Internet DRAFT - draft-josefsson-krb-tcp-expansion
draft-josefsson-krb-tcp-expansion
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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