INTERNET-DRAFT Kurt D. Zeilenga
Intended Category: Standard Track OpenLDAP Foundation
Expires: 4 January 2001 4 July 2000
LDAPv3bis Suggestions:
Extension for Transport Layer Security
<draft-zeilenga-ldapv3bis-rfc2830-00.txt>
Status of Memo
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.
This document is intended to be, after appropriate review and
revision, submitted to the RFC Editor as a Standard Track document.
Distribution of this memo is unlimited. Technical discussion of this
document will take place on the IETF LDAP Extension Working Group
mailing list <ietf-ldapext@netscape.com>. Please send editorial
comments directly to the author <Kurt@OpenLDAP.org>.
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.
Copyright 2000, The Internet Society. All Rights Reserved.
Please see the Copyright section near the end of this document for
more information.
Forward
This Internet Draft suggests a number of updates to the "Lightweight
Directory Access Protocol: Extension for Transport Layer Security"
[RFC 2830]. This document is not intended to be published as an RFC
but used to identify LDAPv3bis work items.
The remainer of this documents incorporates the substantive portion of
RFC 2830 text (less status of memo, appendices, etc). Comments and
Zeilenga [Page 1]
INTERNET-DRAFT draft-zeilenga-ldapv3bis-rfc2830-00 4 July 2000
suggested updates to this text are inserted as inline notes prefixed
with '//'.
// Start of RFC 2830 text
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 }
Zeilenga [Page 2]
INTERNET-DRAFT draft-zeilenga-ldapv3bis-rfc2830-00 4 July 2000
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:
// s/will/may/ to allow other appropriate result
// unwillingToPerform, busy, other
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
Zeilenga [Page 3]
INTERNET-DRAFT draft-zeilenga-ldapv3bis-rfc2830-00 4 July 2000
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.
// other result codes should be allowed.
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
Zeilenga [Page 4]
INTERNET-DRAFT draft-zeilenga-ldapv3bis-rfc2830-00 4 July 2000
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 server MAY send a Notice of Disconnect if it choose.
// notices MAY specify inappropriateAuthentication,
// confidentialityRequired, or other appropriate result code.
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
Zeilenga [Page 5]
INTERNET-DRAFT draft-zeilenga-ldapv3bis-rfc2830-00 4 July 2000
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.
Zeilenga [Page 6]
INTERNET-DRAFT draft-zeilenga-ldapv3bis-rfc2830-00 4 July 2000
// For client initiated closure:
Before closing a TLS connection, the client MUST either wait for any
outstanding LDAP operations to complete, or explicitly abandon them
[LDAPv3].
// If the server receives a TLS closure alert, the server MUST
// immediately abandon all outstanding operations associated with
// the connection and return the session to an anonymous
// authentication/authorization state. If the server is unable
// to do so it must abruptly close TLS per 4.2.
// For server initiated closure:
// Before closing a TLS connection, the server MUST abandon all
// outstanding operations associated with the connection and return
// the session to an anonymous authentication/authorization state.
// If the server is unable to do so it MUST abruptly close TLS per
// 4.2.
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.
// Or, if unwilling to accept the security downgrade, abruptly
// close TLS per 4.2
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.
Zeilenga [Page 7]
INTERNET-DRAFT draft-zeilenga-ldapv3bis-rfc2830-00 4 July 2000
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
Zeilenga [Page 8]
INTERNET-DRAFT draft-zeilenga-ldapv3bis-rfc2830-00 4 July 2000
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.
Zeilenga [Page 9]
INTERNET-DRAFT draft-zeilenga-ldapv3bis-rfc2830-00 4 July 2000
// In particular, vendors should take special care in
// implementing alert and error handling.
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.
// Clients SHOULD either warn the user upon receipt of a graceful
// closure alert or abrupt close TLS upon receipt of the alert.
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.
Zeilenga [Page 10]
INTERNET-DRAFT draft-zeilenga-ldapv3bis-rfc2830-00 4 July 2000
[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.
// remainder trimmed
// End of RFC 2830 text
Additional Information
Discussions regarding these suggestions may directed to the author:
Kurt D. Zeilenga
OpenLDAP Foundation
<Kurt@OpenLDAP.org>
or the LDAPext Working Group mailing list:
<ietf-ldapext@netscape.com>
Copyright 2000, The Internet Society. All Rights Reserved.
This document and translations of it may be copied and furnished
to others, and derivative works that comment on or otherwise explain
it or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph
are included on all such copies and derivative works. However,
this document itself may not be modified in any way, such as by
removing the copyright notice or references to the Internet Society
or other Internet organizations, except as needed for the purpose
of developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not
be revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on
an "AS IS" basis and THE AUTHORS, THE INTERNET SOCIETY, AND THE
INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE
OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY
IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR
PURPOSE.
Zeilenga [Page 11]
INTERNET-DRAFT draft-zeilenga-ldapv3bis-rfc2830-00 4 July 2000
Zeilenga [Page 12]