Network Working Group                                       S. Josefsson
Internet-Draft                                                       SJD
Updates: 4120 (if approved)                                 May 10, 2006
Expires: November 11, 2006

Extended Kerberos Version 5 Key Distribution Center (KDC) Exchanges Over

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   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-

   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

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on November 11, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).


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

Josefsson               Expires November 11, 2006               [Page 1]

Internet-Draft          Kerberos V5 TCP extension               May 2006

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

Josefsson               Expires November 11, 2006               [Page 2]

Internet-Draft          Kerberos V5 TCP extension               May 2006

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",
   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 SHOULD NOT close the
   connection, and SHOULD 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.

   Resource avaibility considerations may influence whether, and for how
   long, the KDC will wait for the client to send requests.

   The behaviour when more than one non-high bit is set depends on the

Josefsson               Expires November 11, 2006               [Page 3]

Internet-Draft          Kerberos V5 TCP extension               May 2006

   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
   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

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.  Implementations MAY 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.  Client and KDC
   policies can be used to reject connections that does not use any

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.

Josefsson               Expires November 11, 2006               [Page 4]

Internet-Draft          Kerberos V5 TCP extension               May 2006

   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 that
   update or obsolete this document.

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

   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

   Copyright (C) 2005, 2006 Simon Josefsson

   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.

Josefsson               Expires November 11, 2006               [Page 5]

Internet-Draft          Kerberos V5 TCP extension               May 2006

Author's Address

   Simon Josefsson


Josefsson               Expires November 11, 2006               [Page 6]

Internet-Draft          Kerberos V5 TCP extension               May 2006

Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at

Disclaimer of Validity

   This document and the information contained herein are provided on an

Copyright Statement

   Copyright (C) The Internet Society (2006).  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.


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

Josefsson               Expires November 11, 2006               [Page 7]