Network Working Group M. Smith, Ed.

Network Working Group M. Smith, Ed.
Request for Comments: 4515 Pearl Crescent, LLC
Obsoletes: 2254 T. Howes
Category: Standards Track Opsware, Inc.
June 2006

Lightweight Directory Access Protocol (LDAP):
String Representation of Search 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

Lightweight Directory Access Protocol (LDAP) search filters are
transmitted in the LDAP protocol using a binary representation that
is appropriate for use on the network. This document defines a
human-readable string representation of LDAP search filters that is
appropriate for use in LDAP URLs (RFC 4516) and in other
applications.

Table of Contents

1. Introduction ....................................................2
2. LDAP Search Filter Definition ...................................2
3. String Search Filter Definition .................................3
4. Examples ........................................................5
5. Security Considerations .........................................7
6. Normative References ............................................7
7. Informative References ..........................................8
8. Acknowledgements ................................................8
Appendix A: Changes Since RFC 2254 .................................9
A.1. Technical Changes ..........................................9
A.2. Editorial Changes ..........................................9

Smith and Howes Standards Track [Page 1]

RFC 4515 LDAP: String Representation of Search Filters June 2006

1. Introduction

The Lightweight Directory Access Protocol (LDAP) [RFC4510] 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; LDAP URLs
[RFC4516] are an example of one such application. This document
defines a human-readable string format for representing the full
range of possible LDAP version 3 search filters, including extended
match filters.

This document is a integral part of the LDAP technical specification
[RFC4510], which obsoletes the previously defined LDAP technical
specification, RFC 3377, in its entirety.

This document replaces RFC 2254. Changes to RFC 2254 are summarized
in Appendix A.

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

2. LDAP Search Filter Definition

An LDAP search filter is defined in Section 4.5.1 of [RFC4511] as
follows:

Filter ::= CHOICE {
and [0] SET SIZE (1..MAX) OF filter Filter,
or [1] SET SIZE (1..MAX) OF filter 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,
-- initial and final can occur at most once
substrings SEQUENCE SIZE (1..MAX) OF substring CHOICE {
initial [0] AssertionValue,
any [1] AssertionValue,
final [2] AssertionValue } }

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RFC 4515 LDAP: String Representation of Search Filters June 2006

AttributeValueAssertion ::= SEQUENCE {
attributeDesc AttributeDescription,
assertionValue AssertionValue }

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

AttributeDescription ::= LDAPString
-- Constrained to <attributedescription>
-- [RFC4512]

AttributeValue ::= OCTET STRING

MatchingRuleId ::= LDAPString

AssertionValue ::= OCTET STRING

LDAPString ::= OCTET STRING -- UTF-8 encoded,
-- [Unicode] characters

The AttributeDescription, as defined in [RFC4511], is a string
representation of the attribute description that is discussed in
[RFC4512]. The AttributeValue and AssertionValue OCTET STRING have
the form defined in [RFC4517]. The Filter is encoded for
transmission over a network using the Basic Encoding Rules (BER)
defined in [X.690], with simplifications described in [RFC4511].

3. String Search Filter Definition

The string representation of an LDAP search filter is a string of
UTF-8 [RFC3629] encoded Unicode characters [Unicode] that is defined
by the following grammar, following the ABNF notation defined in
[RFC4234]. The productions used that are not defined here are
defined in Section 1.4 (Common ABNF Productions) of [RFC4512] unless
otherwise noted. The filter format uses a prefix notation.

filter = LPAREN filtercomp RPAREN
filtercomp = and / or / not / item
and = AMPERSAND filterlist
or = VERTBAR filterlist
not = EXCLAMATION filter
filterlist = 1*filter
item = simple / present / substring / extensible
simple = attr filtertype assertionvalue
filtertype = equal / approx / greaterorequal / lessorequal

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RFC 4515 LDAP: String Representation of Search Filters June 2006

equal = EQUALS
approx = TILDE EQUALS
greaterorequal = RANGLE EQUALS
lessorequal = LANGLE EQUALS
extensible = ( attr [dnattrs]
[matchingrule] COLON EQUALS assertionvalue )
/ ( [dnattrs]
matchingrule COLON EQUALS assertionvalue )
present = attr EQUALS ASTERISK
substring = attr EQUALS [initial] any [final]
initial = assertionvalue
any = ASTERISK *(assertionvalue ASTERISK)
final = assertionvalue
attr = attributedescription
; The attributedescription rule is defined in
; Section 2.5 of [RFC4512].
dnattrs = COLON "dn"
matchingrule = COLON oid
assertionvalue = valueencoding
; The <valueencoding> rule is used to encode an <AssertionValue>
; from Section 4.1.6 of [RFC4511].
valueencoding = 0*(normal / escaped)
normal = UTF1SUBSET / UTFMB
escaped = ESC HEX HEX
UTF1SUBSET = %x01-27 / %x2B-5B / %x5D-7F
; UTF1SUBSET excludes 0x00 (NUL), LPAREN,
; RPAREN, ASTERISK, and ESC.
EXCLAMATION = %x21 ; exclamation mark ("!")
AMPERSAND = %x26 ; ampersand (or AND symbol) ("&")
ASTERISK = %x2A ; asterisk ("*")
COLON = %x3A ; colon (":")
VERTBAR = %x7C ; vertical bar (or pipe) ("|")
TILDE = %x7E ; tilde ("~")

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.

