    Network Working Group                                        K. Zeilenga
    Request for Comments: 4526                           OpenLDAP Foundation
    Category: Standards Track                                      June 2006

                  Lightweight Directory Access Protocol (LDAP)
                        Absolute True and False Filters

    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 (2006).

    Abstract

       This document extends the Lightweight Directory Access Protocol
       (LDAP) to support absolute True and False filters based upon similar
       capabilities found in X.500 directory systems.  The document also
       extends the String Representation of LDAP Search Filters to support
       these filters.

    Table of Contents

       1. Background ......................................................1
       2. Absolute True and False Filters .................................2
       3. Security Considerations .........................................2
       4. IANA Considerations .............................................3
       5. References ......................................................3
          5.1. Normative References .......................................3
          5.2. Informative References .....................................3

    1.  Background

       The X.500 Directory Access Protocol (DAP) [X.511] supports absolute
       True and False assertions.  An 'and' filter with zero elements always
       evaluates to True.  An 'or' filter with zero elements always
       evaluates to False.  These filters are commonly used when requesting
       DSA-specific Entries (DSEs) that do not necessarily have
       'objectClass' attributes; that is, where "(objectClass=*)" may
       evaluate to False.

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       Although LDAPv2 [RFC1777][RFC3494] placed no restriction on the
       number of elements in 'and' and 'or' filter sets, the LDAPv2 string
       representation [RFC1960][RFC3494] could not represent empty 'and' and
       'or' filter sets.  Due to this, absolute True or False filters were
       (unfortunately) eliminated from LDAPv3 [RFC4510].

       This documents extends LDAPv3 to support absolute True and False
       assertions by allowing empty 'and' and 'or' in Search filters
       [RFC4511] and extends the filter string representation [RFC4515] to
       allow empty filter lists.

       It is noted that certain search operations, such as those used to
       retrieve subschema information [RFC4512], require use of particular
       filters.  This document does not change these requirements.

       This feature is intended to allow a more direct mapping between DAP
       and LDAP (as needed to implement DAP-to-LDAP gateways).

       In this document, the key words "MUST", "MUST NOT", "REQUIRED",
       "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
       and "OPTIONAL" are to be interpreted as described in BCP 14
       [RFC2119].

    2.  Absolute True and False Filters

       Implementations of this extension SHALL allow 'and' and 'or' choices
       with zero filter elements.

       An 'and' filter consisting of an empty set of filters SHALL evaluate
       to True.  This filter is represented by the string "(&)".

       An 'or' filter consisting of an empty set of filters SHALL evaluate
       to False.  This filter is represented by the string "(|)".

       Servers supporting this feature SHOULD publish the Object Identifier
       1.3.6.1.4.1.4203.1.5.3 as a value of the 'supportedFeatures'
       [RFC4512] attribute in the root DSE.

       Clients supporting this feature SHOULD NOT use the feature unless
       they know that the server supports it.

    3.  Security Considerations

       The (re)introduction of absolute True and False filters is not
       believed to raise any new security considerations.

       Implementors of this (or any) LDAPv3 extension should be familiar
       with general LDAPv3 security considerations [RFC4510].

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    RFC 4526          LDAP Absolute True and False Filters         June 2006

    4.  IANA Considerations

       Registration of this feature has been completed by the IANA
       [RFC4520].

       Subject: Request for LDAP Protocol Mechanism Registration Object
       Identifier: 1.3.6.1.4.1.4203.1.5.3 Description: True/False filters
       Person & email address to contact for further information:
            Kurt Zeilenga <kurt@openldap.org> Usage: Feature Specification:
       RFC 4526 Author/Change Controller: IESG Comments: none

       This OID was assigned [ASSIGN] by OpenLDAP Foundation, under its
       IANA-assigned private enterprise allocation [PRIVATE], for use in
       this specification.

    5.  References

    5.1.  Normative References

       [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
                     Requirement Levels", BCP 14, RFC 2119, March 1997.

       [RFC4510]     Zeilenga, K., Ed, "Lightweight Directory Access
                     Protocol (LDAP): Technical Specification Road Map", RFC
                     4510, June 2006.

       [RFC4511]     Sermersheim, J., Ed., "Lightweight Directory Access
                     Protocol (LDAP): The Protocol", RFC 4511, June 2006.

       [RFC4512]     Zeilenga, K., "Lightweight Directory Access Protocol
                     (LDAP): Directory Information Models", RFC 4512, June
                     2006.

       [RFC4515]     Smith, M., Ed. and T. Howes, "Lightweight Directory
                     Access Protocol (LDAP): String Representation of Search
                     Filters", RFC 4515, June 2006.

    5.2.  Informative References

       [RFC1777]     Yeong, W., Howes, T., and S. Kille, "Lightweight
                     Directory Access Protocol", RFC 1777, March 1995.

       [RFC1960]     Howes, T., "A String Representation of LDAP Search
                     Filters", RFC 1960, June 1996.

       [RFC3494]     Zeilenga, K., "Lightweight Directory Access Protocol
                     version 2 (LDAPv2) to Historic Status", RFC 3494, March
                     2003.

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    RFC 4526          LDAP Absolute True and False Filters         June 2006

       [RFC4520]     Zeilenga, K., "Internet Assigned Numbers Authority
                     (IANA) Considerations for the Lightweight Directory
                     Access Protocol (LDAP)", BCP 64, RFC 4520, June 2006.

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

       [X.511]       International Telecommunication Union -
                     Telecommunication Standardization Sector, "The
                     Directory: Abstract Service Definition", X.511(1993)
                     (also ISO/IEC 9594-3:1993).

       [ASSIGN]      OpenLDAP Foundation, "OpenLDAP OID Delegations",
                     http://www.openldap.org/foundation/oid-delegate.txt.

       [PRIVATE]     IANA, "Private Enterprise Numbers",
                     http://www.iana.org/assignments/enterprise-numbers.

    Author's Address

       Kurt D. Zeilenga
       OpenLDAP Foundation

       EMail: Kurt@OpenLDAP.org

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    RFC 4526          LDAP Absolute True and False Filters         June 2006

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       Copyright (C) The Internet Society (2006).

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