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RFC 2830 - Lightweight Directory Access Protocol (v3): Extension


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Network Working Group                                          J. Hodges
Request for Comments: 2830                                    Oblix Inc.
Category: Standards Track                                      R. Morgan
                                                      Univ of Washington
                                                                 M. Wahl
                                                  Sun Microsystems, Inc.
                                                                May 2000

              Lightweight Directory Access Protocol (v3):
                 Extension for Transport Layer Security

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 (2000).  All Rights Reserved.

Abstract

   This document defines the "Start Transport Layer Security (TLS)
   Operation" for LDAP [LDAPv3, TLS]. This operation provides for TLS
   establishment in an LDAP association and is defined in terms of an
   LDAP extended request.

1.  Conventions Used in this Document

   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 [ReqsKeywords].

2.  The Start TLS Request

   This section describes the Start TLS extended request and extended
   response themselves: how to form the request, the form of the
   response, and enumerates the various result codes the client MUST be
   prepared to handle.

   The section following this one then describes how to sequence an
   overall Start TLS Operation.

2.1.  Requesting TLS Establishment

   A client may perform a Start TLS operation by transmitting an LDAP
   PDU containing an ExtendedRequest [LDAPv3] specifying the OID for the
   Start TLS operation:

     1.3.6.1.4.1.1466.20037

   An LDAP ExtendedRequest is defined as follows:

     ExtendedRequest ::= [APPLICATION 23] SEQUENCE {
             requestName             [0] LDAPOID,
             requestValue            [1] OCTET STRING OPTIONAL }

   A Start TLS extended request is formed by setting the requestName
   field to the OID string given above.  The requestValue field is
   absent.  The client MUST NOT send any PDUs on this connection
   following this request until it receives a Start TLS extended
   response.

   When a Start TLS extended request is made, the server MUST return an
   LDAP PDU containing a Start TLS extended response.  An LDAP
   ExtendedResponse is defined as follows:

     ExtendedResponse ::= [APPLICATION 24] SEQUENCE {
             COMPONENTS OF LDAPResult,
             responseName     [10] LDAPOID OPTIONAL,
             response         [11] OCTET STRING OPTIONAL }

   A Start TLS extended response MUST contain a responseName field which
   MUST be set to the same string as that in the responseName field
   present in the Start TLS extended request. The response field is
   absent. The server MUST set the resultCode field to either success or
   one of the other values outlined in section 2.3.

2.2.  "Success" Response

   If the ExtendedResponse contains a resultCode of success, this
   indicates that the server is willing and able to negotiate TLS. Refer
   to section 3, below, for details.

2.3.  Response other than "success"

   If the ExtendedResponse contains a resultCode other than success,
   this indicates that the server is unwilling or unable to negotiate
   TLS.

   If the Start TLS extended request was not successful, the resultCode
   will be one of:

   operationsError  (operations sequencing incorrect; e.g. TLS already
                    established)

   protocolError    (TLS not supported or incorrect PDU structure)

   referral         (this server doesn't do TLS, try this one)

   unavailable      (e.g. some major problem with TLS, or server is
                    shutting down)

   The server MUST return operationsError if the client violates any of
   the Start TLS extended operation sequencing requirements described in
   section 3, below.

   If the server does not support TLS (whether by design or by current
   configuration), it MUST set the resultCode to protocolError (see
   section 4.1.1 of [LDAPv3]), or to referral. The server MUST include
   an actual referral value in the LDAP Result if it returns a
   resultCode of referral. The client's current session is unaffected if
   the server does not support TLS. The client MAY proceed with any LDAP
   operation, or it MAY close the connection.

   The server MUST return unavailable if it supports TLS but cannot
   establish a TLS connection for some reason, e.g. the certificate
   server not responding, it cannot contact its TLS implementation, or
   if the server is in process of shutting down. The client MAY retry
   the StartTLS operation, or it MAY proceed with any other LDAP
   operation, or it MAY close the connection.

3.  Sequencing of the Start TLS Operation

   This section describes the overall procedures clients and servers
   MUST follow for TLS establishment. These procedures take into
   consideration various aspects of the overall security of the LDAP
   association including discovery of resultant security level and
   assertion of the client's authorization identity.