The <valueencoding> rule ensures that the entire filter string is a
valid UTF-8 string and provides that the octets that represent the
ASCII characters "*" (ASCII 0x2a), "(" (ASCII 0x28), ")" (ASCII
0x29), "\" (ASCII 0x5c), and NUL (ASCII 0x00) are represented as a
backslash "\" (ASCII 0x5c) followed by the two hexadecimal digits
representing the value of the encoded octet.

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RFC 4515 LDAP: String Representation of Search Filters June 2006

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 octets that are part
of the <normal> set may be escaped using this mechanism, for example,
non-printing ASCII characters.

For AssertionValues that contain UTF-8 character data, 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. For
example, the filter checking whether the "cn" attribute contained a
value with the character "*" anywhere in it would be represented as
"(cn=*\2a*)".

As indicated by the <valueencoding> rule, implementations MUST escape
all octets greater than 0x7F that are not part of a valid UTF-8
encoding sequence when they generate a string representation of a
search filter. Implementations SHOULD accept as input strings that
are not valid UTF-8 strings. This is necessary because RFC 2254 did
not clearly define the term "string representation" (and in
particular did not mention that the string representation of an LDAP
search filter is a string of UTF-8-encoded Unicode characters).

4. 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*)))
(o=univ*of*mich*)
(seeAlso=)

The following examples illustrate the use of extensible matching.

(cn:caseExactMatch:=Fred Flintstone)
(cn:=Betty Rubble)
(sn:dn:2.4.6.8.10:=Barney Rubble)
(o:dn:=Ace Industry)
(:1.2.3:=Wilma Flintstone)
(:DN:2.4.6.8.10:=Dino)

The first example shows use of the matching rule "caseExactMatch."

The second example demonstrates use of a MatchingRuleAssertion form
without a matchingRule.

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RFC 4515 LDAP: String Representation of Search Filters June 2006

The third example illustrates the use of the ":oid" notation to
indicate that the matching rule identified by the OID "2.4.6.8.10"
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 (indicated by the use of ":dn").

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

The fifth example is a filter that should be applied to any attribute
supporting the matching rule given (since the <attr> has been
omitted).

The sixth and final example is also a filter that should be applied
to any attribute supporting the matching rule given. 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)
(cn=*\2A*)
(filename=C:\5cMyFile)
(bin=\00\00\00\04)
(sn=Lu\c4\8di\c4\87)
(1.3.6.1.4.1.1466.0=\04\02\48\69)

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

The fourth example shows a filter searching for the four-octet value
00 00 00 04 (hex), illustrating the use of the escaping mechanism to
represent arbitrary data, including NUL characters.

The fifth example illustrates the use of the escaping mechanism to
represent various non-ASCII UTF-8 characters. Specifically, there
are 5 characters in the <assertionvalue> portion of this example:
LATIN CAPITAL LETTER L (U+004C), LATIN SMALL LETTER U (U+0075), LATIN
SMALL LETTER C WITH CARON (U+010D), LATIN SMALL LETTER I (U+0069),
and LATIN SMALL LETTER C WITH ACUTE (U+0107).

The sixth and final example demonstrates assertion of a BER-encoded
value.

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RFC 4515 LDAP: String Representation of Search Filters June 2006

5. 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.

Please refer to the Security Considerations sections of [RFC4511] and
[RFC4513] for more information.

6. Normative References

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

[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.

[RFC4234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.

[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.

[RFC4513] Harrison, R., Ed., "Lightweight Directory Access Protocol
(LDAP): Authentication Methods and Security Mechanisms",
RFC 4513, June 2006.

[RFC4517] Legg, S., Ed., "Lightweight Directory Access Protocol
(LDAP): Syntaxes and Matching Rules", RFC 4517, June
2006.

[Unicode] The Unicode Consortium, "The Unicode Standard, Version
3.2.0" is defined by "The Unicode Standard, Version 3.0"
(Reading, MA, Addison-Wesley, 2000. ISBN 0-201-61633-5),
as amended by the "Unicode Standard Annex #27: Unicode
3.1" (http://www.unicode.org/reports/tr27/) and by the
"Unicode Standard Annex #28: Unicode 3.2."

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RFC 4515 LDAP: String Representation of Search Filters June 2006

7. Informative References

[RFC4516] Smith, M., Ed. and T. Howes, "Lightweight Directory
Access Protocol (LDAP): Uniform Resource Locator", RFC
4516, June 2006.

