RFC2255 - The LDAP URL Format

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　　Network Working Group T. Howes
Request for Comments: 2255 M. Smith
Category: Standards Track Netscape Communications Corp.
December 1997

The LDAP URL Format

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 Notice

Copyright (C) The Internet Society (1997). All Rights Reserved.

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

LDAP is the Lightweight Directory Access Protocol, defined in [1],
[2] and [3]. This document describes a format for an LDAP Uniform
Resource Locator. The format describes an LDAP search operation to
perform to retrieve information from an LDAP directory. This document
replaces RFC1959. It updates 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, so that future
documents can extend their functionality, for example, to provide
access to new LDAPv3 extensions as they are defined.

The key words "MUST", "MAY", and "SHOULD" used in this document are
to be interpreted as described in [6].

3. URL Definition

An LDAP URL begins with the protocol prefix "ldap" and is defined by
the following grammar.

ldapurl = scheme "://" [hostport] ["/"
[dn ["?" [attributes] ["?" [scope]
["?" [filter] ["?" extensions]]]]]]
scheme = "ldap"
attributes = attrdesc *("," attrdesc)
scope = "base" / "one" / "sub"
dn = distinguishedName from Section 3 of [1]
hostport = hostport from Section 5 of RFC1738 [5]
attrdesc = AttributeDescription from Section 4.1.5 of [2]
filter = filter from Section 4 of [4]
extensions = extension *("," extension)
extension = ["!"] extype ["=" exvalue]
extype = token / xtoken
exvalue = LDAPString from section 4.1.2 of [2]
token = oid from section 4.1 of [3]
xtoken = ("X-" / "x-") token

The "ldap" prefix indicates an entry or entries residing in the LDAP
server running on the given hostname at the given portnumber. The
default LDAP port is TCP port 389. If no hostport is given, the
client must have some apriori knowledge of an appropriate LDAP server
to contact.

The dn is an LDAP Distinguished Name using the string format
described in [1]. It identifies the base object of the LDAP search.

ldapurl = scheme "://" [hostport] ["/"
[dn ["?" [attributes] ["?" [scope]
["?" [filter] ["?" extensions]]]]]]
scheme = "ldap"
attributes = attrdesc *("," attrdesc)
scope = "base" / "one" / "sub"
dn = distinguishedName from Section 3 of [1]
hostport = hostport from Section 5 of RFC1738 [5]
attrdesc = AttributeDescription from Section 4.1.5 of [2]
filter = filter from Section 4 of [4]
extensions = extension *("," extension)
extension = ["!"] extype ["=" exvalue]
extype = token / xtoken
exvalue = LDAPString from section 4.1.2 of [2]
token = oid from section 4.1 of [3]
xtoken = ("X-" / "x-") token

The "ldap" prefix indicates an entry or entries residing in the LDAP
server running on the given hostname at the given portnumber. The
default LDAP port is TCP port 389. If no hostport is given, the
client must have some apriori knowledge of an appropriate LDAP server
to contact.

The dn is an LDAP Distinguished Name using the string format
described in [1]. It identifies the base object of the LDAP search.

The attributes construct is used to indicate which attributes should
be returned from the entry or entries. Individual attrdesc names are
as defined for AttributeDescription in [2]. 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 (in LDAPv3) by requesting the special attribute name "*").

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. If scope is omitted, a scope of "base" is assumed.

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 [4]. If filter is omitted, a filter of
"(objectClass=*)" is assumed.

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 LDAPv3
extension mechanisms. Extensions may be supported or unsupported by
the client resolving the URL. An extension prefixed with a '!'
character (ASCII 33) is critical. An extension not prefixed with a '
!' character is non-critical.

If an extension is supported by the client, the client MUST obey the
extension if the extension is critical. The client SHOULD obey
supported extensions that are non-critical.

If an extension is unsupported by the client, the client MUST NOT
process the URL if the extension is critical. If an unsupported
extension is non-critical, the client MUST ignore the extension.

If a critical extension cannot be processed successfully by the
client, the client MUST NOT process the URL. If a non-critical
extension cannot be processed successfully by the client, the client
SHOULD ignore the extension.

Extension types prefixed by "X-" or "x-" are reserved for use in
bilateral agreements between communicating parties. Other extension
types MUST be defined in this document, or in other standards-track
documents.

One LDAP URL extension is defined in this document in the next
section. Other documents or a future version of this document MAY
define other extensions.

Note that any URL-illegal characters (e.g., spaces), URL special
characters (as defined in section 2.2 of RFC1738) and the reserved
character '?' (ASCII 63) occurring inside a dn, filter, or other
element of an LDAP URL MUST be escaped using the % method described
in RFC1738 [5]. If a comma character ',' occurs inside an extension
value, the character MUST also be escaped using the % method.

4. The Bindname Extension

This section defines an LDAP URL extension for representing the
distinguished name for a client to use when authenticating to an LDAP
directory during resolution of an LDAP URL. Clients MAY implement
this extension.

The extension type is "bindname". The extension value is the
distinguished name of the directory entry to authenticate as, in the
same form as described for dn in the grammar above. The dn may be the
NULL string to specify unauthenticated access. The extension may be
either critical (prefixed with a '!' character) or non-critical (not
prefixed with a '!' character).

