Network Working Group K. Zeilenga
Request for Comments: 3296 OpenLDAP Foundation
Category: Standards Track July 2002
Named Subordinate References in
Lightweight Directory Access Protocol (LDAP) Directories
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 (2002). All Rights Reserved.
Abstract
This document details schema and protocol elements for representing
and managing named subordinate references in Lightweight Directory
Access Protocol (LDAP) Directories.
Conventions
Schema definitions are provided using LDAPv3 description formats
[RFC2252]. Definitions provided here are formatted (line wrapped)
for readability.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" used in
this document are to be interpreted as described in BCP 14 [RFC2119].
1. Background and Intended Usage
The broadening of interest in LDAP (Lightweight Directory Access
Protocol) [RFC2251] directories beyond their use as front ends to
X.500 [X.500] directories has created a need to represent knowledge
information in a more general way. Knowledge information is
information about one or more servers maintained in another server,
used to link servers and services together.
This document details schema and protocol elements for representing
and manipulating named subordinate references in LDAP directories. A
referral object is used to hold subordinate reference information in
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the directory. These referral objects hold one or more URIs
[RFC2396] contained in values of the ref attribute type and are used
to generate protocol referrals and continuations.
A control, ManageDsaIT, is defined to allow manipulation of referral
and other special objects as normal objects. As the name of control
implies, it is intended to be analogous to the ManageDsaIT service
option described in X.511(97) [X.511].
Other forms of knowledge information are not detailed by this
document. These forms may be described in subsequent documents.
This document details subordinate referral processing requirements
for servers. This document does not describe protocol syntax and
semantics. This is detailed in RFC 2251 [RFC2251].
This document does not detail use of subordinate knowledge references
to support replicated environments nor distributed operations (e.g.,
chaining of operations from one server to other servers).
2. Schema
2.1. The referral Object Class
A referral object is a directory entry whose structural object class
is (or is derived from) the referral object class.
( 2.16.840.1.113730.3.2.6
NAME 'referral'
DESC 'named subordinate reference object'
STRUCTURAL
MUST ref )
The referral object class is a structural object class used to
represent a subordinate reference in the directory. The referral
object class SHOULD be used in conjunction with the extensibleObject
object class to support the naming attributes used in the entry's
Distinguished Name (DN) [RFC2253].
Referral objects are normally instantiated at DSEs immediately
subordinate to object entries within a naming context held by the
DSA. Referral objects are analogous to X.500 subordinate knowledge
(subr) DSEs [X.501].
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In the presence of a ManageDsaIT control, referral objects are
treated as normal entries as described in section 3. Note that the
ref attribute is operational and will only be returned in a search
entry response when requested.
In the absence of a ManageDsaIT control, the content of referral
objects are used to construct referrals and search references as
described in Section 4 and, as such, the referral entries are not
themselves visible to clients.
2.2 The ref Attribute Type
( 2.16.840.1.113730.3.1.34
NAME 'ref'
DESC 'named reference - a labeledURI'
EQUALITY caseExactMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.15
USAGE distributedOperation )
The ref attribute type has directoryString syntax and is case
sensitive. The ref attribute is multi-valued. Values placed in the
attribute MUST conform to the specification given for the labeledURI
attribute [RFC2079]. The labeledURI specification defines a format
that is a URI, optionally followed by whitespace and a label. This
document does not make use of the label portion of the syntax.
Future documents MAY enable new functionality by imposing additional
structure on the label portion of the syntax as it appears in the ref
attribute.
If the URI contained in a ref attribute value refers to a LDAP
[RFC2251] server, it MUST be in the form of a LDAP URL [RFC2255].
The LDAP URL SHOULD NOT contain an explicit scope specifier, filter,
attribute description list, or any extensions. The LDAP URL SHOULD
contain a non-empty DN. The handling of LDAP URLs with absent or
empty DN parts or with explicit scope specifier is not defined by
this specification.
Other URI schemes MAY be used so long as all operations returning
referrals based upon the value could be performed. This document
does not detail use of non-LDAP URIs. This is left to future
specifications.
The referential integrity of the URI SHOULD NOT be validated by the
server holding or returning the URI (whether as a value of the
attribute or as part of a referral result or search reference
response).
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When returning a referral result or search continuation, the server
MUST NOT return the separator or label portions of the attribute
values as part of the reference. When the attribute contains
multiple values, the URI part of each value is used to construct the
referral result or search continuation.
The ref attribute values SHOULD NOT be used as a relative name-
component of an entry's DN [RFC2253].
