RFC2254 - The String Representation of LDAP Search Filters

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.

IESG Note

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 RFC2026, 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.

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

UNLIKELY TO INTEROPERATE, or MAY INTEROPERATE ONLY IF AUTHENTICATION

IS REDUCED TO AN UNACCEPTABLY WEAK LEVEL.

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 RFC1960, 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.

3. LDAP Search Filter Definition

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

follows:

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

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.

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

"(cn=*\2a*)".

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*)))

(o=univ*of*mich*)

The following examples illustrate the use of extensible matching.

(cn:1.2.3.4.5:=Fred Flintstone)

(sn:dn:2.4.6.8.10:=Barney Rubble)

(o:dn:=Ace Industry)

(:dn:2.4.6.8.10:=Dino)

The second example illustrates the use of the ":dn" notation to

indicate that matching rule "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.

The third example denotes an equality match, except that DN

components should be considered part of the entry when doing the

match.

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)

(cn=*\2A*)

(filename=C:\5cMyFile)

(bin=\00\00\00\04)

(sn=Lu\c4\8di\c4\87)

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

character.

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.

7. References

[1] Wahl, M., Howes, T., and S. Kille, "Lightweight Directory Access

Protocol (v3)", RFC2251, 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", RFC2044, October 1996.

[5] Crocker, D., "Standard for the Format of ARPA Internet Text

Messages", STD 11, RFC822, August 1982.

8. Author's Address

Tim Howes

Netscape Communications Corp.

501 E. Middlefield Road

Mountain View, CA 94043

USA

Phone: +1 415 937-3419

EMail: howes@netscape.com

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