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

8. Acknowledgements

This document replaces RFC 2254 by Tim Howes. RFC 2254 was a product
of the IETF ASID Working Group.

Changes included in this revised specification are based upon
discussions among the authors, discussions within the LDAP (v3)
Revision Working Group (ldapbis), and discussions within other IETF
Working Groups. The contributions of individuals in these working
groups is gratefully acknowledged.

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RFC 4515 LDAP: String Representation of Search Filters June 2006

Appendix A: Changes Since RFC 2254

A.1. Technical Changes

Replaced [ISO 10646] reference with [Unicode].

The following technical changes were made to the contents of the
"String Search Filter Definition" section:

Added statement that the string representation is a string of UTF-8-
encoded Unicode characters.

Revised all of the ABNF to use common productions from [RFC4512].

Replaced the "value" rule with a new "assertionvalue" rule within the
"simple", "extensible", and "substring" ("initial", "any", and
"final") rules. This matches a change made in [RFC4517].

Added "(" and ")" around the components of the <extensible>
subproductions for clarity.

Revised the "attr", "matchingrule", and "assertionvalue" ABNF to more
precisely reference productions from the [RFC4512] and [RFC4511]
documents.

"String Search Filter Definition" section: replaced "greater" and
"less" with "greaterorequal" and "lessorequal" to avoid confusion.

Introduced the "valueencoding" and associated "normal" and "escaped"
rules to reduce the dependence on descriptive text. The "normal"
production restricts filter strings to valid UTF-8 sequences.

Added a statement about expected behavior in light of RFC 2254's lack
of a clear definition of "string representation."

A.2. Editorial Changes

Changed document title to include "LDAP:" prefix.

IESG Note: removed note about lack of satisfactory mandatory
authentication mechanisms.

Header and "Authors' Addresses" sections: added Mark Smith as the
document editor and updated affiliation and contact information.

"Table of Contents" and "Intellectual Property" sections: added.

Copyright: updated per latest IETF guidelines.

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RFC 4515 LDAP: String Representation of Search Filters June 2006

"Abstract" section: separated from introductory material.

"Introduction" section: new section; separated from the Abstract.
Updated second paragraph to indicate that RFC 2254 is replaced by
this document (instead of RFC 1960). Added reference to the
[RFC4510] document.

"LDAP Search Filter Definition" section: made corrections to the LDAP
search filter ABNF so it matches that used in [RFC4511].

Clarified the definition of 'value' (now 'assertionvalue') to take
into account the fact that it is not precisely an AttributeAssertion
from [RFC4511] Section 4.1.6 (special handling is required for some
characters). Added a note that each octet of a character to be
escaped is replaced by a backslash and two hex digits, which
represent a single octet.

"Examples" section: added four additional examples: (seeAlso=),
(cn:=Betty Rubble), (:1.2.3:=Wilma Flintstone), and
(1.3.6.1.4.1.1466.0=\04\02\48\69). Replaced one occurrence of "a
value" with "an assertion value". Corrected the description of this
example: (sn:dn:2.4.6.8.10:=Barney Rubble). Replaced the numeric OID
in the first extensible match example with "caseExactMatch" to
demonstrate use of the descriptive form. Used "DN" (uppercase) in
the last extensible match example to remind the reader to treat the
<dnattrs> production as case insensitive. Reworded the description
of the fourth escaping mechanism example to avoid making assumptions
about byte order. Added text to the fifth escaping mechanism example
to spell out what the non-ASCII characters are in Unicode terms.

"Security Considerations" section: added references to [RFC4511] and
[RFC4513].

"Normative References" section: renamed from "References" per new RFC
guidelines. Changed from [1] style to [RFC4511] style throughout the
document. Added entries for [Unicode], [RFC2119], [RFC4513],
[RFC4512], and [RFC4510] and updated the UTF-8 reference. Replaced
RFC 822 reference with a reference to RFC 4234.

"Informative References" section: (new section) moved [X.690] to this
section. Added a reference to [RFC4516].

"Acknowledgements" section: added.

"Appendix A: Changes Since RFC 2254" section: added.

Surrounded the names of all ABNF productions with "<" and ">" where
they are used in descriptive text.

Smith and Howes Standards Track [Page 10]

RFC 4515 LDAP: String Representation of Search Filters June 2006

Replaced all occurrences of "LDAPv3" with "LDAP."

Authors' Addresses

Mark Smith, Editor
Pearl Crescent, LLC
447 Marlpool Dr.
Saline, MI 48176
USA

Phone: +1 734 944-2856
EMail: [email protected]

Tim Howes
Opsware, Inc.
599 N. Mathilda Ave.
Sunnyvale, CA 94085
USA

Phone: +1 408 744-7509
EMail: [email protected]

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RFC 4515 LDAP: String Representation of Search Filters June 2006

Full Copyright Statement

Copyright (C) The Internet Society (2006).

This document is subject to the rights, licenses and restrictions
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Smith and Howes Standards Track [Page 12]