This document uses the ref attribute in conjunction with the referral
object class to represent subordinate references. The ref attribute
may be used for other purposes as defined by other documents.
3. The ManageDsaIT Control
The client may provide the ManageDsaIT control with an operation to
indicate that the operation is intended to manage objects within the
DSA (server) Information Tree. The control causes Directory-specific
entries (DSEs), regardless of type, to be treated as normal entries
allowing clients to interrogate and update these entries using LDAP
operations.
A client MAY specify the following control when issuing an add,
compare, delete, modify, modifyDN, search request or an extended
operation for which the control is defined.
The control type is 2.16.840.1.113730.3.4.2. The control criticality
may be TRUE or, if FALSE, absent. The control value is absent.
When the control is present in the request, the server SHALL NOT
generate a referral or continuation reference based upon information
held in referral objects and instead SHALL treat the referral object
as a normal entry. The server, however, is still free to return
referrals for other reasons. When not present, referral objects
SHALL be handled as described above.
The control MAY cause other objects to be treated as normal entries
as defined by subsequent documents.
4. Named Subordinate References
A named subordinate reference is constructed by instantiating a
referral object in the referencing server with ref attribute values
which point to the corresponding subtree maintained in the referenced
server. In general, the name of the referral object is the same as
the referenced object and this referenced object is a context prefix
[X.501].
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That is, if server A holds "DC=example,DC=net" and server B holds
"DC=sub,DC=example,DC=net", server A may contain a referral object
named "DC=sub,DC=example,DC=net" which contains a ref attribute with
value of "ldap://B/DC=sub,DC=example,DC=net".
dn: DC=sub,DC=example,DC=net
dc: sub
ref: ldap://B/DC=sub,DC=example,DC=net
objectClass: referral
objectClass: extensibleObject
Typically the DN of the referral object and the DN of the object in
the referenced server are the same.
If the ref attribute has multiple values, all the DNs contained
within the LDAP URLs SHOULD be equivalent. Administrators SHOULD
avoid configuring naming loops using referrals.
Named references MUST be treated as normal entries if the request
includes the ManageDsaIT control as described in section 3.
5. Scenarios
The following sections contain specifications of how referral objects
should be used in different scenarios followed by examples that
illustrate that usage. The scenarios described here consist of
referral object handling when finding target of a non-search
operation, when finding the base of a search operation, and when
generating search references. Lastly, other operation processing
considerations are presented.
It is to be noted that, in this document, a search operation is
conceptually divided into two distinct, sequential phases: (1)
finding the base object where the search is to begin, and (2)
performing the search itself. The first phase is similar to, but not
the same as, finding the target of a non-search operation.
It should also be noted that the ref attribute may have multiple
values and, where these sections refer to a single ref attribute
value, multiple ref attribute values may be substituted and SHOULD be
processed and returned (in any order) as a group in a referral or
search reference in the same way as described for a single ref
attribute value.
Search references returned for a given request may be returned in any
order.
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5.1. Example Configuration
For example, suppose the contacted server (hosta) holds the entry
"O=MNN,C=WW" and the entry "CN=Manager,O=MNN,C=WW" and the following
referral objects:
dn: OU=People,O=MNN,C=WW
ou: People
ref: ldap://hostb/OU=People,O=MNN,C=US
ref: ldap://hostc/OU=People,O=MNN,C=US
objectClass: referral
objectClass: extensibleObject
dn: OU=Roles,O=MNN,C=WW
ou: Roles
ref: ldap://hostd/OU=Roles,O=MNN,C=WW
objectClass: referral
objectClass: extensibleObject
The first referral object provides the server with the knowledge that
subtree "OU=People,O=MNN,C=WW" is held by hostb and hostc (e.g., one
is the master and the other a shadow). The second referral object
provides the server with the knowledge that the subtree
"OU=Roles,O=MNN,C=WW" is held by hostd.
Also, in the context of this document, the "nearest naming context"
means the deepest context which the object is within. That is, if
the object is within multiple naming contexts, the nearest naming
context is the one which is subordinate to all other naming contexts
the object is within.
5.2. Target Object Considerations
This section details referral handling for add, compare, delete,
modify, and modify DN operations. If the client requests any of
these operations, there are four cases that the server must handle
with respect to the target object.
The DN part MUST be modified such that it refers to the appropriate
target in the referenced server (as detailed below). Even where the
DN to be returned is the same as the target DN, the DN part SHOULD
NOT be trimmed.
In cases where the URI to be returned is a LDAP URL, the server
SHOULD trim any present scope, filter, or attribute list from the URI
before returning it. Critical extensions MUST NOT be trimmed or
modified.
