IETF LDAPEXT Working Group Christopher Lukas [Editor] INTERNET-DRAFT Internet Scout Project Tim Howes Netscape Communications Corp. Michael Roszkowski Internet Scout Project Mark C. Smith Netscape Communications Corp. Mark Wahl Critial Angle, Inc. June 1999 Named Referrals in LDAP Directories 1. Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Distribution of this document is unlimited. Please send comments to the authors or the LDAPEXT mailing list, ietf-ldapext@netscape.com. Copyright Notice: Copyright (C) The Internet Society (1999). All Rights Reserved. This draft is a revision of a draft formerly published as draft-ietf- ldapext-referral-00.txt. This draft expires December 6, 1999. Howes, et al. IETF LDAPEXT Working Group [Page 1] INTERNET-DRAFT LDAPv3 Named Referrals March 1999 2. Abstract This document defines a "ref" attribute and associated "referral" object class for representing generic knowledge information in LDAP directories [RFC2251]. The attribute uses URIs [RFC1738] to represent knowledge, enabling LDAP and non-LDAP services alike to be referenced. The object class can be used to construct entries in an LDAP directory containing references to other directories or services. This document also defines procedures directory servers should follow when supporting these schema elements and when responding to requests for which the directory server does not contain the requested object but may contain some knowledge of the location of the requested object. 3. Background and intended usage The broadening of interest in LDAP directories beyond their use as front ends to 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 defines a general method of representing knowledge infor- mation in LDAP directories, based on URIs. The key words "MUST", "SHOULD", and "MAY" used in this document are to be interpreted as described in [RFC2119]. 4. The ref attribute type This section defines the ref attribute type for holding general knowledge reference information. ( 2.16.840.1.113730.3.1.34 NAME 'ref' DESC 'URL reference' EQUALITY caseExactIA5Match SYNTAX 1.3.6.1.4.1.1466.115.121.1.26 USAGE distributedOperation ) The ref attribute type has IA5 syntax and is case sensitive. The ref attribute is multivalued. Values placed in the attribute MUST conform to the specification given for the labeledURI attribute defined in [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 the ref attribute refers to an LDAPv3 server, it must be in the LDAP URI format described in [RFC2255]. Howes, et al. IETF LDAPEXT Working Group [Page 2] INTERNET-DRAFT LDAPv3 Named Referrals March 1999 When returning a referral result, the server must not return the label portion of the labeledURI as part of the referral. Only the URI portion of the ref attribute should be returned. 5. Use of the ref attribute One usage of the ref attribute is defined in this document. Other uses of the ref attribute MAY be defined in subsequent documents, or by bila- teral agreement between cooperating clients and servers. Except when the manageDsaIT control (documented in section 8 of this document) is present in the operation request, the ref attribute is not visible to clients, except as its value is returned in referrals or con- tinuation references. If the manageDsaIT control is not set, and the entry named in a request contains the ref attribute, and the entry is not the root DSE, the server returns an LDAPResult with the resultCode field set to "referral" and the referral field set to contain the value(s) of the ref attribute minus any optional trailing whitespace and labels that might be present. If the manageDsaIT control is not set, and an entry containing the ref attribute is in the scope of a one level or subtree search request, the server returns a SearchResultReference for each such entry containing the value(s) of the entry's ref attribute. When the manageDsaIT control is present in a request, the server will treat an entry containing the ref attribute as an ordinary entry, and the ref attribute as an ordinary attribute, and the server will not return referrals or continuation references corresponding to ref attri- butes. The following sections detail these usages of the ref attribute. 5.1. Named reference This use of the ref attribute is to facilitate distributed name resolu- tion or search across multiple servers. The ref attribute appears in an entry named in the referencing server. The value of the ref attribute points to the corresponding entry maintained in the referenced server. While the distinguished name in a value of the ref attribute is typi- cally that of an entry in a naming context below the naming context held by the referencing server, it is permitted to be the distinguished name of any entry. If the ref attribute is multi-valued all the DNs in the values of the ref attribute SHOULD have the same value. It is the responsibility of clients to not loop repeatedly if a naming loop is present in the directory. Administrators SHOULD avoid configuring Howes, et al. IETF LDAPEXT Working Group [Page 3] INTERNET-DRAFT LDAPv3 Named Referrals March 1999 naming loops using referrals. Clients SHOULD perform at least simple "depth-of-referral count" loop detection by incrementing a counter each time a new set of referrals is received. Clients MAY perform more sophisticated loop detection, for example not chasing the same URI twice. If an entry containing the ref attribute is immediately subordinate to the base object named in a one level search request, then the referring server MUST include a scope of "base" in any LDAP URIs returned in the corresponding SearchResultReference. 