Additional Kerberos Naming Constraints This document defines new naming constraints for well-known Kerberos principal names and well-known Kerberos realm names

L. Zhu

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 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
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    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
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    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] 

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