


Network Working Group                                       S. Josefsson
Internet-Draft                                                       SJD
Updates: 4120 (if approved)                               April 23, 2006
Expires: October 25, 2006


Extended Kerberos Version 5 Key Distribution Center (KDC) Exchanges Over
                                  TCP
                  draft-josefsson-krb-tcp-expansion-02

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Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document describes an extensibility mechanism for the Kerberos
   v5 protocol when used over TCP transports.








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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Conventions used in this document . . . . . . . . . . . . . . . 3
   3.  Extension Mechanism for TCP transport . . . . . . . . . . . . . 3
   4.  Interoperability Consideration  . . . . . . . . . . . . . . . . 4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 4
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 5
   8.  Normative References  . . . . . . . . . . . . . . . . . . . . . 5
   Appendix A.  Copying conditions . . . . . . . . . . . . . . . . . . 5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 6
   Intellectual Property and Copyright Statements  . . . . . . . . . . 7






































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1.  Introduction

   The Kerberos 5 [3] specification, in section 7.2.2, reserve the high
   order bit in the initial length field for TCP transport for future
   expansion.  This document update [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, 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 support.  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.

   If a KDC receive 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
   indicate which extensions it supports.  The KDC MUST NOT close the
   connection, and MUST wait for the client to then send a second
   4-octet value, with the high bit set and the remaining bits as the
   bitmask, to request a particular extension.  If the second 4-octet
   value is a PROBE or an unsupported extension, the KDC MUST close the
   connection.  This is used by the client to shutdown a session when
   the KDC did not support a, by the client, required extension.

   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 interact with another requested extensions
   (bit Y), the KDC MUST treat the request as a PROBE or unsupported



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   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.  If an
   extension mechanism reserve 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.  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 implementations.
   Implementations experiencing trouble getting the expected responses
   from a server SHOULD assume that it does not support this extension
   mechanism.  Clients MAY remember this semi-permanently, to avoid
   excessive logging in the server.  Care should be taken to avoid
   unexpected behaviour for the user when the KDC is eventually
   upgraded.  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.  Client and KDC
   policies can be used to reject connections that does not use any
   expansion.


6.  IANA Considerations

   IANA needs to create a new registry for "Kerberos 5 TCP Extensions".
   The initial contents of this registry should be:

   [[RFC Editor: Replace xxxx below with the number of this RFC.]]

   Bit #         Meaning                             Reference
   -----         -------                             ---------
   0..29         AVAILABLE for registration.
   30            RESERVED.                           RFC XXXX

   IANA will register values 0 to 29 after IESG Approval, as defined in
   BCP 64 [2].  Assigning value 30 requires a Standards Action.



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7.  Acknowledgements

   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.

   Nicolas Williams and Jeffrey Hutzelman provided comments that
   improved the document.

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.


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.



















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Author's Address

   Simon Josefsson
   SJD

   Email: simon@josefsson.org













































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