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).
AbstractThis 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. 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
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  to describe the behaviour when that bit is set. This mechanism is intended for extensions that are specific for the TCP transport. RFC 2119 . RFC 4120 and that does not support this extension mechanism, because a KRB-ERROR is always of non-zero size.)
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. 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
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. BCP 64 . 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.
RFC 4120 properly with regards to the high bit, which resulted in an Interoperability Consideration.  Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The Kerberos Network Authentication Service (V5)", RFC 4120, July 2005.
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