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rfc:rfc5021

Network Working Group S. Josefsson Request for Comments: 5021 SJD Updates: 4120 August 2007 Category: Standards Track

     Extended Kerberos Version 5 Key Distribution Center (KDC)
                         Exchanges over TCP

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 IETF Trust (2007).

Abstract

 This document describes an extensibility mechanism for the Kerberos
 V5 protocol when used over TCP transports.  The mechanism uses the
 reserved high-bit in the length field.  It can be used to negotiate
 TCP-specific Kerberos extensions.

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
 2.  Conventions Used in This Document . . . . . . . . . . . . . . . 2
 3.  Extension Mechanism for TCP Transport . . . . . . . . . . . . . 2
 4.  Interoperability Consideration  . . . . . . . . . . . . . . . . 3
 5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 4
 6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
 7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 5
 8.  Normative References  . . . . . . . . . . . . . . . . . . . . . 5
 Appendix A.  Copying Conditions . . . . . . . . . . . . . . . . . . 6

Josefsson Standards Track [Page 1] RFC 5021 Kerberos V5 TCP Extension August 2007

1. Introduction

 The Kerberos V5 [3] specification, in section 7.2.2, reserves the
 high order bit in the initial length field for TCP transport for
 future expansion.  This document updates [3] to describe the
 behaviour when that bit is set.  This mechanism is intended for
 extensions that are specific for the TCP transport.

2. 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 RFC 2119 [1].

3. Extension Mechanism for TCP Transport

 The reserved high bit of the request length field is used to signal
 the use of this extension mechanism.  When the reserved high bit is
 set in the length field, the remaining 31 bits of the initial 4
 octets are interpreted as a bitmap.  Each bit in the bitmask can be
 used to request a particular extension.  The 31 bits form the
 "extension bitmask".  It is expected that other documents will
 describe the details associated with particular bits.
 A 4-octet value with only the high bit set, and thus the extension
 bitmask all zeros, is called a PROBE.  A client may send a probe to
 find out which extensions a KDC supports.  A client may also set
 particular bits in the extension bitmask directly, if it does not
 need to query the KDC for available extensions before deciding which
 extension to request.
 Note that clients are not forced to use this extension mechanism, and
 further, clients are expected to only use it when they wish to
 negotiate a particular extension.
 The protocol is as follows.  The client MUST begin by sending a
 4-octet value with the high bit set.  The packet is thus either a
 PROBE or a specific request for some extension(s).  The client MUST
 NOT send additional data before the server has responded.
 If a KDC receives a request for a set of extensions that it supports,
 it MUST respond by sending a 4-octet zero value, i.e., 0x00000000.
 The KDC MAY directly send additional data after the zero value,
 without waiting for the client to respond, as specified by the
 particular negotiated extension.  (Note: A 4-octet zero value can
 never be sent by an implementation that conforms to RFC 4120 and that
 does not support this extension mechanism, because a KRB-ERROR is
 always of non-zero size.)

Josefsson Standards Track [Page 2] RFC 5021 Kerberos V5 TCP Extension August 2007

 If a KDC receives a PROBE, or if a KDC does not support all
 extensions corresponding to set bits in the extension bitmask, the
 KDC MUST return 4 octets with the high bit set, and with the
 remaining bitmask indicating which extensions it supports.  The KDC
 then MUST wait, and the client MUST send a second 4-octet value with
 the high bit set.  If the second 4-octet value is a PROBE or an
 unsupported extension, the KDC MUST close the connection.  This can
 be used by the client to shut down a session when the KDC did not
 support an extension that is required by the client.  If the second
 4-octet value is a supported extension, the KDC MUST respond by
 sending a 4-octet zero value, i.e., 0x00000000.  The KDC MAY directly
 send additional data after the zero value, as specified by the
 particular negotiated extension.
 The client and KDC SHOULD wait for the other side to respond
 according to this protocol, and the client and KDC SHOULD NOT close
 the connection prematurely.  Resource availability considerations may
 influence whether, and for how long, the client and KDC will wait for
 the other side to respond to a request.
 The KDC MUST NOT support the extension mechanism if it does not
 support any extensions.  If no extensions are supported, the KDC MUST
 return a KRB-ERROR message with the error KRB_ERR_FIELD_TOOLONG and
 MUST close the TCP stream, similar to what an implementation that
 does not understand this extension mechanism would do.
 The behaviour when more than one non-high bit is set depends on the
 particular extension mechanisms.  If a requested extension (bit X)
 does not specify how it interacts with another requested extension
 (bit Y), the KDC MUST treat the request as a PROBE or unsupported
 extension, and proceed as above.
 Each extension MUST describe the structure of protocol data beyond
 the length field, and the behaviour of the client and KDC.  In
 particular, the structure may be a protocol with its own message
 framing.  If an extension mechanism reserves multiple bits, it MUST
 describe how they interact.