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Case 1: The target object is not held by the server and is not within
or subordinate to any naming context nor subordinate to any
referral object held by the server.
The server SHOULD process the request normally as appropriate for
a non-existent base which is not within any naming context of the
server (generally return noSuchObject or a referral based upon
superior knowledge reference information). This document does not
detail management or processing of superior knowledge reference
information.
Case 2: The target object is held by the server and is a referral
object.
The server SHOULD return the URI value contained in the ref
attribute of the referral object appropriately modified as
described above.
Example: If the client issues a modify request for the target object
of "OU=People,O=MNN,c=WW", the server will return:
ModifyResponse (referral) {
ldap://hostb/OU=People,O=MNN,C=WW
ldap://hostc/OU=People,O=MNN,C=WW
}
Case 3: The target object is not held by the server, but the nearest
naming context contains no referral object which the target object
is subordinate to.
If the nearest naming context contains no referral object which
the target is subordinate to, the server SHOULD process the
request as appropriate for a nonexistent target (generally return
noSuchObject).
Case 4: The target object is not held by the server, but the nearest
naming context contains a referral object which the target object
is subordinate to.
If a client requests an operation for which the target object is
not held by the server and the nearest naming context contains a
referral object which the target object is subordinate to, the
server SHOULD return a referral response constructed from the URI
portion of the ref value of the referral object.
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Example: If the client issues an add request where the target object
has a DN of "CN=Manager,OU=Roles,O=MNN,C=WW", the server will
return:
AddResponse (referral) {
ldap://hostd/CN=Manager,OU=Roles,O=MNN,C=WW"
}
Note that the DN part of the LDAP URL is modified such that it
refers to the appropriate entry in the referenced server.
5.3. Base Object Considerations
This section details referral handling for base object processing
within search operations. Like target object considerations for
non-search operations, there are the four cases.
In cases where the URI to be returned is a LDAP URL, the server MUST
provide an explicit scope specifier from the LDAP URL prior to
returning it. In addition, the DN part MUST be modified such that it
refers to the appropriate target in the referenced server (as
detailed below).
If aliasing dereferencing was necessary in finding the referral
object, the DN part of the URI MUST be replaced with the base DN as
modified by the alias dereferencing such that the return URL refers
to the new target object per [RFC2251, 4.1.11].
Critical extensions MUST NOT be trimmed nor modified.
Case 1: The base object is not held by the server and is not within
nor subordinate to any naming context held by the server.
The server SHOULD process the request normally as appropriate for
a non-existent base which not within any naming context of the
server (generally return a superior referral or noSuchObject).
This document does not detail management or processing of superior
knowledge references.
Case 2: The base object is held by the server and is a referral
object.
The server SHOULD return the URI value contained in the ref
attribute of the referral object appropriately modified as
described above.
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Example: If the client issues a subtree search in which the base
object is "OU=Roles,O=MNN,C=WW", the server will return
SearchResultDone (referral) {
ldap://hostd/OU=Roles,O=MNN,C=WW??sub
}
If the client were to issue a base or oneLevel search instead of
subtree, the returned LDAP URL would explicitly specify "base" or
"one", respectively, instead of "sub".
Case 3: The base object is not held by the server, but the nearest
naming context contains no referral object which the base object
is subordinate to.
If the nearest naming context contains no referral object which
the base is subordinate to, the request SHOULD be processed
normally as appropriate for a nonexistent base (generally return
noSuchObject).
Case 4: The base object is not held by the server, but the nearest
naming context contains a referral object which the base object is
subordinate to.
If a client requests an operation for which the target object is
not held by the server and the nearest naming context contains a
referral object which the target object is subordinate to, the
server SHOULD return a referral response which is constructed from
the URI portion of the ref value of the referral object.
Example: If the client issues a base search request for
"CN=Manager,OU=Roles,O=MNN,C=WW", the server will return
SearchResultDone (referral) {
ldap://hostd/CN=Manager,OU=Roles,O=MNN,C=WW??base"
}
If the client were to issue a subtree or oneLevel search instead
of subtree, the returned LDAP URL would explicitly specify "sub"
or "one", respectively, instead of "base".
Note that the DN part of the LDAP URL is modified such that it
refers to the appropriate entry in the referenced server.
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5.4. Search Continuation Considerations
For search operations, once the base object has been found and
determined not to be a referral object, the search may progress. Any
entry matching the filter and scope of the search which is not a
referral object is returned to the client normally as described in
[RFC2251].
