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Versions: 00 01 02 03 04 05 06 07 RFC 6111
NETWORK WORKING GROUP L. Zhu
Internet-Draft Microsoft Corporation
Updates: 4120 (if approved) August 12, 2008
Intended status: Standards Track
Expires: February 13, 2009
Additional Kerberos Naming Constraints
draft-ietf-krb-wg-naming-07
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Abstract
This document defines new naming constraints for well-known Kerberos
principal name and well-known Kerberos realm names.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . . 3
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Well-known Kerberos Principal Names . . . . . . . . . . . . 3
3.2. Well-known Kerberos Realm Names . . . . . . . . . . . . . . 4
4. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7
Intellectual Property and Copyright Statements . . . . . . . . . . 8
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1. Introduction
Occasionally protocol designers need to designate a Kerberos
principal name or a Kerberos realm name to have special meanings,
other than identifying a particular instance. An example is that the
the anonymous principal name and the anonymous realm name are defined
for the Kerberos anonymity support [ANON]. This anonymity name pair
conveys no more meaning than that the client's identity is not
disclosed. In the case of the anonymity support, it is critical that
deployed Kerberos implementations that do not support anonymity MUST
fail the authentication if the anonymity name pair is used, therefore
no access is granted accidentally to a principal who's name happens
to match with that of the anonymous identity.
However Kerberos as defined in [RFC4120] does not have such reserved
names. As such, protocol designers have resolved to use exceedingly-
unlikely-to-have-been-used names to avoid collision. Even if a
registry were setup to avoid collision for new implementations, there
is no guarantee for deployed implementations to prevent accidental
reuse of names that can lead to access being granted unexpectedly.
The Kerberos realm name in [RFC4120] has a reserved name space
although no specific name is defined and the criticality of unknown
reserved realm names is not specified.
This document is to remedy these issues by defining well-known
Kerberos names and the protocol behavior when a well-known name is
used but not supported.
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 [RFC2119].
3. Definitions
In this section, well-known names are defined for both the Kerberos
principal name and the Kerberos realm name.
3.1. Well-known Kerberos Principal Names
A new name type KRB_NT_WELLKNOWN is defined for well-known principal
names. The Kerberos principal name is defined in Section 6.2 of
[RFC4120].
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KRB_NT_WELLKNOWN 11
A well-known principal name MUST have at least two or more
KerberosString components, and the first component must be the string
literal "WELLKNOWN".
If a well-known principal name is used as the client principal name
or the server principal name but not supported, the Authentication
Service (AS) [RFC4120] and the application server MUST reject the
authentication attempt. Similarly, the Ticket Granting Service (TGS)
[RFC4120] MAY reject the authentication attempt if a well-known
principal name is used as the client principal name but not
supported, and SHOULD reject the authentication attempt if a well-
known principal name is used as the server principal name but not
supported. These rules were designed to allow incremental updates
and ease migration. More specifically, if a well-known principal is
accepted in one realm, it is desirable to allow the cross-realm TGT
to work when not all of the realms in the cross-realm authentication
path are updated; if the server principal with an identically-named
well-known name was created before the KDC is updated, it might be
acceptable to allow authentication to work within a reasonably-
limited time window. However unless otherwise specified, if a well-
known principal name is used but not supported in any other places of
Kerberos messages, authentication MUST fail. The error code is
KRB_AP_ERR_PRINCIPAL_UNKNOWN, and there is no accompanying error data
defined in this document for this error.
KRB_AP_ERR_PRINCIPAL_UNKNOWN 82
-- A well-known Kerberos principal name is used but not
-- supported.
3.2. Well-known Kerberos Realm Names
Section 6.1 of [RFC4120] defines the "other" style realm name, a new
realm type WELLKNOWN is defined as a name of type "other", with the
NAMETYPE part filled in with the string literal "WELLKNOWN".
other: WELLKNOWN:realm-name
This name type is designated for well-known Kerberos realms.
