Network Working Group M. Wahl Request for Comments: 2251 Critical Angle Inc. Category: Standards Track T. Howes Netscape Communications Corp. S. Kille Isode Limited December 1997 Lightweight Directory Access Protocol (v3) 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. Wahl, et. al. Standards Track [Page 1] RFC 2251 LDAPv3 December 1997 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. Table of Contents 1. Status of this Memo .................................... 1 Copyright Notice ....................................... 1 IESG Note .............................................. 1 2. Abstract ............................................... 3 3. Models ................................................. 4 3.1. Protocol Model ........................................ 4 3.2. Data Model ............................................ 5 3.2.1. Attributes of Entries ............................... 5 3.2.2. Subschema Entries and Subentries .................... 7 3.3. Relationship to X.500 ................................. 8 3.4. Server-specific Data Requirements ..................... 8 4. Elements of Protocol ................................... 9 4.1. Common Elements ....................................... 9 4.1.1. Message Envelope .................................... 9 4.1.1.1. Message ID ........................................ 11 4.1.2. String Types ........................................ 11 4.1.3. Distinguished Name and Relative Distinguished Name .. 11 4.1.4. Attribute Type ...................................... 12 4.1.5. Attribute Description ............................... 13 4.1.5.1. Binary Option ..................................... 14 4.1.6. Attribute Value ..................................... 14 4.1.7. Attribute Value Assertion ........................... 15 4.1.8. Attribute ........................................... 15 4.1.9. Matching Rule Identifier ............................ 15 4.1.10. Result Message ..................................... 16 4.1.11. Referral ........................................... 18 4.1.12. Controls ........................................... 19 4.2. Bind Operation ........................................ 20 4.2.1. Sequencing of the Bind Request ...................... 21 4.2.2. Authentication and Other Security Services .......... 22 4.2.3. Bind Response ....................................... 23 4.3. Unbind Operation ...................................... 24 4.4. Unsolicited Notification .............................. 24 4.4.1. Notice of Disconnection ............................. 24 4.5. Search Operation ...................................... 25 Wahl, et. al. Standards Track [Page 2] RFC 2251 LDAPv3 December 1997 4.5.1. Search Request ...................................... 25 4.5.2. Search Result ....................................... 29 4.5.3. Continuation References in the Search Result ........ 31 4.5.3.1. Example ........................................... 31 4.6. Modify Operation ...................................... 32 4.7. Add Operation ......................................... 34 4.8. Delete Operation ...................................... 35 4.9. Modify DN Operation ................................... 36 4.10. Compare Operation .................................... 37 4.11. Abandon Operation .................................... 38 4.12. Extended Operation ................................... 38 5. Protocol Element Encodings and Transfer ................ 39 5.1. Mapping Onto BER-based Transport Services ............. 39 5.2. Transfer Protocols .................................... 40 5.2.1. Transmission Control Protocol (TCP) ................. 40 6. Implementation Guidelines .............................. 40 6.1. Server Implementations ................................ 40 6.2. Client Implementations ................................ 40 7. Security Considerations ................................ 41 8. Acknowledgements ....................................... 41 9. Bibliography ........................................... 41 10. Authors' Addresses ..................................... 42 Appendix A - Complete ASN.1 Definition ..................... 44 Full Copyright Statement ................................... 50 2. Abstract The protocol described in this document is designed to provide access to directories supporting the X.500 models, while not incurring the resource requirements of the X.500 Directory Access Protocol (DAP). This protocol is specifically targeted at management applications and browser applications that provide read/write interactive access to directories. When used with a directory supporting the X.500 protocols, it is intended to be a complement to the X.500 DAP. