Internet Engineering Task Force (IETF) L. Zhu
Request for Comments: 6111 Microsoft Corporation
Updates: 4120 April 2011
Category: Standards Track
Additional Kerberos Naming Constraints
This document defines new naming constraints for well-known Kerberos
principal names and well-known Kerberos realm names.
Status of This Memo
This is an Internet Standards Track document.
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(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
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Table of Contents
1. Introduction ....................................................22. Conventions Used in This Document ...............................33. Definitions .....................................................33.1. Well-Known Kerberos Principal Names ........................33.2. Well-Known Kerberos Realm Names ............................44. Security Considerations .........................................55. Acknowledgements ................................................66. IANA Considerations .............................................67. References ......................................................67.1. Normative References .......................................67.2. Informative References .....................................61. Introduction
Occasionally, protocol designers need to designate a Kerberos
principal name or a Kerberos realm name to have a special meaning
other than identifying a particular instance. An example is that the
anonymous principal name and the anonymous realm name are defined for
the Kerberos anonymity support [RFC6112]. 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 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
names that are exceedingly unlikely to have been used to avoid
collision. Even if a registry were set up to avoid collision of new
implementations, there is no guarantee for deployed implementations
preventing accidental reuse of names that can lead to access being
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 remedies these issues by defining well-known Kerberos
names and the protocol behavior when a well-known name is used but
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].
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
A well-known principal name MUST have at least two or more
KerberosString components, and the first component MUST be the string
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
Ticket Granting Ticket (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 Key Distribution Center (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.
-- A well-known Kerberos principal name is used but not
3.2. Well-Known Kerberos Realm Names
Section 6.1 of [RFC4120] defines the "other" style of 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".
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 it 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 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.
-- A well-known Kerberos realm name is used but not
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.
-- A reserved Kerberos principal name is used but not
There is no accompanying error data defined in this document for this
According to Section 18.104.22.168 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.
4. Security Considerations
It is possible to have a name collision with well-known names because
Kerberos, as defined in [RFC4120], does not reserve names that have
special meanings; accidental reuse of names MUST be avoided. 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 [RFC6112]. 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 that 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.
The initial document was mostly based on the author's conversation
with Clifford Newman and Sam Hartman.
Jeffrey 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. Two new IANA registries have been created
to contain well-known Kerberos principal names and Kerberos realm
names that are defined based on this document. The evaluation policy
for each is "Specification Required", as specified in [RFC5226].
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,
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
7.2. Informative References
[RFC6112] Zhu, L., Leach, P., and S. Hartman, "Anonymity Support for
Kerberos", RFC 6112, April 2011.
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