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RFC 2255 - 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 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.

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 RFC 1959. 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 RFC 1738 [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 RFC 1738 [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 RFC 1738) 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 RFC 1738 [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",
   RFC 2253, December 1997.

   [2] Wahl, M., Howes, T., and S. Kille, "Lightweight Directory Access
   Protocol (v3)", RFC 2251, 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)," RFC 1738, December 1994.

   [6] Bradner, S., "Key Words for use in RFCs to Indicate Requirement
   Levels," RFC 2119, 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

Full Copyright Statement

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

   This document and translations of it may be copied and furnished to
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   or assist in its implementation may be prepared, copied, published
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   The limited permissions granted above are perpetual and will not be
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