Network Working Group M. Smith, Ed.
Request for Comments: 4516 Pearl Crescent, LLC
Obsoletes: 2255 T. Howes
Category: Standards Track Opsware, Inc.
June 2006 Lightweight Directory Access Protocol (LDAP):
Uniform Resource Locator
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 (2006).
This document describes a format for a Lightweight Directory Access
Protocol (LDAP) Uniform Resource Locator (URL). An LDAP URL
describes an LDAP search operation that is used to retrieve
information from an LDAP directory, or, in the context of an LDAP
referral or reference, an LDAP URL describes a service where an LDAP
operation may be progressed.
Table of Contents
1. Introduction ....................................................22. URL Definition ..................................................22.1. Percent-Encoding ...........................................43. Defaults for Fields of the LDAP URL .............................54. Examples ........................................................65. Security Considerations .........................................86. Normative References ............................................97. Informative References .........................................108. Acknowledgements ...............................................10
Appendix A: Changes Since RFC 2255 ................................11A.1. Technical Changes .........................................11A.2. Editorial Changes .........................................11
LDAP is the Lightweight Directory Access Protocol [RFC4510]. This
document specifies the LDAP URL format for version 3 of LDAP and
clarifies how LDAP URLs are resolved. This document also defines an
extension mechanism for LDAP URLs. This mechanism may be used to
provide access to new LDAP extensions.
Note that not all the parameters of the LDAP search operation
described in [RFC4511] can be expressed using the format defined in
this document. Note also that URLs may be used to represent
reference knowledge, including that for non-search operations.
This document is an 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 2255. See Appendix A for a list of
changes relative to RFC 2255.
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. URL Definition
An LDAP URL begins with the protocol prefix "ldap" and is defined by
the following grammar, following the ABNF notation defined in
ldapurl = scheme COLON SLASH SLASH [host [COLON port]]
[SLASH dn [QUESTION [attributes]
[QUESTION [scope] [QUESTION [filter]
; <host> and <port> are defined
; in Sections 3.2.2 and 3.2.3
; of [RFC3986].
; <filter> is from Section 3 of
; [RFC4515], subject to the
; provisions of the
; "Percent-Encoding" section
scheme = "ldap"
dn = distinguishedName ; From Section 3 of [RFC4514],
; subject to the provisions of
; the "Percent-Encoding"
; section below.
attributes = attrdesc *(COMMA attrdesc)
attrdesc = selector *(COMMA selector)
selector = attributeSelector ; From Section 4.5.1 of
; [RFC4511], subject to the
; provisions of the
; "Percent-Encoding" section
scope = "base" / "one" / "sub"
extensions = extension *(COMMA extension)
extension = [EXCLAMATION] extype [EQUALS exvalue]
extype = oid ; From section 1.4 of [RFC4512].
exvalue = LDAPString ; From section 4.1.2 of
; [RFC4511], subject to the
; provisions of the
; "Percent-Encoding" section
EXCLAMATION = %x21 ; exclamation mark ("!")
SLASH = %x2F ; forward slash ("/")
COLON = %x3A ; colon (":")
QUESTION = %x3F ; question mark ("?")
The "ldap" prefix indicates an entry or entries accessible from the
LDAP server running on the given hostname at the given portnumber.
Note that the <host> may contain literal IPv6 addresses as specified
in Section 3.2.2 of [RFC3986].
The <dn> is an LDAP Distinguished Name using the string format
described in [RFC4514]. It identifies the base object of the LDAP
search or the target of a non-search operation.
The <attributes> construct is used to indicate which attributes
should be returned from the entry or entries.
The <scope> construct is used to specify the scope of the search to
perform in the given LDAP server. The allowable scopes are "base"
for a base object search, "one" for a one-level search, or "sub" for
a subtree search.
The <filter> is used to specify the search filter to apply to entries
within the specified scope during the search. It has the format
specified in [RFC4515].
The <extensions> construct provides the LDAP URL with an
extensibility mechanism, allowing the capabilities of the URL to be
extended in the future. Extensions are a simple comma-separated list
of type=value pairs, where the =value portion MAY be omitted for
options not requiring it. Each type=value pair is a separate
extension. These LDAP URL extensions are not necessarily related to
any of the LDAP extension mechanisms. Extensions may be supported or
unsupported by the client resolving the URL. An extension prefixed
with a '!' character (ASCII 0x21) is critical. An extension not
prefixed with a '!' character is non-critical.
