Network Working Group                                        Tim Howes
INTERNET DRAFT                                              Mark Smith
OBSOLETES: RFC 1959                      Netscape Communications Corp.
                                                             June 1997

                          The LDAP URL Format

1.  Status of this Memo

This document is an Internet-Draft.  Internet-Drafts are  working  docu-
ments  of the Internet Engineering Task Force (IETF), its areas, and its
working groups.  Note that other  groups  may  also  distribute  working
documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum  of  six  months
and  may  be  updated,  replaced, or obsoleted by other documents at any
time.  It is inappropriate to use Internet- Drafts as reference material
or to cite them other than as ``work in progress.''

To learn the current status of  any  Internet-Draft,  please  check  the
``1id-abstracts.txt''  listing  contained in the Internet- Drafts Shadow
Directories on (US East Coast),  (Europe), (US West Coast), or (Pacific Rim).

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 func-
tionality,  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].

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3.  URL Definition

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

    ldapurl    =3D scheme "://" [hostport] ["/"
                 [dn ["?" [attributes] ["?" [scope]
                 ["?" [filter] ["?" extensions]]]]]]
    scheme     =3D "ldap"
    attributes =3D attrdesc *("," attrdesc)
    scope      =3D "base" / "one" / "sub"
    dn         =3D distinguishedName from Section 3 of [1]
    hostport   =3D hostport from Section 5 of RFC 1738 [5]
    attrdesc   =3D AttributeDescription from Section 4.1.5 of [2]
    filter     =3D filter from Section 4 of [4]
    extensions =3D extension *("," extension)
    extension  =3D ["!"] extype "=3D" exvalue
    extype     =3D token / xtoken
    exvalue    =3D LDAPString from section 4.1.2 of [2]
    token      =3D <keystring> from section 4.2.1 of [3]
    xtoken     =3D ("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 con-

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  per-
form  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 speci-
fied in [4].  If filter is omitted, a  filter  of  "(objectClass=3D*)"  i=

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RFC DRAFT                                                      June 1997

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=3Dvalu=
pairs.  Each  type=3Dvalue  pair  is  a separate extension. These LDAP UR=
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-

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  pro-
cess  the URL if the extension is critical.  If an unsupported extension
is non-critical, the client MUST ignore the extension.

Extension types prefixed by "X-" or "x-" are reserved for use  in  bila-
teral  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

Note that any URL-illegal characters (e.g.,  spaces)  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  dis-
tinguished  name  for  a  client  to  use when authenticating to an LDAP
directory during resolution of an LDAP URL. Clients MAY  implement  this

The extension type is  "bindname".  The  extension  value  is  the  dis-
tinguished  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

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appropriate authentication method. Note that for  a  NULL  distinguished
name,  no  bind MAY be required to obtain anonymous access to the direc-
tory. 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. This is not specified in the URL and is
at the client's discretion.

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 discre-

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

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:


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


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RFC DRAFT                                                      June 1997

Both  of  these  URLs  correspond  to  a  base  object  search  of   the
"o=3DUniversity   of   Michigan,   c=3DUS"   entry   using   a   filter  =
"(objectclass=3D*)", 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 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:


The next example is an LDAP URL referring to all children  of  the  c=3DG=


The objectClass attribute is requested to be  returned  along  with  the
entries, and the default filter of "(objectclass=3D*)" is used.

The next example is an LDAP URL to retrieve the mail attribute  for  the
LDAP entry named "o=3DQuestion?,c=3DUS" is given below, illustrating the =
of the escaping mechanism on the reserved character '?'.


The next example illustrates the interaction between LDAP and URL  quot-
ing 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  (int=3D\00\00\00\04).  Because  the  \  character  must  b=
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.


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RFC DRAFT                                                      June 1997

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

The use of security mechanisms when processing LDAP URLs  requires  par-
ticular  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 per-
mitting 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  cir-
cumstances.   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  Michi-
gan.  This material is based upon work supported by the National Science
Foundation under Grant No. NCR-9416667. The support of both the  Univer-
sity  of Michigan and the National Science Foundation is gratefully ack-

Several people  have  made  valuable  comments  on  this  document.   In

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particular  RL  "Bob"  Morgan  and  Mark Wahl deserve special thanks for
their contributions.

9.  References

[1]  Lightweight Directory Access Protocol (v3): UTF-8 String  Represen-
     tation  of  Distinguished  Names.   M.  Wahl, S. Kille, draft-ietf-
     asid-ldapv3-dn-02.txt, March 1997.

[2]  Lightweight Directory Access Protocol (v3).  M. Wahl, T. Howes,  S.
     Kille, draft-ietf-asid-ldapv3-protocol-04.txt, March 1997.

[3]  Lightweight Directory Access Protocol (v3): Attribute Syntax Defin-
     itions.   M.  Wahl,  A.  Coulbeck,  T. Howes, S. Kille, draft-ietf-
     asid-ldapv3-attributes-04.txt, March 1997.

[4]  A String Representation of LDAP Search Filters.  T.  Howes,  draft-
     ietf-asid-ldapv3-filter.01.txt, March 1997.

[5]  Uniform Resource Locators (URL). T. Berners-Lee,  L.  Masinter,  M.
     McCahill, Request for Comment (RFC) 1738, December 1994.

[6]  Key Words for use  in  RFCs  to  Indicate  Requirement  Levels,  S.
     Bradner, RFC 2119, March 1997.

10.  Author's Address

   Tim Howes
   Netscape Communications Corp.
   501 E. Middlefield Rd.
   Mountain View, CA 94043
   +1 415 937-3419

   Mark Smith
   Netscape Communications Corp.
   501 E. Middlefield Rd.
   Mountain View, CA 94043
   +1 415 937-3477

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