5.1.1. Scenarios The following sections contain specifications of how the ref attribute should be used in different scenarios followed by examples that illus- trate that usage. The scenarios described consist of referral operation when finding a base or target object, referral operation when performing a one level search, and referral operation when performing a subtree search. It is to be noted that, in this document, a search operation is concep- tually divided into two distinct, sequential phases: (1) finding the base object where the search is to begin, and (2) performing the search itself. The operation of the server with respect to referrals in phase (1) is almost identical to the operation of the server while finding the target object for a non-search operation. It is to also be noted that multiple ref attributes are allowed in any entry and, where these sections refer to a single ref attribute, multi- ple ref attributes may be substituted and should be processed and returned as a group in an LDAPResult or search result in the same way as described for a single attribute. The order of the returned continuation references within a result is not defined. 5.1.1.1. Example configuration Howes, et al. IETF LDAPEXT Working Group [Page 4] INTERNET-DRAFT LDAPv3 Named Referrals March 1999 |------------------------------------------------------------| | Server A | | dn: o=abc,c=us dn: o=xyz,c=us | | o: abc o: xyz | | ref: ldap://hostB/o=abc,c=us ref: ldap://hostD/o=xyz,c=us | | ref: ldap://hostC/o=abc,c=us objectclass: referral | | objectclass: referral objectclass: extensibleObject| | objectclass: extensibleObject | |____________________________________________________________| |---------------------| |---------------------| |---------------------| | Server B | | Server D | | Server C | | dn: o=abc,c=us | | dn: o=xyz,c=us | | dn: o=abc,c=us | | o: abc | | o: xyz | | o: abc | | other attributes... | | other attributes... | | other attributes... | |_____________________| |_____________________| |_____________________| In this example, Server A holds references for two entries: "o=abc,c=us" and "o=xyz,c=us". For the "o=abc,c=us" entry, Server A holds two refer- ences, one to Server B and one to Server C. The entries referenced are replicas of each other. For the "o=xyz,c=us" entry, Server A holds a single reference to the entry contained in Server D. In the following protocol interaction examples, the client has contacted Server A. Server A holds the naming context "c=us". 5.1.1.2. Base or target object considerations As previously described, the process of generating referrals for a search can be described in two phases. The first, which is described in this section, is generating referrals based on the base object specified in the search. This process is identical to the process of generating referrals based on the target object while processing other operations (modify, add, delete, modify DN, and compare) with the sole exception that for these other operations, the DN in the referral must be modified in some cases. If a client requests any of these operations, there are four cases that the server must handle with respect to the base or target object speci- fied in the request. Case 1: The base or target object is not held by the server and is not subordinate to any object held by the server with a ref attribute. The handling of this case is described in section 6. Case 2: The base or target object is held by the server and contains a ref attribute Howes, et al. IETF LDAPEXT Working Group [Page 5] INTERNET-DRAFT LDAPv3 Named Referrals March 1999 In this case, if the type of operation requested is a search or the URI contained in the ref attribute of the requested base object is NOT an LDAP URI as defined in [RFC2255], the server should return the URI value contained in the ref attribute of the base object whose DN is the DN requested by the client as the base for the operation. Example: If the client issues a search in which the base object is "o=xyz,c=us", server A will return SearchResultDone "referral" { ldap://hostD/o=xyz,c=us } If the type of operation requested is not a search and the URI contained in the ref attribute of the requested target object is an LDAP URI [RFC2255], the server should return a modified form of this URL. The returned URL must have only the protocol, host, port, and trailing "/" portion of the URL contained in the ref attribute. The server should strip any dn, attributes, scope, and filter parts of the URL. Example: If the client issues a modify request for the target object of "o=abc,c=us", server A will return ModifyResponse "referral" { ldap://hostB/ ldap://hostC/ } Case 3: The base or target object is not held by the server, but is subordinate to an object with a ref attribute held by the server. If a client requests an operation for which the base or target object is not held by the server, but is subordinate to one or more objects with a ref attribute held by the server, the server must return the referral from the superior held object nearest to the requested base or target object. Nearest superior object with a referral, in this document, means an object superior to the base or target object with the DN that has the most attribute values in common with the DN of the base or target object and contains a ref attribute. The process of finding the nearest superior object can be envisioned as walking up the locally held part of the DIT from the requested base or target object checking each superior object until either an object with Howes, et al. IETF LDAPEXT Working Group [Page 6] INTERNET-DRAFT LDAPv3 Named Referrals March 1999 a ref attribute is found or the top-most locally held object is reached. Once possible implementation of this algorithm is as follows: 1. Remove the leftmost attribute/value pair from the DN of the requested base or target object. 