draft-ietf-krb-wg-tcp-expansion-02.txt   rfc5021.txt 
Network Working Group S. Josefsson Network Working Group S. Josefsson
Internet-Draft SJD Request for Comments: 5021 SJD
Updates: 4120 (if approved) May 2, 2007 Updates: 4120 August 2007
Intended status: Standards Track Category: Standards Track
Expires: November 3, 2007
Extended Kerberos Version 5 Key Distribution Center (KDC) Exchanges Over
TCP
draft-ietf-krb-wg-tcp-expansion-02
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Copyright Notice Copyright Notice
Copyright (C) The IETF Trust (2007). Copyright (C) The IETF Trust (2007).
Abstract Abstract
This document describes an extensibility mechanism for the Kerberos This document describes an extensibility mechanism for the Kerberos
V5 protocol when used over TCP transports. The mechanism uses the V5 protocol when used over TCP transports. The mechanism uses the
reserved high-bit in the length field. It can be used to negotiate reserved high-bit in the length field. It can be used to negotiate
TCP-specific Kerberos extensions. TCP-specific Kerberos extensions.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions used in this document . . . . . . . . . . . . . . . 3 2. Conventions Used in This Document . . . . . . . . . . . . . . . 2
3. Extension Mechanism for TCP transport . . . . . . . . . . . . . 3 3. Extension Mechanism for TCP Transport . . . . . . . . . . . . . 2
4. Interoperability Consideration . . . . . . . . . . . . . . . . 4 4. Interoperability Consideration . . . . . . . . . . . . . . . . 3
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 5 5. Security Considerations . . . . . . . . . . . . . . . . . . . . 4
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5
8. Normative References . . . . . . . . . . . . . . . . . . . . . 6 8. Normative References . . . . . . . . . . . . . . . . . . . . . 5
Appendix A. Copying conditions . . . . . . . . . . . . . . . . . . 6 Appendix A. Copying Conditions . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 6
Intellectual Property and Copyright Statements . . . . . . . . . . 7
1. Introduction 1. Introduction
The Kerberos V5 [3] specification, in section 7.2.2, reserve the high The Kerberos V5 [3] specification, in section 7.2.2, reserves the
order bit in the initial length field for TCP transport for future high order bit in the initial length field for TCP transport for
expansion. This document update [3] to describe the behaviour when future expansion. This document updates [3] to describe the
that bit is set. This mechanism is intended for extensions that are behaviour when that bit is set. This mechanism is intended for
specific for the TCP transport. extensions that are specific for the TCP transport.
2. Conventions used in this document 2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1]. document are to be interpreted as described in RFC 2119 [1].
3. Extension Mechanism for TCP transport 3. Extension Mechanism for TCP Transport
The reserved high bit of the request length field is used to signal 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 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 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 octets are interpreted as a bitmap. Each bit in the bitmask can be
used to request a particular extension. The 31 bits form the used to request a particular extension. The 31 bits form the
"extension bitmask". It is expected that other documents will "extension bitmask". It is expected that other documents will
describe the details associated with particular bits. describe the details associated with particular bits.
A 4-octet value with only the high bit set, and thus the extension 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 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 find out which extensions a KDC supports. A client may also set
particular bits in the extension bitmask directly, if it does not particular bits in the extension bitmask directly, if it does not
need to query the KDC for available extensions before deciding which need to query the KDC for available extensions before deciding which
extension to request. extension to request.
Note that clients are not forced to use this extension mechanism, and Note that clients are not forced to use this extension mechanism, and
further, clients are expected to only use it when they wish to further, clients are expected to only use it when they wish to
negotiate a particular extension. negotiate a particular extension.
The protocol is as follows. The client MUST begin by sending a 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 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 PROBE or a specific request for some extension(s). The client MUST
NOT send additional data before the server has responded. NOT send additional data before the server has responded.
If a KDC receive a request for a set of extensions that it supports, 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. it MUST respond by sending a 4-octet zero value, i.e., 0x00000000.
The KDC MAY directly send additional data after the zero value, The KDC MAY directly send additional data after the zero value,
without waiting for the client to respond, as specified by the without waiting for the client to respond, as specified by the
particular negotiated extension. (Note: A 4-octet zero value can particular negotiated extension. (Note: A 4-octet zero value can
never be sent by a RFC 4120 conforming implementation that does not never be sent by an implementation that conforms to RFC 4120 and that
support this extension mechanism, because a KRB-ERROR is always of does not support this extension mechanism, because a KRB-ERROR is
non-zero size.) always of non-zero size.)
