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

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 5021.
Author Simon Josefsson
Last updated 2015-10-14 (Latest revision 2007-05-03)
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Additional resources Mailing list discussion
Stream WG state (None)
Document shepherd (None)
IESG IESG state Became RFC 5021 (Proposed Standard)
Action Holders
Consensus boilerplate Unknown
Telechat date (None)
Responsible AD Sam Hartman
Send notices to (None)
Network Working Group                                       S. Josefsson
Internet-Draft                                                       SJD
Updates: 4120 (if approved)                                  May 2, 2007
Intended status: Standards Track
Expires: November 3, 2007

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
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   The list of current Internet-Drafts can be accessed at

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   This Internet-Draft will expire on November 3, 2007.

Copyright Notice

   Copyright (C) The IETF Trust (2007).


   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.

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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 . . . . . . . . . . . . . . . . . . . . 5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 6
   8.  Normative References  . . . . . . . . . . . . . . . . . . . . . 6
   Appendix A.  Copying conditions . . . . . . . . . . . . . . . . . . 6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 6
   Intellectual Property and Copyright Statements  . . . . . . . . . . 7

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1.  Introduction

   The Kerberos V5 [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 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 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.

   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 receive 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 a RFC 4120 conforming implementation that does not

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   support this extension mechanism, because a KRB-ERROR is always of
   non-zero size.)

   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 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
   shutdown a session when the KDC did not support a, by the client,
   required extension.  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 avaibility 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 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.  In
   particular, the structure may be a protocol with its own message
   framing.  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.  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.

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   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
   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

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

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

   Bit #                                             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 that
   update or obsolete 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.

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7.  Acknowledgements

   Nicolas Williams, Jeffrey Hutzelman, and Sam Hartman 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

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.

Author's Address

   Simon Josefsson


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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.

   This document and the information contained herein are provided on an

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