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", and "MAY" in this document are to be interpreted as described in RFC 2119 [10]. Key aspects of this version of LDAP are: - All protocol elements of LDAPv2 (RFC 1777) are supported. The protocol is carried directly over TCP or other transport, bypassing much of the session/presentation overhead of X.500 DAP. - Most protocol data elements can be encoded as ordinary strings (e.g., Distinguished Names). Wahl, et. al. Standards Track [Page 3] RFC 2251 LDAPv3 December 1997 - Referrals to other servers may be returned. - SASL mechanisms may be used with LDAP to provide association security services. - Attribute values and Distinguished Names have been internationalized through the use of the ISO 10646 character set. - The protocol can be extended to support new operations, and controls may be used to extend existing operations. - Schema is published in the directory for use by clients. 3. Models Interest in X.500 [1] directory technologies in the Internet has led to efforts to reduce the high cost of entry associated with use of these technologies. This document continues the efforts to define directory protocol alternatives, updating the LDAP [2] protocol specification. 3.1. Protocol Model The general model adopted by this protocol is one of clients performing protocol operations against servers. In this model, a client transmits a protocol request describing the operation to be performed to a server. The server is then responsible for performing the necessary operation(s) in the directory. Upon completion of the operation(s), the server returns a response containing any results or errors to the requesting client. In keeping with the goal of easing the costs associated with use of the directory, it is an objective of this protocol to minimize the complexity of clients so as to facilitate widespread deployment of applications capable of using the directory. Note that although servers are required to return responses whenever such responses are defined in the protocol, there is no requirement for synchronous behavior on the part of either clients or servers. Requests and responses for multiple operations may be exchanged between a client and server in any order, provided the client eventually receives a response for every request that requires one. In LDAP versions 1 and 2, no provision was made for protocol servers returning referrals to clients. However, for improved performance and distribution this version of the protocol permits servers to return to clients referrals to other servers. This allows servers to offload the work of contacting other servers to progress operations. Wahl, et. al. Standards Track [Page 4] RFC 2251 LDAPv3 December 1997 Note that the core protocol operations defined in this document can be mapped to a strict subset of the X.500(1997) directory abstract service, so it can be cleanly provided by the DAP. However there is not a one-to-one mapping between LDAP protocol operations and DAP operations: server implementations acting as a gateway to X.500 directories may need to make multiple DAP requests. 3.2. Data Model This section provides a brief introduction to the X.500 data model, as used by LDAP. The LDAP protocol assumes there are one or more servers which jointly provide access to a Directory Information Tree (DIT). The tree is made up of entries. Entries have names: one or more attribute values from the entry form its relative distinguished name (RDN), which MUST be unique among all its siblings. The concatenation of the relative distinguished names of the sequence of entries from a particular entry to an immediate subordinate of the root of the tree forms that entry's Distinguished Name (DN), which is unique in the tree. An example of a Distinguished Name is CN=Steve Kille, O=Isode Limited, C=GB Some servers may hold cache or shadow copies of entries, which can be used to answer search and comparison queries, but will return referrals or contact other servers if modification operations are requested. Servers which perform caching or shadowing MUST ensure that they do not violate any access control constraints placed on the data by the originating server. The largest collection of entries, starting at an entry that is mastered by a particular server, and including all its subordinates and their subordinates, down to the entries which are mastered by different servers, is termed a naming context. The root of the DIT is a DSA-specific Entry (DSE) and not part of any naming context: each server has different attribute values in the root DSE. (DSA is an X.500 term for the directory server). 