If a KDC receives a PROBE, or if a KDC does not support all
If a KDC receive a PROBE, or if a KDC does not support all extensions extensions corresponding to set bits in the extension bitmask, the
corresponding to set bits in the extension bitmask, the KDC MUST KDC MUST return 4 octets with the high bit set, and with the
return 4 octets with the high bit set, and with the remaining bitmask remaining bitmask indicating which extensions it supports. The KDC
indicate which extensions it supports. The KDC then MUST wait and then MUST wait, and the client MUST send a second 4-octet value with
the client MUST send a second 4-octet value, with the high bit set. the high bit set. If the second 4-octet value is a PROBE or an
If the second 4-octet value is a PROBE or an unsupported extension, unsupported extension, the KDC MUST close the connection. This can
the KDC MUST close the connection. This can be used by the client to be used by the client to shut down a session when the KDC did not
shutdown a session when the KDC did not support a, by the client, support an extension that is required by the client. If the second
required extension. If the second 4-octet value is a supported 4-octet value is a supported extension, the KDC MUST respond by
extension, the KDC MUST respond by sending a 4-octet zero value, sending a 4-octet zero value, i.e., 0x00000000. The KDC MAY directly
i.e., 0x00000000. The KDC MAY directly send additional data after send additional data after the zero value, as specified by the
the zero value, as specified by the particular negotiated extension. particular negotiated extension.
The client and KDC SHOULD wait for the other side to respond The client and KDC SHOULD wait for the other side to respond
according to this protocol, and the client and KDC SHOULD NOT close according to this protocol, and the client and KDC SHOULD NOT close
the connection prematurely. Resource avaibility considerations may the connection prematurely. Resource availability considerations may
influence whether, and for how long, the client and KDC will wait for influence whether, and for how long, the client and KDC will wait for
the other side to respond to a request. the other side to respond to a request.
The KDC MUST NOT support the extension mechanism if it does not The KDC MUST NOT support the extension mechanism if it does not
support any extensions. If no extensions are supported, the KDC MUST support any extensions. If no extensions are supported, the KDC MUST
return a KRB-ERROR message with the error KRB_ERR_FIELD_TOOLONG and return a KRB-ERROR message with the error KRB_ERR_FIELD_TOOLONG and
MUST close the TCP stream, similar to what an implementation that MUST close the TCP stream, similar to what an implementation that
does not understand this extension mechanism would do. does not understand this extension mechanism would do.
The behaviour when more than one non-high bit is set depends on the The behaviour when more than one non-high bit is set depends on the
particular extension mechanisms. If a requested extension (bit X) particular extension mechanisms. If a requested extension (bit X)
does not specify how it interact with another requested extensions does not specify how it interacts with another requested extension
(bit Y), the KDC MUST treat the request as a PROBE or unsupported (bit Y), the KDC MUST treat the request as a PROBE or unsupported
extension, and proceed as above. extension, and proceed as above.
Each extension MUST describe the structure of protocol data beyond Each extension MUST describe the structure of protocol data beyond
the length field, and the behaviour of the client and KDC. In the length field, and the behaviour of the client and KDC. In
particular, the structure may be a protocol with its own message particular, the structure may be a protocol with its own message
framing. If an extension mechanism reserve multiple bits, it MUST framing. If an extension mechanism reserves multiple bits, it MUST
describe how they interact. describe how they interact.
4. Interoperability Consideration 4. Interoperability Consideration
Implementations with support for TCP that do not claim to conform to Implementations with support for TCP that do not claim to conform to
RFC 4120 may not handle the high bit correctly. The KDC behaviour RFC 4120 may not handle the high bit correctly. The KDC behaviour
may include closing the TCP connection without any response, and may include closing the TCP connection without any response, and
logging an error message in the KDC log. When this was written, this logging an error message in the KDC log. When this was written, this
problem existed in modern versions of popular KDC implementations. problem existed in modern versions of popular KDC implementations.
Implementations experiencing trouble getting the expected responses Implementations experiencing trouble getting the expected responses
from a KDC might assume that the KDC does not support this extension from a KDC might assume that the KDC does not support this extension
mechanism. A client might remember this semi-permanently, to avoid mechanism. A client might remember this semi-permanently, to avoid
triggering the same problematic behaviour on the KDC every time. triggering the same problematic behaviour on the KDC every time.
Care should be taken to avoid unexpected behaviour for the user when Care should be taken to avoid unexpected behaviour for the user when
the KDC is eventually upgraded. Implementations might also provide a the KDC is eventually upgraded. Implementations might also provide a
way to enable and disable this extension on a per-realm basis. How way to enable and disable this extension on a per-realm basis. How
to handle these backwards compatibility quirks are in general left to handle these backwards compatibility quirks are in general left
unspecified. unspecified.
skipping to change at page 5, line 21 skipping to change at page 4, line 17
way to enable and disable this extension on a per-realm basis. How way to enable and disable this extension on a per-realm basis. How
to handle these backwards compatibility quirks are in general left to handle these backwards compatibility quirks are in general left
unspecified. unspecified.