If the bindname extension is critical, the client resolving the URL
MUST authenticate to the directory using the given distinguished name
and an appropriate authentication method. Note that for a NULL
distinguished name, no bind MAY be required to obtain anonymous
access to the directory. If the extension is non-critical, the client
MAY bind to the directory using the given distinguished name.

5. URL Processing

This section describes how an LDAP URL SHOULD be resolved by a
client.

First, the client obtains a connection to the LDAP server referenced
in the URL, or an LDAP server of the client's choice if no LDAP
server is explicitly referenced. This connection MAY be opened
specifically for the purpose of resolving the URL or the client MAY
reuse an already open connection. The connection MAY provide
confidentiality, integrity, or other services, e.g., using TLS. Use
of security services is at the client's discretion if not specified
in the URL.

Next, the client authenticates itself to the LDAP server. This step
is optional, unless the URL contains a critical bindname extension
with a non-NULL value. If a bindname extension is given, the client
proceeds according to the section above.

If a bindname extension is not specified, the client MAY bind to the
directory using a appropriate dn and authentication method of its own
choosing (including NULL authentication).

Next, the client performs the LDAP search operation specified in the
URL. Additional fields in the LDAP protocol search request, such as
sizelimit, timelimit, deref, and anything else not specified or
defaulted in the URL specification, MAY be set at the client's
discretion.

Once the search has completed, the client MAY close the connection to
the LDAP server, or the client MAY keep the connection open for
future use.

6. Examples

The following are some example LDAP URLs using 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
choosing:

ldap:///o=University%20of%20Michigan,c=US

The next example is an LDAP URL referring to the University of
Michigan entry in a particular ldap server:

ldap://ldap.itd.umich.edu/o=University%20of%20Michigan,c=US

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:

ldap://ldap.itd.umich.edu/o=University%20of%20Michigan,
c=US?postalAddress

The corresponding LDAP search operation is the same as in the
previous example, except that only the postalAddress attribute is
requested.

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:

ldap://host.com:6666/o=University%20of%20Michigan,
c=US??sub?(cn=Babs%20Jensen)

The next example is an LDAP URL referring to all children of the c=GB
entry:

ldap://ldap.itd.umich.edu/c=GB?objectClass?one

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" is given below, illustrating
the use of the escaping mechanism on the reserved character '?'.

ldap://ldap.question.com/o=Question%3f,c=US?mail

The next example illustrates the interaction between LDAP and URL
quoting mechanisms.

ldap://ldap.netscape.com/o=Babsco,c=US??(int=%5c00%5c00%5c00%5c04)

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 (int=\00\00\00\04). Because the \ character must
be escaped in a URL, the \'s are escaped as %5c in the URL encoding.

The final example shows the use of the bindname extension to specify
the dn a client should use for authentication when resolving the URL.

ldap:///??sub??bindname=cn=Manager%2co=Foo
ldap:///??sub??!bindname=cn=Manager%2co=Foo

The two URLs are the same, except that the second one marks the
bindname extension as critical. Notice the use of the % encoding
method to encode the comma in the distinguished name value in the

bindname extension.

7. Security Considerations

General URL security considerations discussed in [5] 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 about which servers to connect to using which security
mechanisms, and SHOULD NOT make connections that are inconsistent
with this policy.

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 connection. (Note that clients
conforming to previous LDAP URL specifications, where all connections
are anonymous and unprotected, are consistent with this
specification; they simply have the default security policy.)

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 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.

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, the initiation of a long-lived
search, etc. The security implications of resolving an LDAP URL are
the same as those of resolving an LDAP search query.

8. Acknowledgements

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
gratefully acknowledged.

Several people have made valuable comments on this document. In
particular RL "Bob" Morgan and Mark Wahl deserve special thanks for
their contributions.

9. References

[1] Wahl, M., Kille, S., and T. Howes, "Lightweight Directory Access
Protocol (v3): UTF-8 String Representation of Distinguished Names",
RFC2253, December 1997.

[2] Wahl, M., Howes, T., and S. Kille, "Lightweight Directory Access
Protocol (v3)", RFC2251, December 1997.

[3] Wahl, M., Coulbeck, A., Howes, T. and S. Kille, "Lightweight
Directory Access Protocol (v3): Attribute Syntax Definitions", RFC
2252, December 1997.

[4] Howes, T., "A String Representation of LDAP Search Filters", RFC
2254, December 1997.

[5] Berners-Lee, T., Masinter, L. and M. McCahill, "Uniform Resource
Locators (URL)," RFC1738, December 1994.

[6] Bradner, S., "Key Words for use in RFCs to Indicate Requirement
Levels," RFC2119, March 1997.

Authors' Addresses

Tim Howes
Netscape Communications Corp.
501 E. Middlefield Rd.
Mountain View, CA 94043
USA

Phone: +1 415 937-3419
EMail: howes@netscape.com

Mark Smith
Netscape Communications Corp.
501 E. Middlefield Rd.
Mountain View, CA 94043
USA

Phone: +1 415 937-3477
EMail: mcs@netscape.com

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