3.2.1. Attributes of Entries Entries consist of a set of attributes. An attribute is a type with one or more associated values. The attribute type is identified by a short descriptive name and an OID (object identifier). The attribute Wahl, et. al. Standards Track [Page 5] RFC 2251 LDAPv3 December 1997 type governs whether there can be more than one value of an attribute of that type in an entry, the syntax to which the values must conform, the kinds of matching which can be performed on values of that attribute, and other functions. An example of an attribute is "mail". There may be one or more values of this attribute, they must be IA5 (ASCII) strings, and they are case insensitive (e.g. "foo@bar.com" will match "FOO@BAR.COM"). Schema is the collection of attribute type definitions, object class definitions and other information which a server uses to determine how to match a filter or attribute value assertion (in a compare operation) against the attributes of an entry, and whether to permit add and modify operations. The definition of schema for use with LDAP is given in [5] and [6]. Additional schema elements may be defined in other documents. Each entry MUST have an objectClass attribute. The objectClass attribute specifies the object classes of an entry, which along with the system and user schema determine the permitted attributes of an entry. Values of this attribute may be modified by clients, but the objectClass attribute cannot be removed. Servers may restrict the modifications of this attribute to prevent the basic structural class of the entry from being changed (e.g. one cannot change a person into a country). When creating an entry or adding an objectClass value to an entry, all superclasses of the named classes are implicitly added as well if not already present, and the client must supply values for any mandatory attributes of new superclasses. Some attributes, termed operational attributes, are used by servers for administering the directory system itself. They are not returned in search results unless explicitly requested by name. Attributes which are not operational, such as "mail", will have their schema and syntax constraints enforced by servers, but servers will generally not make use of their values. Servers MUST NOT permit clients to add attributes to an entry unless those attributes are permitted by the object class definitions, the schema controlling that entry (specified in the subschema - see below), or are operational attributes known to that server and used for administrative purposes. Note that there is a particular objectClass 'extensibleObject' defined in [5] which permits all user attributes to be present in an entry. Entries MAY contain, among others, the following operational attributes, defined in [5]. These attributes are maintained automatically by the server and are not modifiable by clients: Wahl, et. al. Standards Track [Page 6] RFC 2251 LDAPv3 December 1997 - creatorsName: the Distinguished Name of the user who added this entry to the directory. - createTimestamp: the time this entry was added to the directory. - modifiersName: the Distinguished Name of the user who last modified this entry. - modifyTimestamp: the time this entry was last modified. - subschemaSubentry: the Distinguished Name of the subschema entry (or subentry) which controls the schema for this entry. 3.2.2. Subschema Entries and Subentries Subschema entries are used for administering information about the directory schema, in particular the object classes and attribute types supported by directory servers. A single subschema entry contains all schema definitions used by entries in a particular part of the directory tree. Servers which follow X.500(93) models SHOULD implement subschema using the X.500 subschema mechanisms, and so these subschemas are not ordinary entries. LDAP clients SHOULD NOT assume that servers implement any of the other aspects of X.500 subschema. A server which masters entries and permits clients to modify these entries MUST implement and provide access to these subschema entries, so that its clients may discover the attributes and object classes which are permitted to be present. It is strongly recommended that all other servers implement this as well. The following four attributes MUST be present in all subschema entries: - cn: this attribute MUST be used to form the RDN of the subschema entry. - objectClass: the attribute MUST have at least the values "top" and "subschema". - objectClasses: each value of this attribute specifies an object class known to the server. - attributeTypes: each value of this attribute specifies an attribute type known to the server. These are defined in [5]. Other attributes MAY be present in subschema entries, to reflect additional supported capabilities. Wahl, et. al. Standards Track [Page 7] RFC 2251 LDAPv3 December 1997 These include matchingRules, matchingRuleUse, dITStructureRules, dITContentRules, nameForms and ldapSyntaxes. Servers SHOULD provide the attributes createTimestamp and modifyTimestamp in subschema entries, in order to allow clients to maintain their caches of schema information. Clients MUST only retrieve attributes from a subschema entry by requesting a base object search of the entry, where the search filter is "(objectClass=subschema)". (This will allow LDAPv3 servers which gateway to X.500(93) to detect that subentry information is being requested.) 