If an LDAP URL extension is implemented (that is, if the
implementation understands it and is able to use it), the
implementation MUST make use of it. If an extension is not
implemented and is marked critical, the implementation MUST NOT
process the URL. If an extension is not implemented and is not
marked critical, the implementation MUST ignore the extension.
The extension type (<extype>) MAY be specified using the numeric OID
<numericoid> form (e.g., 126.96.36.199) or the descriptor <descr> form
(e.g., myLDAPURLExtension). Use of the <descr> form SHOULD be
restricted to registered object identifier descriptive names. See
[RFC4520] for registration details and usage guidelines for
No LDAP URL extensions are defined in this document. Other documents
or a future version of this document MAY define one or more
A generated LDAP URL MUST consist only of the restricted set of
characters included in one of the following three productions defined
Implementations SHOULD accept other valid UTF-8 strings [RFC3629] as
input. An octet MUST be encoded using the percent-encoding mechanism
described in section 2.1 of [RFC3986] in any of these situations:
The octet is not in the reserved set defined in section 2.2 of
[RFC3986] or in the unreserved set defined in section 2.3 of
It is the single Reserved character '?' and occurs inside a <dn>,
<filter>, or other element of an LDAP URL.
It is a comma character ',' that occurs inside an <exvalue>.
Note that before the percent-encoding mechanism is applied, the
extensions component of the LDAP URL may contain one or more null
(zero) bytes. No other component may.
3. Defaults for Fields of the LDAP URL
Some fields of the LDAP URL are optional, as described above. In the
absence of any other specification, the following general defaults
SHOULD be used when a field is absent. Note that other documents MAY
specify different defaulting rules; for example, section 4.1.10 of
[RFC4511] specifies a different rule for determining the correct DN
to use when it is absent in an LDAP URL that is returned as a
If no <host> is given, the client must have some a priori
knowledge of an appropriate LDAP server to contact.
The default LDAP port is TCP port 389.
If no <dn> is given, the default is the zero-length DN, "".
If the <attributes> part is omitted, all user attributes of the
entry or entries should be requested (e.g., by setting the
attributes field AttributeDescriptionList in the LDAP search
request to a NULL list, or by using the special <alluserattrs>
If <scope> is omitted, a <scope> of "base" is assumed.
If <filter> is omitted, a filter of "(objectClass=*)" is assumed.
If <extensions> is omitted, no extensions are assumed.
The following are some example LDAP URLs that use the format defined
above. The first example is an LDAP URL referring to the University
of Michigan entry, available from an LDAP server of the client's
The next example is an LDAP URL referring to the University of
Michigan entry in a particular ldap server:
Both of these URLs correspond to a base object search of the
"o=University of Michigan,c=US" entry using a filter of
"(objectclass=*)", requesting all attributes.
The next example is an LDAP URL referring to only the postalAddress
attribute of the University of Michigan entry:
The corresponding LDAP search operation is the same as in the
previous example, except that only the postalAddress attribute is
The next example is an LDAP URL referring to the set of entries found
by querying the given LDAP server on port 6666 and doing a subtree
search of the University of Michigan for any entry with a common name
of "Babs Jensen", retrieving all attributes:
The next example is an LDAP URL referring to all children of the c=GB
The objectClass attribute is requested to be returned along with the
entries, and the default filter of "(objectclass=*)" is used.
The next example is an LDAP URL to retrieve the mail attribute for
the LDAP entry named "o=Question?,c=US", illustrating the use of the
percent-encoding mechanism on the reserved character '?'.
The next example (which is broken into two lines for readability)
illustrates the interaction between the LDAP string representation of
the filters-quoting mechanism and the URL-quoting mechanisms.
The filter in this example uses the LDAP escaping mechanism of \ to
encode three zero or null bytes in the value. In LDAP, the filter
would be written as (four-octet=\00\00\00\04). Because the \
character must be escaped in a URL, the \s are percent-encoded as %5c
(or %5C) in the URL encoding.
The next example illustrates the interaction between the LDAP string
representation of the DNs-quoting mechanism and URL-quoting
The DN encoded in the above URL is:
o=An Example\2C Inc.,c=US
That is, the left-most RDN value is:
An Example, Inc.
