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Network Working Group                                       S. JosefssonRequest for Comments: 5021                                           SJDUpdates:4120                                                August 2007Category: Standards TrackExtended Kerberos Version 5 Key Distribution Center (KDC)Exchanges over TCPStatus 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).Abstract   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 Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .22.  Conventions Used in This Document . . . . . . . . . . . . . . .23.  Extension Mechanism for TCP Transport . . . . . . . . . . . . .24.  Interoperability Consideration  . . . . . . . . . . . . . . . .35.  Security Considerations . . . . . . . . . . . . . . . . . . . .46.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . .47.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . .58.  Normative References  . . . . . . . . . . . . . . . . . . . . .5Appendix A.  Copying Conditions . . . . . . . . . . . . . . . . . .6Josefsson                   Standards Track                     [Page 1]

RFC 5021               Kerberos V5 TCP Extension             August 20071.  Introduction   The Kerberos V5 [3] specification, insection 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",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described inRFC 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 toRFC 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 toRFC 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 avoidJosefsson                   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   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 (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   extensions.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 inBCP 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 20077.  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 followRFC 4120 properly with   regards to the high bit, which resulted in an Interoperability   Consideration.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 2007Appendix 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   SJD   EMail: simon@josefsson.orgJosefsson                   Standards Track                     [Page 6]

RFC 5021               Kerberos V5 TCP Extension             August 2007Full Copyright Statement   Copyright (C) The IETF Trust (2007).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.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 inBCP 78 andBCP 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 athttp://www.ietf.org/ipr.   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   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Josefsson                   Standards Track                     [Page 7]