Extended Kerberos Version 5 Key Distribution Center (KDC) Exchanges
Over TCP
SJDsimon@josefsson.orgThis 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.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.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.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
shutdown a session when the KDC did not support a, by the
client, required extension. 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. 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
unspecified.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.IANA has created a new registry for "Kerberos TCP
Extensions". The initial contents of this registry are: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 Consideration.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.