The following three URLs are equivalent, assuming that the defaulting
rules specified in Section 3 of this document are used:
These three URLs point to the root DSE on the ldap.example.net
The final two examples show use of a hypothetical, experimental bind
name extension (the value associated with the extension is an LDAP
The two URLs are the same, except that the second one marks the
e-bindname extension as critical. Notice the use of the percent-
encoding mechanism to encode the commas within the distinguished name
value in the e-bindname extension.
5. Security Considerations
The general URL security considerations discussed in [RFC3986] are
relevant for LDAP URLs.
The use of security mechanisms when processing LDAP URLs requires
particular care, since clients may encounter many different servers
via URLs, and since URLs are likely to be processed automatically,
without user intervention. A client SHOULD have a user-configurable
policy that controls which servers the client will establish LDAP
sessions with and with which security mechanisms, and SHOULD NOT
establish LDAP sessions that are inconsistent with this policy. If a
client chooses to reuse an existing LDAP session when resolving one
or more LDAP URLs, it MUST ensure that the session is compatible with
the URL and that no security policies are violated.
Sending authentication information, no matter the mechanism, may
violate a user's privacy requirements. In the absence of specific
policy permitting authentication information to be sent to a server,
a client should use an anonymous LDAP session. (Note that clients
conforming to previous LDAP URL specifications, where all LDAP
sessions are anonymous and unprotected, are consistent with this
specification; they simply have the default security policy.) Simply
opening a transport connection to another server may violate some
users' privacy requirements, so clients should provide the user with
a way to control URL processing.
Some authentication methods, in particular, reusable passwords sent
to the server, may reveal easily-abused information to the remote
server or to eavesdroppers in transit and should not be used in URL
processing unless they are explicitly permitted by policy.
Confirmation by the human user of the use of authentication
information is appropriate in many circumstances. Use of strong
authentication methods that do not reveal sensitive information is
much preferred. If the URL represents a referral for an update
operation, strong authentication methods SHOULD be used. Please
refer to the Security Considerations section of [RFC4513] for more
The LDAP URL format allows the specification of an arbitrary LDAP
search operation to be performed when evaluating the LDAP URL.
Following an LDAP URL may cause unexpected results, for example, the
retrieval of large amounts of data or the initiation of a long-lived
search. The security implications of resolving an LDAP URL are the
same as those of resolving an LDAP search query.
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.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, January 2005.
[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
[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
[RFC4513] Harrison, R., Ed., "Lightweight Directory Access Protocol
(LDAP): Authentication Methods and Security Mechanisms",
RFC 4513, June 2006.
[RFC4514] Zeilenga, K., Ed., "Lightweight Directory Access Protocol
(LDAP): String Representation of Distinguished Names", RFC
4514, June 2006.
[RFC4515] Smith, M. Ed. and T. Howes, "Lightweight Directory Access
Protocol (LDAP): String Representation of Search Filters",
RFC 4515, June 2006.
7. Informative References
[RFC2396] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396,
[RFC4520] Zeilenga, K., "Internet Assigned Numbers Authority (IANA)
Considerations for the Lightweight Directory Access
Protocol (LDAP)", BCP 64, RFC 4520, June 2006.
The LDAP URL format was originally defined at the University of
Michigan. This material is based upon work supported by the National
Science Foundation under Grant No. NCR-9416667. The support of both
the University of Michigan and the National Science Foundation is
This document obsoletes RFC 2255 by Tim Howes and Mark Smith.
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. Several people in particular have
made valuable comments on this document: RL "Bob" Morgan, Mark Wahl,
Kurt Zeilenga, Jim Sermersheim, and Hallvard Furuseth deserve special
thanks for their contributions.
Appendix A: Changes Since RFC 2255
A.1. Technical Changes
The following technical changes were made to the contents of the "URL
Revised all of the ABNF to use common productions from [RFC4512].
Replaced references to [RFC2396] with a reference to [RFC3986] (this
allows literal IPv6 addresses to be used inside the <host> portion of
the URL, and a note was added to remind the reader of this
enhancement). Referencing [RFC3986] required changes to the ABNF and
text so that productions that are no longer defined by [RFC3986] are
not used. For example, <hostport> is not defined by [RFC3986] so it
has been replaced with host [COLON port]. Note that [RFC3986]
includes new definitions for the "Reserved" and "Unreserved" sets of
characters, and the net result is that the following two additional
characters should be percent-encoded when they appear anywhere in the
data used to construct an LDAP URL: "[" and "]" (these two characters
were first added to the Reserved set by RFC 2732).