4. Interoperability Consideration

 Implementations with support for TCP that do not claim to conform to
 RFC 4120 may not handle the high bit correctly.  The KDC behaviour
 may include closing the TCP connection without any response, and
 logging an error message in the KDC log.  When this was written, this
 problem existed in modern versions of popular KDC implementations.
 Implementations experiencing trouble getting the expected responses
 from a KDC might assume that the KDC does not support this extension
 mechanism.  A client might remember this semi-permanently, to avoid

Josefsson Standards Track [Page 3] RFC 5021 Kerberos V5 TCP Extension August 2007

 triggering the same problematic behaviour on the KDC every time.
 Care should be taken to avoid unexpected behaviour for the user when
 the KDC is eventually upgraded.  Implementations might also provide a
 way to enable and disable this extension on a per-realm basis.  How
 to handle these backwards compatibility quirks are in general left
 unspecified.

5. Security Considerations

 Because the initial length field is not protected, it is possible for
 an active attacker (i.e., one that is able to modify traffic between
 the client and the KDC) to make it appear to the client that the
 server does not support this extension mechanism (a downgrade
 attack).  Further, active attackers can also interfere with the
 negotiation of which extensions are supported, which may also result
 in a downgrade attack.  This problem can be solved by having a policy
 in the clients and in the KDC to reject connections that do not have
 the desired properties.  The problem can also be mitigated by having
 the negotiated extension send a cryptographic checksum of the offered
 extensions.

6. IANA Considerations

 IANA has created a new registry for "Kerberos TCP Extensions".  The
 initial contents of this registry are:
 Bit #                                             Reference
 -----                                             ---------
 0..29         AVAILABLE for registration.
 30            RESERVED.                           RFC 5021
 IANA will register values 0 to 29 after IESG Approval, as defined in
 BCP 64 [2].  Assigning value 30 requires a Standards Action that
 updates or obsoletes this document.
 Registration policy: The IESG will act as a steward for the
 namespace, considering whether the registration is justified given
 the limited size of the namespace.  The IESG will also confirm that
 proposed registrations are not harmful to the Internet.

Josefsson Standards Track [Page 4] RFC 5021 Kerberos V5 TCP Extension August 2007

7. Acknowledgements

 Nicolas Williams, Jeffrey Hutzelman, Sam Hartman, and Chris Newman
 provided comments that improved the protocol and document.
 Thanks to Andrew Bartlett who pointed out that some implementations
 (MIT Kerberos and Heimdal) did not follow RFC 4120 properly with
 regards to the high bit, which resulted in an Interoperability
 Consideration.

8. Normative References

 [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.
 [2]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
      Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.
 [3]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The Kerberos
      Network Authentication Service (V5)", RFC 4120, July 2005.

Josefsson Standards Track [Page 5] RFC 5021 Kerberos V5 TCP Extension August 2007

Appendix A. Copying Conditions

 Regarding this entire document or any portion of it, the author makes
 no guarantees and is not responsible for any damage resulting from
 its use.  The author grants irrevocable permission to anyone to use,
 modify, and distribute it in any way that does not diminish the
 rights of anyone else to use, modify, and distribute it, provided
 that redistributed derivative works do not contain misleading author
 or version information.  Derivative works need not be licensed under
 similar terms.

Author's Address

 Simon Josefsson
 SJD
 EMail: simon@josefsson.org

Josefsson Standards Track [Page 6] RFC 5021 Kerberos V5 TCP Extension August 2007

Full Copyright Statement

 Copyright (C) The IETF Trust (2007).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Josefsson Standards Track [Page 7]

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