The Kerberos V5  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.
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 .
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 to 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.
4. Interoperability Consideration
Implementations with support for TCP that do not claim to conform to
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
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
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 in
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.
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 follow RFC 4120 properly with
regards to the high bit, which resulted in an Interoperability
8. Normative References
 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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