Skip to main content

Extended Kerberos Version 5 Key Distribution Center (KDC) Exchanges over TCP
RFC 5021

Document Type RFC - Proposed Standard (August 2007)
Updates RFC 4120
Author Simon Josefsson
Last updated 2015-10-14
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
IESG Responsible AD Sam Hartman
Send notices to (None)
RFC 5021
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).


   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",
   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

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

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

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


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.

   This document and the information contained herein are provided on an

Intellectual Property

   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


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

Josefsson                   Standards Track                     [Page 7]