx     = "~="
        greater    = ">="
        less       = "<="
        extensible = attr [":dn"] [":" matchingrule] ":=" value
                     / [":dn"] ":" matchingrule ":=" value
        present    = attr "=*"
        substring  = attr "=" [initial] any [final]
        initial    = value
        any        = "*" *(value "*")
        final      = value
        attr       = AttributeDescription from Section 4.1.5 of [1]
        matchingrule = MatchingRuleId from Section 4.1.9 of [1]
        value      = AttributeValue from Section 4.1.6 of [1]

   The attr, matchingrule, and value constructs are as described in the
   corresponding section of [1] given above.

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   If a value should contain any of the following characters

           Character       ASCII value
           *               0x2a
           (               0x28
           )               0x29
           \               0x5c
           NUL             0x00

   the character must be encoded as the backslash '\' character (ASCII
   0x5c) followed by the two hexadecimal digits representing the ASCII
   value of the encoded character. The case of the two hexadecimal
   digits is not significant.

   This simple escaping mechanism eliminates filter-parsing ambiguities
   and allows any filter that can be represented in LDAP to be
   represented as a NUL-terminated string. Other characters besides the
   ones listed above may be escaped using this mechanism, for example,
   non-printing characters.

   For example, the filter checking whether the "cn" attribute contained
   a value with the character "*" anywhere in it would be represented as

   Note that although both the substring and present productions in the
   grammar above can produce the "attr=*" construct, this construct is
   used only to denote a presence filter.

5. Examples

   This section gives a few examples of search filters written using
   this notation.

        (cn=Babs Jensen)
        (!(cn=Tim Howes))
        (&(objectClass=Person)(|(sn=Jensen)(cn=Babs J*)))

   The following examples illustrate the use of extensible matching.

        (cn: Flintstone)
        (sn:dn: Rubble)
        (o:dn:=Ace Industry)

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   The second example illustrates the use of the ":dn" notation to
   indicate that matching rule "" should be used when making
   comparisons, and that the attributes of an entry's distinguished name
   should be considered part of the entry when evaluating the match.

   The third example denotes an equality match, except that DN
   components should be considered part of the entry when doing the

   The fourth example is a filter that should be applied to any
   attribute supporting the matching rule given (since the attr has been
   left off). Attributes supporting the matching rule contained in the
   DN should also be considered.

   The following examples illustrate the use of the escaping mechanism.

        (o=Parens R Us \28for all your parenthetical needs\29)

   The first example shows the use of the escaping mechanism to
   represent parenthesis characters. The second shows how to represent a
   "*" in a value, preventing it from being interpreted as a substring
   indicator. The third illustrates the escaping of the backslash

   The fourth example shows a filter searching for the four-byte value
   0x00000004, illustrating the use of the escaping mechanism to
   represent arbitrary data, including NUL characters.

   The final example illustrates the use of the escaping mechanism to
   represent various non-ASCII UTF-8 characters.

6. Security Considerations

   This memo describes a string representation of LDAP search filters.
   While the representation itself has no known security implications,
   LDAP search filters do. They are interpreted by LDAP servers to
   select entries from which data is retrieved.  LDAP servers should
   take care to protect the data they maintain from unauthorized access.

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7. References

   [1] Wahl, M., Howes, T., and S. Kille, "Lightweight Directory Access
   Protocol (v3)", RFC 2251, December 1997.

   [2] Wahl, M., Coulbeck, A., Howes, T., and S. Kille, "Lightweight
   Directory Access Protocol (v3): Attribute Syntax Definitions", RFC
   2252, December 1997.

   [3] Specification of ASN.1 encoding rules: Basic, Canonical, and
   Distinguished Encoding Rules, ITU-T Recommendation X.690, 1994.

   [4] Yergeau, F., "UTF-8, a transformation format of Unicode and ISO
   10646", RFC 2044, October 1996.

   [5] Crocker, D., "Standard for the Format of ARPA Internet Text
   Messages", STD 11, RFC 822, August 1982.

8. Author's Address

   Tim Howes
   Netscape Communications Corp.
   501 E. Middlefield Road
   Mountain View, CA 94043

   Phone: +1 415 937-3419

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9. Full Copyright Statement

   Copyright (C) The Internet Society (1997).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than

   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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