2. If the remaining DN represents a locally held object that contains a ref attribute, that object is the nearest superior object with a referral. Stop and process the referral as described below. 3. If the remaining DN is the root of the locally held part of the DIT, stop and proceed as described in section 6. 4. Continue with step 1. Once the nearest superior object has been identified, if the referral contained in that object is not an LDAP URI [RFC2255], it should be returned as-is. If the referral is an LDAP URI, the referral must be modified, regardless of the type of operation, as case 2 describes for a non-search requuest. That is, the dn, attributes, scope, and filter parts of the URL must be stripped from the referral and the referral returned. Example: If the client issues an add request where the target object has a DN of "cn=Chris Lukas,o=abc,c=us", server A will return AddResponse "referral" { ldap://hostB/ ldap://hostC/ } 5.1.1.3. Search with one level scope For search operations, once the base object has been found and deter- mined not to contain a ref attribute, the search may progress. Any entries matching the filter and scope of the search that do NOT contain a ref attribute are returned to the client normally as described in [RFC2251]. Any entries matching the filter and one level scope that do contain a ref attribute must be returned as referrals as described here. If a matching entry contains a ref attribute and the URI contained in the ref attribute is NOT an LDAP URI [RFC2255], the server should return the URI value contained in the ref attribute of that entry in a Sear- chResultReference. If a matching entry contains a ref attribute in the LDAP URI syntax Howes, et al. IETF LDAPEXT Working Group [Page 7] INTERNET-DRAFT LDAPv3 Named Referrals March 1999 [RFC2255], the URL from the ref attribute must be modified before it is returned by adding or substituting a "base" scope into the URL. If the URL does not contain a scope specifier, the "base" scope specifier must be added. If the URL does contain a scope specifier, the existing scope specifier must be replaced by the "base" scope. Example: If a client requests a one level search of "c=US" then, in addition to any entries one level below the "c=US" naming context matching the filter (shown below as "... SearchResultEntry responses ..."), the server will also return referrals modified to include the "base" scope to maintain the one level search semantics. The order of the SearchResultEntry responses and the SearchResultRefer- ence responses is undefined. One possible sequence is shown. ... SearchResultEntry responses ... SearchResultReference { ldap://hostB/o=abc,c=us??base ldap://hostC/o=abc,c=us??base } SearchResultReference { ldap://hostD/o=xyz,c=us??base } SearchResultDone "success" 5.1.1.4. Search with subtree scope For a search operation with a subtree scope, once the base object has been found, the search progresses. As with the one level search, any entries matching the filter and scope of the search that do NOT contain a ref attribute are returned to the client normally as described in [RFC2251]. If an entry matching the requested scope and filter contains a ref attribute, the server should return the URI value in a SearchResul- tReference. Example: If a client requests a subtree search of "c=us", then in addition to any entries in the "c=us" naming context which match the filter, Server A will also return two continuation references. As described in the Howes, et al. IETF LDAPEXT Working Group [Page 8] INTERNET-DRAFT LDAPv3 Named Referrals March 1999 preceding section, the order of the responses is not defined. One possible response might be: ... SearchResultEntry responses ... SearchResultReference { ldap://hostB/o=abc,c=us ldap://hostC/o=abc,c=us } SearchResultReference { ldap://hostD/o=xyz,c=us } SearchResultDone "success" 6. Superior Reference An LDAP server may be configured to return a superior reference in the case where the server does not hold either the requested base object or an object containing a ref attribute that is superior to that base object. An LDAP server's root DSE MAY contain a ref attribute. The values of the ref attribute in the root DSE that are LDAP URIs SHOULD NOT contain any dn part, just the host name and optional port number. If the LDAP server's root DSE contains a ref attribute and a client requests an object not held by the server and not subordinate to any held object, the server must return the URI component of the values in the ref attribute of the root DSE as illustrated in the example. If the LDAP server's root DSE does not contain a ref attribute, the server may return one or more references that the server determines via a method not defined in this document to be appropriate. The default reference may be to any server that might contain more knowledge of the namespace than the responding server. In particular, the client must not expect the superior reference to be identical from session to session as the reference may be dynamically created by the server based on the details of the query submitted by the client. When the server receives an operation for which the base or target entry of the request is not contained in or subordinate to any naming context held by the server or a referral entry, the server will return an LDAPResult with the resultCode set to "referral", and with the referral Howes, et al. IETF LDAPEXT Working Group [Page 9] INTERNET-DRAFT LDAPv3 Named Referrals March 1999 field filled with a referral that the server has determined