   Note that the precise effects, on a client's authorization identity,
   of establishing TLS on an LDAP association are described in detail in
   section 5.

3.1.  Requesting to Start TLS on an LDAP Association

   The client MAY send the Start TLS extended request at any time after
   establishing an LDAP association, except that in the following cases
   the client MUST NOT send a Start TLS extended request:

     - if TLS is currently established on the connection, or
     - during a multi-stage SASL negotiation, or
     - if there are any LDAP operations outstanding on the connection.

   The result of violating any of these requirements is a resultCode of
   operationsError, as described above in section 2.3.

   The client MAY have already performed a Bind operation when it sends
   a Start TLS request, or the client might have not yet bound.

   If the client did not establish a TLS connection before sending any
   other requests, and the server requires the client to establish a TLS
   connection before performing a particular request, the server MUST
   reject that request with a confidentialityRequired or
   strongAuthRequired result. The client MAY send a Start TLS extended
   request, or it MAY choose to close the connection.

3.2.  Starting TLS

   The server will return an extended response with the resultCode of
   success if it is willing and able to negotiate TLS.  It will return
   other resultCodes, documented above, if it is unable.

   In the successful case, the client, which has ceased to transfer LDAP
   requests on the connection, MUST either begin a TLS negotiation or
   close the connection. The client will send PDUs in the TLS Record
   Protocol directly over the underlying transport connection to the
   server to initiate TLS negotiation [TLS].

3.3.  TLS Version Negotiation

   Negotiating the version of TLS or SSL to be used is a part of the TLS
   Handshake Protocol, as documented in [TLS]. Please refer to that
   document for details.

3.4.  Discovery of Resultant Security Level

   After a TLS connection is established on an LDAP association, both
   parties MUST individually decide whether or not to continue based on
   the privacy level achieved. Ascertaining the TLS connection's privacy
   level is implementation dependent, and accomplished by communicating
   with one's respective local TLS implementation.

   If the client or server decides that the level of authentication or
   privacy is not high enough for it to continue, it SHOULD gracefully
   close the TLS connection immediately after the TLS negotiation has
   completed (see sections 4.1 and 5.2, below).

   The client MAY attempt to Start TLS again, or MAY send an unbind
   request, or send any other LDAP request.

3.5.  Assertion of Client's Authorization Identity

   The client MAY, upon receipt of a Start TLS extended response
   indicating success, assert that a specific authorization identity be
   utilized in determining the client's authorization status. The client
   accomplishes this via an LDAP Bind request specifying a SASL
   mechanism of "EXTERNAL" [SASL]. See section 5.1.2, below.

3.6.  Server Identity Check

   The client MUST check its understanding of the server's hostname
   against the server's identity as presented in the server's
   Certificate message, in order to prevent man-in-the-middle attacks.

   Matching is performed according to these rules:

   - The client MUST use the server hostname it used to open the LDAP
     connection as the value to compare against the server name as
     expressed in the server's certificate.  The client MUST NOT use the
     server's canonical DNS name or any other derived form of name.

   - If a subjectAltName extension of type dNSName is present in the
     certificate, it SHOULD be used as the source of the server's
     identity.

   - Matching is case-insensitive.

   - The "*" wildcard character is allowed.  If present, it applies only
     to the left-most name component.

   E.g. *.bar.com would match a.bar.com, b.bar.com, etc. but not
   bar.com.  If more than one identity of a given type is present in the
   certificate (e.g. more than one dNSName name), a match in any one of
   the set is considered acceptable.

   If the hostname does not match the dNSName-based identity in the
   certificate per the above check, user-oriented clients SHOULD either
   notify the user (clients MAY give the user the opportunity to

   continue with the connection in any case) or terminate the connection
   and indicate that the server's identity is suspect. Automated clients
   SHOULD close the connection, returning and/or logging an error
   indicating that the server's identity is suspect.

   Beyond the server identity checks described in this section, clients
   SHOULD be prepared to do further checking to ensure that the server
   is authorized to provide the service it is observed to provide. The
   client MAY need to make use of local policy information.

