Network Working Group S. Josefsson Internet-Draft April 8, 2006 Updates: 4120 (if approved) Expires: October 10, 2006 Extended Kerberos Version 5 Key Distribution Center (KDC) Exchanges Over TCP draft-josefsson-krb-tcp-expansion-01 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 Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on October 10, 2006. 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. Josefsson Expires October 10, 2006 [Page 1]
Internet-Draft Kerberos V5 TCP extension April 2006 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions used in this document . . . . . . . . . . . . . . . 3 3. Expansion 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 Josefsson Expires October 10, 2006 [Page 2]
Internet-Draft Kerberos V5 TCP extension April 2006 1. Introduction The Kerberos 5 [3] specification, in section 7.2.2, reserve the high order bit in the length field for TCP transport for future expansion. This document update [3] to describe the behaviour when that bit is set. This expansion mechanism is intended for features that are specific for the TCP transport. The required behaviour regarding the reserved bit is described as follows in [3]: Each request (KRB_KDC_REQ) and response (KRB_KDC_REP or KRB_ERROR) sent over the TCP stream is preceded by the length of the request as 4 octets in network byte order. The high bit of the length is reserved for future expansion and MUST currently be set to zero. If a KDC that does not understand how to interpret a set high bit of the length encoding receives a request with the high order bit of the length set, it MUST return a KRB-ERROR message with the error KRB_ERR_FIELD_TOOLONG and MUST close the TCP stream. 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. Expansion Mechanism for TCP transport The reserved high-order bit of the request length field is used to provide an extension mechanism. When the reserved high bit is set, the remaining 31 bits of the 4 octets are treated as an extension identifier, and thus form a 31 bit integer enumerating various expansions. Each of the values indicate a specific extended operation mode, which are to be described elsewhere. If the KDC does not understand a requested expansion, it MUST return a KRB-ERROR with a KRB_ERR_FIELD_TOOLONG value (prefixed by the 4 octet length integer, with the high bit clear, as usual) and close the TCP stream. The following table specifies the meaning of the 31 lower bits in the 4 octet field, when the high bit is set: Josefsson Expires October 10, 2006 [Page 3]
Internet-Draft Kerberos V5 TCP extension April 2006 0 RESERVED. 1...2147483647 AVAILABLE for registration, through IANA. 2147483648 RESERVED. Each expansion mechanism MUST describe the structure of protocol data beyond the length field, and also the behaviour of the client and KDC. 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 expansion 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 expansion. 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 Expansions". The initial contents of this registry should be: [[RFC Editor: Replace xxxx below with the number of this RFC.]] Decimal Meaning Reference ------- ------- --------- 0 RESERVED. RFC XXXX 1...2147483647 AVAILABLE for registration. 2147483648 RESERVED. RFC XXXX IANA will register new values on a First Come First Served basis, as Josefsson Expires October 10, 2006 [Page 4]
Internet-Draft Kerberos V5 TCP extension April 2006 defined in BCP 64 [2]. Changing the RESERVED values (0 and 2147483648) will require IETF Consensus. While the registration procedures do not require expert review, authors of expansion are encouraged to seek community review and comment whenever that is feasible. Authors may seek community review by posting a specification of their proposed mechanism as an Internet-Draft. Expansions intended for widespread use should be standardized through the normal IETF process, when appropriate. 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. Josefsson Expires October 10, 2006 [Page 5]
Internet-Draft Kerberos V5 TCP extension April 2006 Author's Address Simon Josefsson Email: simon@josefsson.org Josefsson Expires October 10, 2006 [Page 6]
Internet-Draft Kerberos V5 TCP extension April 2006 Intellectual Property Statement 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 http://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. Disclaimer of Validity 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 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. Copyright Statement Copyright (C) The Internet Society (2006). 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. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Josefsson Expires October 10, 2006 [Page 7]