to be appropriate. Example: If a client requests a subtree search of "c=de" from server A in the example configuration, and server A has the following ref attribute defined in it's root DSE: ref: ldap://hostG/ then server A will return SearchResultDone "referral" { ldap://hostG/ } 7. The referral object class The referral object class is defined as follows. ( 2.16.840.1.113730.3.2.6 NAME 'referral' SUP top STRUCTURAL MAY ( ref ) ) The referral object class is a subclass of top and may contain the referral attribute. The referral object class should, in general, be used in conjunction with the extensibleObject object class to support the naming attributes used in the entry's distinguished name. Servers must support the ref attribute through use of the referral object class. Any named reference must be of the referral object class and will likely also be of the extensibleObject object class to support naming and use of other attributes. 8. The manageDsaIT control A client MAY specify the following control when issuing a search, com- pare, add, delete, modify, or modifyDN 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 SHOULD be marked as critical. There is no value; the controlValue field is absent. This control causes entries with the "ref" attribute to be treated as normal entries, allowing clients to read and modify these entries. Howes, et al. IETF LDAPEXT Working Group [Page 10] INTERNET-DRAFT LDAPv3 Named Referrals March 1999 This control is not needed if the entry containing the referral attri- bute is one used for directory administrative purposes, such as the root DSE, or the server change log entries. Operations on these entries never cause referrals or continuation references to be returned. 9. Relationship to X.500 Knowledge References The X.500 standard defines several types of knowledge references, used to bind together different parts of the X.500 namespace. In X.500, knowledge references can be associated with a set of unnamed entries (e.g., a reference, associated with an entry, to a server containing the descendants of that entry). This creates a potential problem for LDAP clients resolving an LDAPv3 URL referral referring to an LDAP directory back-ended by X.500. Sup- pose the search is a subtree search, and that server A holds the base object of the search, and server B holds the descendants of the base object. The behavior of X.500(1993) subordinate references is that the base object on server A is searched, and a single continuation reference is returned pointing to all of the descendants held on server B. An LDAP URI only allows the base object to be specified. It is not pos- sible using standard LDAP URIs to indicate a search of several entries whose names are not known to the server holding the superior entry. X.500 solves this problem by having two fields, one indicating the pro- gress of name resolution and the other indicating the target of the search. In the above example, name resolution would be complete by the time the query reached server B, indicating that it should not refer the request. This document does not address this problem. This problem will be addressed in separate documents which define the changes to the X.500 distribution model and LDAPv3 extensions to indicate the progress of name resolution. 10. Security Considerations This document defines mechanisms that can be used to "glue" LDAP (and other) servers together. The information used to specify this glue information should be protected from unauthorized modification. If the server topology information itself is not public information, the infor- mation should be protected from unauthorized access as well. Clients should use caution when re-using credentials while following referrals as the client may be directed to any server which may or may not respect or use those credentials appropriately. Howes, et al. IETF LDAPEXT Working Group [Page 11] INTERNET-DRAFT LDAPv3 Named Referrals March 1999 11. References [RFC1738] Berners-Lee, T., Masinter, L., and McCahill, M., "Uniform Resource Locators (URL)", RFC 1738, CERN, Xerox Corporation, University of Minnesota, December 1994. [RFC2079] M. Smith, "Definition of an X.500 Attribute Type and an Object Class to Hold Uniform Resource Identifiers (URIs)", RFC 2079, January 1997. [RFC2119] S. Bradner, "Key Words for use in RFCs to Indicate Requirement Lev- els", RFC 2119, March 1997. (Format: TXT=4723 bytes) (Also BCP0014) (Status: BEST CURRENT PRACTICE) [RFC2251] M. Wahl, T. Howes, S. Kille, "Lightweight Directory Access Protocol (v3)", RFC 2251, December 1997. [RFC2255] T. Howes, M. Smith, "The LDAP URL Format", RFC 2255, December, 1997. (Format: TXT=20685 bytes) (Status: PROPOSED STANDARD) [X500] ITU-T Rec. X.501, "The Directory: Models", 1993. 12. Author's Address Tim Howes Netscape Communications Corp. 501 E. Middlefield Rd. Mailstop MV068 Mountain View, CA 94043 USA +1 650 937-3419 EMail: howes@netscape.com Christopher E. Lukas Internet Scout Project Computer Sciences Dept. University of Wisconsin-Madison 1210 W. Dayton St. Madison, WI 53706 USA EMail: lukas@cs.wisc.edu Howes, et al. IETF LDAPEXT Working Group [Page 12] INTERNET-DRAFT LDAPv3 Named Referrals March 1999 Michael Roszkowski Internet Scout Project Computer Sciences Dept. University of Wisconsin-Madison 1210 W. Dayton St. Madison, WI 53706 USA EMail: mfr@cs.wisc.edu Mark C. Smith Netscape Communications Corp. 501 E. Middlefield Rd. Mailstop MV068 Mountain View, CA 94043 USA EMail: mcs@netscape.com Mark Wahl Innosoft International, Inc. 8911 Capital of Texas Hwy #4140 Austin TX 78759 EMail: M.Wahl@innosoft.com This draft expires December 6, 1999. Howes, et al. IETF LDAPEXT Working Group [Page 13]