3.7.  Refresh of Server Capabilities Information

   The client MUST refresh any cached server capabilities information
   (e.g. from the server's root DSE; see section 3.4 of [LDAPv3]) upon
   TLS session establishment. This is necessary to protect against
   active-intermediary attacks which may have altered any server
   capabilities information retrieved prior to TLS establishment. The
   server MAY advertise different capabilities after TLS establishment.

4.  Closing a TLS Connection

4.1.  Graceful Closure

   Either the client or server MAY terminate the TLS connection on an
   LDAP association by sending a TLS closure alert. This will leave the
   LDAP association intact.

   Before closing a TLS connection, the client MUST either wait for any
   outstanding LDAP operations to complete, or explicitly abandon them
   [LDAPv3].

   After the initiator of a close has sent a closure alert, it MUST
   discard any TLS messages until it has received an alert from the
   other party.  It will cease to send TLS Record Protocol PDUs, and
   following the receipt of the alert, MAY send and receive LDAP PDUs.

   The other party, if it receives a closure alert, MUST immediately
   transmit a TLS closure alert.  It will subsequently cease to send TLS
   Record Protocol PDUs, and MAY send and receive LDAP PDUs.

4.2.  Abrupt Closure

   Either the client or server MAY abruptly close the entire LDAP
   association and any TLS connection established on it by dropping the
   underlying TCP connection. A server MAY beforehand send the client a
   Notice of Disconnection [LDAPv3] in this case.

5.  Effects of TLS on a Client's Authorization Identity

   This section describes the effects on a client's authorization
   identity brought about by establishing TLS on an LDAP association.
   The default effects are described first, and next the facilities for
   client assertion of authorization identity are discussed including
   error conditions. Lastly, the effects of closing the TLS connection
   are described.

   Authorization identities and related concepts are defined in
   [AuthMeth].

5.1.  TLS Connection Establishment Effects

5.1.1.  Default Effects

   Upon establishment of the TLS connection onto the LDAP association,
   any previously established authentication and authorization
   identities MUST remain in force, including anonymous state. This
   holds even in the case where the server requests client
   authentication via TLS -- e.g. requests the client to supply its
   certificate during TLS negotiation (see [TLS]).

5.1.2.  Client Assertion of Authorization Identity

   A client MAY either implicitly request that its LDAP authorization
   identity be derived from its authenticated TLS credentials or it MAY
   explicitly provide an authorization identity and assert that it be
   used in combination with its authenticated TLS credentials. The
   former is known as an implicit assertion, and the latter as an
   explicit assertion.

5.1.2.1.  Implicit Assertion

   An implicit authorization identity assertion is accomplished after
   TLS establishment by invoking a Bind request of the SASL form using
   the "EXTERNAL" mechanism name [SASL, LDAPv3] that SHALL NOT include
   the optional credentials octet string (found within the
   SaslCredentials sequence in the Bind Request). The server will derive
   the client's authorization identity from the authentication identity
   supplied in the client's TLS credentials (typically a public key
   certificate) according to local policy. The underlying mechanics of
   how this is accomplished are implementation specific.

5.1.2.2.  Explicit Assertion

   An explicit authorization identity assertion is accomplished after
   TLS establishment by invoking a Bind request of the SASL form using
   the "EXTERNAL" mechanism name [SASL, LDAPv3] that SHALL include the
   credentials octet string. This string MUST be constructed as
   documented in section 9 of [AuthMeth].

5.1.2.3.  Error Conditions

   For either form of assertion, the server MUST verify that the
   client's authentication identity as supplied in its TLS credentials
   is permitted to be mapped to the asserted authorization identity. The
   server MUST reject the Bind operation with an invalidCredentials
   resultCode in the Bind response if the client is not so authorized.

   Additionally, with either form of assertion, if a TLS session has not
   been established between the client and server prior to making the
   SASL EXTERNAL Bind request and there is no other external source of
   authentication credentials (e.g.  IP-level security [IPSEC]), or if,
   during the process of establishing the TLS session, the server did
   not request the client's authentication credentials, the SASL
   EXTERNAL bind MUST fail with a result code of
   inappropriateAuthentication.