5. Security Considerations 5. Security Considerations
Because the initial length field is not protected, it is possible for 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 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 the client and the KDC) to make it appear to the client that the
server does not support this extension mechanism (a downgrade server does not support this extension mechanism (a downgrade
attack). Further, active attackers can also inferfere with the attack). Further, active attackers can also interfere with the
negotiation of which extensions are supported, which may also result negotiation of which extensions are supported, which may also result
in a downgrade attack. This problem can be solved by having a policy 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 in the clients and in the KDC to reject connections that do not have
have the desired properties. The problem can also be mitigated by the desired properties. The problem can also be mitigated by having
having the negotiated extension send a cryptographic checksum of the the negotiated extension send a cryptographic checksum of the offered
offered extensions. extensions.
6. IANA Considerations 6. IANA Considerations
IANA needs to create a new registry for "Kerberos TCP Extensions". IANA has created a new registry for "Kerberos TCP Extensions". The
The initial contents of this registry should be: initial contents of this registry are:
[[RFC Editor: Replace xxxx below with the number of this RFC.]]
Bit # Reference Bit # Reference
----- --------- ----- ---------
0..29 AVAILABLE for registration. 0..29 AVAILABLE for registration.
30 RESERVED. RFC XXXX 30 RESERVED. RFC 5021
IANA will register values 0 to 29 after IESG Approval, as defined in IANA will register values 0 to 29 after IESG Approval, as defined in
BCP 64 [2]. Assigning value 30 requires a Standards Action that BCP 64 [2]. Assigning value 30 requires a Standards Action that
update or obsolete this document. updates or obsoletes this document.
Registration policy: The IESG will act as a steward for the Registration policy: The IESG will act as a steward for the
namespace, considering whether the registration is justified given namespace, considering whether the registration is justified given
the limited size of the namespace. The IESG will also confirm that the limited size of the namespace. The IESG will also confirm that
proposed registrations are not harmful to the Internet. proposed registrations are not harmful to the Internet.
7. Acknowledgements 7. Acknowledgements
Nicolas Williams, Jeffrey Hutzelman, and Sam Hartman provided Nicolas Williams, Jeffrey Hutzelman, Sam Hartman, and Chris Newman
comments that improved the protocol and document. provided comments that improved the protocol and document.
Thanks to Andrew Bartlett who pointed out that some implementations Thanks to Andrew Bartlett who pointed out that some implementations
(MIT Kerberos and Heimdal) did not follow RFC 4120 properly with (MIT Kerberos and Heimdal) did not follow RFC 4120 properly with
regards to the high bit, which resulted in an Interoperability regards to the high bit, which resulted in an Interoperability
Consideration. Consideration.
8. Normative References 8. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997. Levels", BCP 14, RFC 2119, March 1997.
[2] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA [2] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October 1998. Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.
[3] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The Kerberos [3] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The Kerberos
Network Authentication Service (V5)", RFC 4120, July 2005. Network Authentication Service (V5)", RFC 4120, July 2005.
Appendix A. Copying conditions Appendix A. Copying Conditions
Regarding this entire document or any portion of it, the author makes Regarding this entire document or any portion of it, the author makes
no guarantees and is not responsible for any damage resulting from no guarantees and is not responsible for any damage resulting from
its use. The author grants irrevocable permission to anyone to use, its use. The author grants irrevocable permission to anyone to use,
modify, and distribute it in any way that does not diminish the modify, and distribute it in any way that does not diminish the
rights of anyone else to use, modify, and distribute it, provided rights of anyone else to use, modify, and distribute it, provided
that redistributed derivative works do not contain misleading author that redistributed derivative works do not contain misleading author
or version information. Derivative works need not be licensed under or version information. Derivative works need not be licensed under
similar terms. similar terms.
Author's Address Author's Address
Simon Josefsson Simon Josefsson
SJD SJD
Email: simon@josefsson.org EMail: simon@josefsson.org
Full Copyright Statement Full Copyright Statement
Copyright (C) The IETF Trust (2007). Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors contained in BCP 78, and except as set forth therein, the authors
retain all their rights. retain all their rights.
This document and the information contained herein are provided on an This document and the information contained herein are provided on an
skipping to change at page 7, line 45 skipping to change at page 7, line 45
such proprietary rights by implementers or users of this such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr. http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at this standard. Please address the information to the IETF at
ietf-ipr@ietf.org. ietf-ipr@ietf.org.
Acknowledgment Acknowledgement
Funding for the RFC Editor function is provided by the IETF Funding for the RFC Editor function is currently provided by the
Administrative Support Activity (IASA). Internet Society.
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