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 (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.
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.
The Lightweight Directory Access Protocol (LDAP)  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
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.
where the LDAPString above is limited to the UTF-8 encoding of the
ISO 10646 character set . The AttributeDescription is a string
representation of the attribute description and is defined in .
The AttributeValue and AssertionValue OCTET STRING have the form
defined in . The Filter is encoded for transmission over a
network using the Basic Encoding Rules defined in , with
simplifications described in .
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 . 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 
matchingrule = MatchingRuleId from Section 4.1.9 of 
value = AttributeValue from Section 4.1.6 of 
The attr, matchingrule, and value constructs are as described in the
corresponding section of  given above.
If a value should contain any of the following characters
Character ASCII value
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,
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.
This section gives a few examples of search filters written using
The following examples illustrate the use of extensible matching.
The second example illustrates the use of the ":dn" notation to
indicate that matching rule "188.8.131.52.10" 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
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.
 Wahl, M., Howes, T., and S. Kille, "Lightweight Directory Access
Protocol (v3)", RFC 2251, December 1997.
 Wahl, M., Coulbeck, A., Howes, T., and S. Kille, "Lightweight
Directory Access Protocol (v3): Attribute Syntax Definitions", RFC
2252, December 1997.
 Specification of ASN.1 encoding rules: Basic, Canonical, and
Distinguished Encoding Rules, ITU-T Recommendation X.690, 1994.
 Yergeau, F., "UTF-8, a transformation format of Unicode and ISO
10646", RFC 2044, October 1996.
 Crocker, D., "Standard for the Format of ARPA Internet Text
Messages", STD 11, RFC 822, August 1982.
8. Author's Address
Netscape Communications Corp.
501 E. Middlefield Road
Mountain View, CA 94043
Phone: +1 415 937-3419
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