   After the above Bind operation failures, any client authentication
   and authorization state of the LDAP association is lost, so the LDAP
   association is in an anonymous state after the failure.  TLS
   connection state is unaffected, though a server MAY end the TLS
   connection, via a TLS close_notify message, based on the Bind failure
   (as it MAY at any time).

5.2.  TLS Connection Closure Effects

   Closure of the TLS connection MUST cause the LDAP association to move
   to an anonymous authentication and authorization state regardless of
   the state established over TLS and regardless of the authentication
   and authorization state prior to TLS connection establishment.

6.  Security Considerations

   The goals of using the TLS protocol with LDAP are to ensure
   connection confidentiality and integrity, and to optionally provide
   for authentication. TLS expressly provides these capabilities, as
   described in [TLS].

   All security gained via use of the Start TLS operation is gained by
   the use of TLS itself. The Start TLS operation, on its own, does not
   provide any additional security.

   The use of TLS does not provide or ensure for confidentiality and/or
   non-repudiation of the data housed by an LDAP-based directory server.
   Nor does it secure the data from inspection by the server
   administrators.  Once established, TLS only provides for and ensures
   confidentiality and integrity of the operations and data in transit
   over the LDAP association, and only if the implementations on the
   client and server support and negotiate it.

   The level of security provided though the use of TLS depends directly
   on both the quality of the TLS implementation used and the style of
   usage of that implementation. Additionally, an active-intermediary
   attacker can remove the Start TLS extended operation from the
   supportedExtension attribute of the root DSE. Therefore, both parties
   SHOULD independently ascertain and consent to the security level
   achieved once TLS is established and before beginning use of the TLS
   connection. For example, the security level of the TLS connection
   might have been negotiated down to plaintext.

   Clients SHOULD either warn the user when the security level achieved
   does not provide confidentiality and/or integrity protection, or be
   configurable to refuse to proceed without an acceptable level of
   security.

   Client and server implementors SHOULD take measures to ensure proper
   protection of credentials and other confidential data where such
   measures are not otherwise provided by the TLS implementation.

   Server implementors SHOULD allow for server administrators to elect
   whether and when connection confidentiality and/or integrity is
   required, as well as elect whether and when client authentication via
   TLS is required.

7.  Acknowledgements

   The authors thank Tim Howes, Paul Hoffman, John Kristian, Shirish
   Rai, Jonathan Trostle, Harald Alvestrand, and Marcus Leech for their
   contributions to this document.

8.  References

   [AuthMeth]     Wahl, M., Alvestrand, H., Hodges, J. and R. Morgan,
                  "Authentication Methods for LDAP", RFC 2829, May 2000.

   [IPSEC]        Kent, S. and R. Atkinson, "Security Architecture for
                  the Internet Protocol", RFC 2401, November 1998.

   [LDAPv3]       Wahl, M., Kille S. and T. Howes, "Lightweight
                  Directory Access Protocol (v3)", RFC 2251, December
                  1997.

   [ReqsKeywords] Bradner, S., "Key Words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119, March 1997.

   [SASL]         Myers, J., "Simple Authentication and Security Layer
                  (SASL)", RFC 2222, October 1997.

   [TLS]          Dierks, T. and C. Allen. "The TLS Protocol Version
                  1.0", RFC 2246, January 1999.

9.  Authors' Addresses

   Jeff Hodges
   Oblix, Inc.
   18922 Forge Drive
   Cupertino, CA 95014
   USA

   Phone: +1-408-861-6656
   EMail: JHodges@oblix.com

   RL "Bob" Morgan
   Computing and Communications
   University of Washington
   Seattle, WA
   USA

   Phone: +1-206-221-3307
   EMail: rlmorgan@washington.edu

   Mark Wahl
   Sun Microsystems, Inc.
   8911 Capital of Texas Hwy #4140
   Austin TX 78759
   USA

   EMail: M.Wahl@innosoft.com

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   proprietary rights by implementors or users of this specification can
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   The IETF invites any interested party to bring to its attention any
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   rights which may cover technology that may be required to practice
   this standard.  Please address the information to the IETF Executive
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   Funding for the RFC Editor function is currently provided by the
   Internet Society.

 

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