





Internet Engineering Task Force (IETF)                            L. Zhu
Request for Comments: 6111                         Microsoft Corporation
Updates: 4120                                                 April 2011
Category: Standards Track
ISSN: 2070-1721


                 Additional Kerberos Naming Constraints

Abstract

   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.

   This document is a product of the Internet Engineering Task Force
   (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.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6111.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.









Zhu                          Standards Track                    [Page 1]

RFC 6111                     Kerberos Naming                  April 2011


   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1. Introduction ....................................................2
   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 .........................................5
   5. Acknowledgements ................................................6
   6. IANA Considerations .............................................6
   7. References ......................................................6
      7.1. Normative References .......................................6
      7.2. Informative References .....................................6

1.  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
   granted unexpectedly.




Zhu                          Standards Track                    [Page 2]

RFC 6111                     Kerberos Naming                  April 2011


   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
   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].

            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
   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-



Zhu                          Standards Track                    [Page 3]

RFC 6111                     Kerberos Naming                  April 2011


   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 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".

            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 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.

            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.





Zhu                          Standards Track                    [Page 4]

RFC 6111                     Kerberos Naming                  April 2011


   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.

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.





Zhu                          Standards Track                    [Page 5]

RFC 6111                     Kerberos Naming                  April 2011


5.  Acknowledgements

   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.  References

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.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

7.2.  Informative References

   [RFC6112]  Zhu, L., Leach, P., and S. Hartman, "Anonymity Support for
              Kerberos", RFC 6112, April 2011.

Author's Address

   Larry Zhu
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   US

   EMail: lzhu@microsoft.com






Zhu                          Standards Track                    [Page 6]