For each referral object within the requested scope, regardless of
the search filter, the server SHOULD return a SearchResultReference
which is constructed from the URI component of values of the ref
attribute. If the URI component is not a LDAP URL, it should be
returned as is. If the LDAP URL's DN part is absent or empty, the DN
part must be modified to contain the DN of the referral object. If
the URI component is a LDAP URL, the URI SHOULD be modified to add an
explicit scope specifier.
Subtree Example:
If a client requests a subtree search of "O=MNN,C=WW", then in
addition to any entries within scope which match the filter, hosta
will also return two search references as the two referral objects
are within scope. One possible response might be:
SearchEntry for O=MNN,C=WW
SearchResultReference {
ldap://hostb/OU=People,O=MNN,C=WW??sub
ldap://hostc/OU=People,O=MNN,C=WW??sub
}
SearchEntry for CN=Manager,O=MNN,C=WW
SearchResultReference {
ldap://hostd/OU=Roles,O=MNN,C=WW??sub
}
SearchResultDone (success)
One Level Example:
If a client requests a one level search of "O=MNN,C=WW" then, in
addition to any entries one level below the "O=MNN,C=WW" entry
matching the filter, the server will also return two search
references as the two referral objects are within scope. One
possible sequence is shown:
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SearchResultReference {
ldap://hostb/OU=People,O=MNN,C=WW??base
ldap://hostc/OU=People,O=MNN,C=WW??base
}
SearchEntry for CN=Manager,O=MNN,C=WW
SearchResultReference {
ldap://hostd/OU=Roles,O=MNN,C=WW??base
}
SearchResultDone (success)
Note: Unlike the examples in Section 4.5.3.1 of RFC 2251, the LDAP
URLs returned with the SearchResultReference messages contain, as
required by this specification, an explicit scope specifier.
5.6. Other Considerations
This section details processing considerations for other operations.
5.6.1 Bind
Servers SHOULD NOT return referral result code if the bind name (or
authentication identity or authorization identity) is (or is
subordinate to) a referral object but MAY use the knowledge
information to process the bind request (such as in support a future
distributed operation specification). Where the server makes no use
of the knowledge information, the server processes the request
normally as appropriate for a non-existent authentication or
authorization identity (e.g., return invalidCredentials).
5.6.2 Modify DN
If the newSuperior is a referral object or is subordinate to a
referral object, the server SHOULD return affectsMultipleDSAs. If
the newRDN already exists but is a referral object, the server SHOULD
return affectsMultipleDSAs instead of entryAlreadyExists.
6. Security Considerations
This document defines mechanisms that can be used to tie LDAP (and
other) servers together. The information used to tie services
together should be protected from unauthorized modification. If the
server topology information is not public information, it should be
protected from unauthorized disclosure as well.
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7. Acknowledgments
This document borrows heavily from previous work by IETF LDAPext
Working Group. In particular, this document is based upon "Named
Referral in LDAP Directories" (an expired Internet Draft) by
Christopher Lukas, Tim Howes, Michael Roszkowski, Mark C. Smith, and
Mark Wahl.
8. Normative References
[RFC2079] Smith, M., "Definition of an X.500 Attribute Type and an
Object Class to Hold Uniform Resource Identifiers (URIs)",
RFC 2079, January 1997.
[RFC2119] Bradner, S., "Key Words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2251] Wahl, M., Howes, T. and S. Kille, "Lightweight Directory
Access Protocol (v3)", RFC 2251, December 1997.
[RFC2252] Wahl, M., Coulbeck, A., Howes, T. and S. Kille,
"Lightweight Directory Access Protocol (v3): Attribute
Syntax Definitions", RFC 2252, December 1997.
[RFC2253] Wahl, M., Kille, S. and T. Howes, "Lightweight Directory
Access Protocol (v3): UTF-8 String Representation of
Distinguished Names", RFC 2253, December 1997.
[RFC2255] Howes, T. and M. Smith, "The LDAP URL Format", RFC 2255,
December, 1997.
[RFC2396] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396,
August 1998.
[X.501] ITU-T, "The Directory: Models", X.501, 1993.
9. Informative References
[X.500] ITU-T, "The Directory: Overview of Concepts, Models, and
Services", X.500, 1993.
[X.511] ITU-T, "The Directory: Abstract Service Definition", X.500,
1997.
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10. Author's Address
Kurt D. Zeilenga
OpenLDAP Foundation
EMail: [email protected]
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11. Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
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The limited permissions granted above are perpetual and will not be
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This document and the information contained herein is provided on an
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TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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