The AS and the application server MUST reject the authentication
attempt if a well-known realm name is used as the client realm or the
server realm but not supported. The TGS [RFC4120] MAY reject the
authentication attempt if a well-known realm name is used as the
client realm but not supported, and SHOULD reject the authentication
attempt if a well-known realm name is used as the server realm but
not supported. Unless otherwise specified, if a well-known realm
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name is used but not supported in any other places of Kerberos
messages, authentication MUST fail. The error code is
KRB_AP_ERR_REALM_UNKNOWN, and there is no accompanying error data
defined in this document for this error.
KRB_AP_ERR_REALM_UNKNOWN 83
-- A well-known Kerberos realm name is used but not
-- supported.
Unless otherwise specified, all principal names involving a well-
known realm name are reserved, and if a reserved principal name is
used but not supported, and if the authentication is rejected, the
error code MUST be KRB_AP_ERR_PRINCIPAL_RESERVED.
KRB_AP_ERR_PRINCIPAL_RESERVED 84
-- A reserved Kerberos principal name is used but not
-- supported.
There is no accompanying error data defined in this document for this
error.
According to Section 3.3.3.2 of [RFC4120], the TGS MUST add the name
of the previous realm into the transited field of the returned
ticket. Typically well-known realms are defined to carry special
meanings, and they are not used to refer to intermediate realms in
the client's authentication path. Consequently, unless otherwise
specified, the TGS MUST NOT encode a well-known Kerberos realm name
into the transited field [RFC4120] of a ticket, and parties checking
the transited realm path MUST reject a transited realm path that
includes a well known realm. In the case of KDCs checking the
transited realm path, this means that the TRANSITED-POLICY-CHECKED
flag MUST NOT be set in the resulting ticket. Aside from the
hierarchical meaning of a null subfield, the DOMAIN-X500-COMPRESS
encoding for transited realms [RFC4120] treats realm names as
strings, although it is optimized for domain style and X.500 realm
names, hence the DOMAIN-X500-COMPRESS encoding can be used when the
client realm or the server realm is reserved or when a reserved realm
is in the transited field. However, if the client's realm is a well-
known realm, the abbreviation forms [RFC4120] that build on the
preceding name cannot be used at the start of the transited encoding.
The null-subfield form (e.g., encoding ending with ",") [RFC4120]
could not be used next to a well-known realm, including potentially
at the beginning and end where the client and server realm names,
respectively, are filled in.
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4. Security Considerations
It is possible to have name collision with well-known names because
Kerberos as defined in [RFC4120] does not reserve names that have
special meanings, consequently care MUST be taken to avoid accidental
reuse of names. If a well-known name is not supported,
authentication MUST fail as specified in Section 3. Otherwise,
access can be granted unintentionally, resulting in a security
weakness. Consider for example, a KDC that supports this
specification but not the anonymous authentication described in
[ANON]. Assume further that the KDC allows a principal to be created
named identically to the anonymous principal. If that principal were
created and given access to resources, then anonymous users might
inadvertently gain access to those resources if the KDC supports
anonymous authentication at some future time. Similar issues may
occur with other well-known names. By requiring KDCs reject
authentication with unknown well-known names, we minimize these
concerns.
If a well-known name was created before the KDC is updated to conform
to this specification, it SHOULD be renamed. The provisioning code
that manages account creation MUST be updated to disallow creation of
principals with unsupported well-known names.
5. Acknowledgements
The initial document was mostly based on the author's conversation
with Clifford Newman and Sam Hartman.
Jeffery Hutzelman, Ken Raeburn, and Stephen Hanna provided helpful
suggestions for improvements to early revisions of this document.
6. IANA Considerations
This document provides the framework for defining well-known Kerberos
names and Kerberos realms. A new IANA registry should be created to
contain well-known Kerberos names and Kerberos realms that are
defined based on this document. The evaluation policy is
"Specification Required".
7. References
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7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005.
7.2. Informative References
[ANON] Zhu, L., Leach, P. and K. Jaganathan, "Kerberos Anonymity
Support", draft-ietf-krb-wg-anon, work in progress.
Author's Address
Larry Zhu
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
Email: lzhu@microsoft.com
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