3.3. Relationship to X.500 This document defines LDAP in terms of X.500 as an X.500 access mechanism. An LDAP server MUST act in accordance with the X.500(1993) series of ITU recommendations when providing the service. However, it is not required that an LDAP server make use of any X.500 protocols in providing this service, e.g. LDAP can be mapped onto any other directory system so long as the X.500 data and service model as used in LDAP is not violated in the LDAP interface. 3.4. Server-specific Data Requirements An LDAP server MUST provide information about itself and other information that is specific to each server. This is represented as a group of attributes located in the root DSE (DSA-Specific Entry), which is named with the zero-length LDAPDN. These attributes are retrievable if a client performs a base object search of the root with filter "(objectClass=*)", however they are subject to access control restrictions. The root DSE MUST NOT be included if the client performs a subtree search starting from the root. Servers may allow clients to modify these attributes. The following attributes of the root DSE are defined in section 5 of [5]. Additional attributes may be defined in other documents. - namingContexts: naming contexts held in the server. Naming contexts are defined in section 17 of X.501 [6]. - subschemaSubentry: subschema entries (or subentries) known by this server. - altServer: alternative servers in case this one is later unavailable. Wahl, et. al. Standards Track [Page 8] RFC 2251 LDAPv3 December 1997 - supportedExtension: list of supported extended operations. - supportedControl: list of supported controls. - supportedSASLMechanisms: list of supported SASL security features. - supportedLDAPVersion: LDAP versions implemented by the server. If the server does not master entries and does not know the locations of schema information, the subschemaSubentry attribute is not present in the root DSE. If the server masters directory entries under one or more schema rules, there may be any number of values of the subschemaSubentry attribute in the root DSE. 4. Elements of Protocol The LDAP protocol is described using Abstract Syntax Notation 1 (ASN.1) [3], and is typically transferred using a subset of ASN.1 Basic Encoding Rules [11]. In order to support future extensions to this protocol, clients and servers MUST ignore elements of SEQUENCE encodings whose tags they do not recognize. Note that unlike X.500, each change to the LDAP protocol other than through the extension mechanisms will have a different version number. A client will indicate the version it supports as part of the bind request, described in section 4.2. If a client has not sent a bind, the server MUST assume that version 3 is supported in the client (since version 2 required that the client bind first). Clients may determine the protocol version a server supports by reading the supportedLDAPVersion attribute from the root DSE. Servers which implement version 3 or later versions MUST provide this attribute. Servers which only implement version 2 may not provide this attribute. 4.1. Common Elements This section describes the LDAPMessage envelope PDU (Protocol Data Unit) format, as well as data type definitions which are used in the protocol operations. 4.1.1. Message Envelope For the purposes of protocol exchanges, all protocol operations are encapsulated in a common envelope, the LDAPMessage, which is defined as follows: LDAPMessage ::= SEQUENCE { Wahl, et. al. Standards Track [Page 9] RFC 2251 LDAPv3 December 1997 messageID MessageID, protocolOp CHOICE { bindRequest BindRequest, bindResponse BindResponse, unbindRequest UnbindRequest, searchRequest SearchRequest, searchResEntry SearchResultEntry, searchResDone SearchResultDone, searchResRef SearchResultReference, modifyRequest ModifyRequest, modifyResponse ModifyResponse, addRequest AddRequest, addResponse AddResponse, delRequest DelRequest, delResponse DelResponse, modDNRequest ModifyDNRequest, modDNResponse ModifyDNResponse, compareRequest CompareRequest, compareResponse CompareResponse, abandonRequest AbandonRequest, extendedReq ExtendedRequest, extendedResp ExtendedResponse }, controls [0] Controls OPTIONAL } MessageID ::= INTEGER (0 .. maxInt) maxInt INTEGER ::= 2147483647 -- (2^^31 - 1) -- The function of the LDAPMessage is to provide an envelope containing common fields required in all protocol exchanges. At this time the only common fields are the message ID and the controls. If the server receives a PDU from the client in which the LDAPMessage SEQUENCE tag cannot be recognized, the messageID cannot be parsed, the tag of the protocolOp is not recognized as a request, or the encoding structures or lengths of data