Changed the definition of <attrdesc> to refer to <attributeSelector>
from [RFC4511]. This allows the use of "*" in the <attrdesc> part of
the URL. It is believed that existing implementations of RFC 2255
already support this.
Avoided use of <prose-val> (bracketed-string) productions in the
<dn>, <host>, <attrdesc>, and <exvalue> rules.
Changed the ABNF for <ldapurl> to group the <dn> component with the
Changed the <extype> rule to be an <oid> from [RFC4512].
Changed the text about extension types so it references [RFC4520].
Reordered rules to more closely follow the order in which the
elements appear in the URL.
"Bindname Extension": removed due to lack of known implementations.
A.2. Editorial Changes
Changed document title to include "LDAP:" prefix.
IESG Note: removed note about lack of satisfactory mandatory
"Status of this Memo" section: updated boilerplate to match current
"Abstract" section: separated from introductory material.
"Table of Contents" and "Intellectual Property" sections: added.
"Introduction" section: new section; separated from the Abstract.
Changed the text indicate that RFC 2255 is replaced by this document
(instead of RFC 1959). Added text to indicate that LDAP URLs are
used for references and referrals. Fixed typo (replaced the nonsense
phrase "to perform to retrieve" with "used to retrieve"). Added a
note to let the reader know that not all of the parameters of the
LDAP search operation described in [RFC4511] can be expressed using
"URL Definition" section: removed second copy of <ldapurl> grammar
and following two paragraphs (editorial error in RFC 2255). Fixed
line break within '!' sequence. Reformatted the ABNF to improve
readability by aligning comments and adding some blank lines.
Replaced "residing in the LDAP server" with "accessible from the LDAP
server" in the sentence immediately following the ABNF. Removed the
sentence "Individual attrdesc names are as defined for
AttributeDescription in [RFC4511]." because [RFC4511]'s
<attributeSelector> is now used directly in the ABNF. Reworded last
paragraph to clarify which characters must be percent-encoded. Added
text to indicate that LDAP URLs are used for references and
referrals. Added text that refers to the ABNF from RFC 4234.
Clarified and strengthened the requirements with respect to
processing of URLs that contain implemented and not implemented
extensions (the approach now closely matches that specified in
[RFC4511] for LDAP controls).
"Defaults for Fields of the LDAP URL" section: added; formed by
moving text about defaults out of the "URL Definition" section.
Replaced direct reference to the attribute name "*" with a reference
to the special <alluserattrs> selector "*" defined in [RFC4511].
"URL Processing" section: removed.
"Examples" section: Modified examples to use example.com and
example.net hostnames. Added missing '?' to the LDAP URL example
whose filter contains three null bytes. Removed space after one
comma within a DN. Revised the bindname example to use e-bindname.
Changed the name of an attribute used in one example from "int" to
"four-octet" to avoid potential confusion. Added an example that
demonstrates the interaction between DN escaping and URL percent-
encoding. Added some examples to show URL equivalence with respect
to the <dn> portion of the URL. Used uppercase in some examples to
remind the reader that some tokens are case-insensitive.
"Security Considerations" section: Added a note about connection
reuse. Added a note about using strong authentication methods for
updates. Added a reference to [RFC4513]. Added note that simply
opening a connection may violate some users' privacy requirements.
Adopted the working group's revised LDAP terminology specification by
replacing the word "connection" with "LDAP session" or "LDAP
connection" as appropriate.
"Acknowledgements" section: added statement that this document
obsoletes RFC 2255. Added Kurt Zeilenga, Jim Sermersheim, and
"Normative References" section: renamed from "References" per new RFC
guidelines. Changed from  style to [RFC4511] style throughout the
document. Added references to RFC 4234 and RFC 3629. Updated all
RFC 1738 references to point to the appropriate sections within
[RFC3986]. Updated the LDAP references to refer to LDAPBis WG
documents. Removed the reference to the LDAP Attribute Syntaxes
document and added references to the [RFC4513], [RFC4520], and
"Informative References" section: added.
Header and "Authors' Addresses" sections: added "editor" next to Mark
Smith's name. Updated affiliation and contact information.
Copyright: updated the year.
Throughout the document: surrounded the names of all ABNF productions
with "<" and ">" where they are used in descriptive text.
Mark Smith, Editor
Pearl Crescent, LLC
447 Marlpool Dr.
Saline, MI 48176
Phone: +1 734 944-2856
599 N. Mathilda Ave.
Sunnyvale, CA 94085
Phone: +1 408 744-7509
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