LDAP Dereference Control

Cross-links between entries are often used to describe relationships between entries. To exploit the uniqueness of entries naming, these links are usually represented by the distinguished name (DN) of the linked entries. In many cases, DUAs need to collect information about linked entries. This requires to explicitly dereference each linked entry in order to collect the desired attributes, resulting in the need to perform a specific sequence of search operations, using the links as search base, with a SearchRequest.scope of baseObject . This document describes a LDAP Control that allows a DUA to request the DSA to return specific attributes of linked entries along with the link, under the assumption that this operation can be performed by the DSA in a more efficient manner than the DUA would itself by performing the complete sequence of required search operations. 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 .

This control allows specifying a dereference attribute and a set of attributes to be dereferenced, as illustrated in . The dereference attribute's syntax MUST be 1.3.6.1.4.1.1466.115.121.1.12 (DN) . Each value of the dereference attribute in a SearchResultEntry SHOULD result in dereferencing the corresponding entry, collecting the values of the attributes to be dereferenced, and returning them as part of the control value in the SearchResultEntry response, in the format detailed in . The control value may contain dereference attribute values without any dereferenced attribute values, as detailed in . The control semantics does not specify whether this is a consequence of a dangling link or of the application of access restrictions on the values of the attributes to be dereferenced. Attribute description hierarchy SHALL NOT be exploited when collecting the values of the attributes to be dereferenced. On the contrary, all of the attribute descriptions in an attribute hierarchy SHOULD be treated as distinct and unrelated descriptions. This control is only appropriate for the search operation . The semantics of the criticality field are specified in . In detail, the criticality of the control determines whether the control will or will not be used, and if it will not be used, whether the operation will continue without returning the control in the response, or fail, returning unavailableCriticalExtension. If the control is appropriate for an operation and, for any reason, it cannot be applied in its entirety to a single SearchResultEntry response, it MUST NOT be applied to that specific SearchResultEntry response, without affecting its application to any subsequent SearchResultEntry response. Servers implementing this technical specification SHOULD publish the object identifier deref-oid (IANA assigned; see ) as a value of the 'supportedControl' attribute in their root DSE. This control is totally unrelated to alias dereferencing .

The control type is deref-oid (IANA assigned; see ). The specification of the Dereference Control request is: controlValue ::= SEQUENCE OF derefSpec DerefSpec DerefSpec ::= SEQUENCE { derefAttr attributeDescription, ; with DN syntax attributes AttributeList } AttributeList ::= SEQUENCE OF attr AttributeDescription Each derefSpec.derefAttr MUST be unique within controlValue.

The control type is deref-oid (IANA assigned; see ). The specification of the Dereference Control response is: controlValue ::= SEQUENCE OF derefRes DerefRes DerefRes ::= SEQUENCE { derefAttr AttributeDescription, derefVal LDAPDN, attrVals [0] PartialAttributeList OPTIONAL } PartialAttributeList ::= SEQUENCE OF partialAttribute PartialAttribute

PartialAttribute is defined in ; the definition is reported here for clarity: PartialAttribute ::= SEQUENCE { type AttributeDescription, vals SET OF value AttributeValue } If partialAttribute.vals is empty, the corresponding partialAttribute is omitted. If all partialAttribute.vals in attrVals are empty, that derefRes.attrVals is omitted.

Given these entries: dn: cn=Howard Chu,ou=people,dc=example,dc=org objectClass: inetOrgPerson cn: Howard Chu sn: Chu uid: hyc dn: cn=Pierangelo Masarati,ou=people,dc=example,dc=org objectClass: inetOrgPerson cn: Pierangelo Masarati sn: Masarati uid: ando dn: cn=Test Group,ou=groups,dc=example,dc=org objectClass: groupOfNames cn: Test Group member: cn=Howard Chu,ou=people,dc=example,dc=org member: cn=Pierangelo Masarati,ou=people,dc=example,dc=org

A search could be performed with a Dereference request control value specified as { member, uid }

and the "cn=Test Group" entry would be returned with the response control value { { member, cn=Howard Chu,ou=people,dc=example,dc=org, { { uid, [hyc] } } }, { member, cn=Pierangelo Masarati,ou=people,dc=example,dc=org, { { uid, [ando] } } } }

This LDAP extension is currently implemented in OpenLDAP software using the temporary OID 1.3.6.1.4.1.4203.666.5.16 under OpenLDAP's experimental OID arc.

The control result MUST NOT disclose information the client's identity could not have accessed by performing the related search operations. The presence of a derefRes.derefVal in the response control, with no derefRes.attrVals, does not imply neither the existence of nor any access privilege to the corresponding entry. It is merely a consequence of the read access the client's identity has on the corresponding value of the derefRes.derefAttr that would be returned as part of the attributes of a SearchResultEntry response . Security considerations described in documents listed in apply.

It is requested that IANA register upon Standards Action an LDAP Object Identifier for use in this technical specification. Subject: Request for LDAP OID Registration Person & email address to contact for further information: Pierangelo Masarati <ando@OpenLDAP.org> Specification: (I-D) Author/Change Controller: IESG Comments: Identifies the LDAP Dereference Control request and response

TBD