fields are found to be incorrect, then the server MUST return the notice of disconnection described in section 4.4.1, with resultCode protocolError, and immediately close the connection. In other cases that the server cannot parse the request received by the client, the server MUST return an appropriate response to the request, with the resultCode set to protocolError. If the client receives a PDU from the server which cannot be parsed, the client may discard the PDU, or may abruptly close the connection. The ASN.1 type Controls is defined in section 4.1.12. Wahl, et. al. Standards Track [Page 10] RFC 2251 LDAPv3 December 1997 4.1.1.1. Message ID All LDAPMessage envelopes encapsulating responses contain the messageID value of the corresponding request LDAPMessage. The message ID of a request MUST have a value different from the values of any other requests outstanding in the LDAP session of which this message is a part. A client MUST NOT send a second request with the same message ID as an earlier request on the same connection if the client has not received the final response from the earlier request. Otherwise the behavior is undefined. Typical clients increment a counter for each request. A client MUST NOT reuse the message id of an abandonRequest or of the abandoned operation until it has received a response from the server for another request invoked subsequent to the abandonRequest, as the abandonRequest itself does not have a response. 4.1.2. String Types The LDAPString is a notational convenience to indicate that, although strings of LDAPString type encode as OCTET STRING types, the ISO 10646 [13] character set (a superset of Unicode) is used, encoded following the UTF-8 algorithm [14]. Note that in the UTF-8 algorithm characters which are the same as ASCII (0x0000 through 0x007F) are represented as that same ASCII character in a single byte. The other byte values are used to form a variable-length encoding of an arbitrary character. LDAPString ::= OCTET STRING The LDAPOID is a notational convenience to indicate that the permitted value of this string is a (UTF-8 encoded) dotted-decimal representation of an OBJECT IDENTIFIER. LDAPOID ::= OCTET STRING For example, 1.3.6.1.4.1.1466.1.2.3 4.1.3. Distinguished Name and Relative Distinguished Name An LDAPDN and a RelativeLDAPDN are respectively defined to be the representation of a Distinguished Name and a Relative Distinguished Name after encoding according to the specification in [4], such that Wahl, et. al. Standards Track [Page 11] RFC 2251 LDAPv3 December 1997 ::= ::= where and are as defined in [4]. LDAPDN ::= LDAPString RelativeLDAPDN ::= LDAPString Only Attribute Types can be present in a relative distinguished name component; the options of Attribute Descriptions (next section) MUST NOT be used in specifying distinguished names. 4.1.4. Attribute Type An AttributeType takes on as its value the textual string associated with that AttributeType in its specification. AttributeType ::= LDAPString Each attribute type has a unique OBJECT IDENTIFIER which has been assigned to it. This identifier may be written as decimal digits with components separated by periods, e.g. "2.5.4.10". A specification may also assign one or more textual names for an attribute type. These names MUST begin with a letter, and only contain ASCII letters, digit characters and hyphens. They are case insensitive. (These ASCII characters are identical to ISO 10646 characters whose UTF-8 encoding is a single byte between 0x00 and 0x7F.) If the server has a textual name for an attribute type, it MUST use a textual name for attributes returned in search results. The dotted- decimal OBJECT IDENTIFIER is only used if there is no textual name for an attribute type. Attribute type textual names are non-unique, as two different specifications (neither in standards track RFCs) may choose the same name. A server which masters or shadows entries SHOULD list all the attribute types it supports in the subschema entries, using the attributeTypes attribute. Servers which support an open-ended set of attributes SHOULD include at least the attributeTypes value for the 'objectClass' attribute. Clients MAY retrieve the attributeTypes value from subschema entries in order to obtain the OBJECT IDENTIFIER and other information associated with attribute types. Wahl, et. al. Standards Track [Page 12] RFC 2251 LDAPv3 December 1997 Some attribute type names which are used in this version of LDAP are described in [5]. Servers may implement additional attribute types. 4.1.5. Attribute Description An AttributeDescription is a superset of the definition of the AttributeType. It has the same ASN.1 definition, but allows additional options to be specified. They are also case insensitive. AttributeDescription ::= LDAPString A value of AttributeDescription is based on the following BNF: ::= [ ";" ] ::=