Additional Kerberos Naming Constraints
draft-ietf-krb-wg-naming-05

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Abstract

This document defines new naming constraints for well-known Kerberos principal name and well-known Kerberos realm names.

Table of Contents

1.  Introduction
2.  Conventions Used in This Document
3.  Definitions
    3.1.  Well-known Kerberos Principal Names
    3.2.  Well-known Kerberos Realm Names
4.  Security Considerations
5.  Acknowledgements
6.  IANA Considerations
7.  References
    7.1.  Normative References
    7.2.  Informative References
§  Author's Address
§  Intellectual Property and Copyright Statements

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] (Raeburn, K., “Kerberos Anonymity Support,” December 2004.). 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] (Neuman, C., Yu, T., Hartman, S., and K. Raeburn, “The Kerberos Network Authentication Service (V5),” July 2005.) 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] (Neuman, C., Yu, T., Hartman, S., and K. Raeburn, “The Kerberos Network Authentication Service (V5),” July 2005.) 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 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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

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] (Neuman, C., Yu, T., Hartman, S., and K. Raeburn, “The Kerberos Network Authentication Service (V5),” July 2005.).

         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] (Neuman, C., Yu, T., Hartman, S., and K. Raeburn, “The Kerberos Network Authentication Service (V5),” July 2005.) and the application server MUST reject the authentication attempt. Similarly, the Ticket Granting Service (TGS) [RFC4120] (Neuman, C., Yu, T., Hartman, S., and K. Raeburn, “The Kerberos Network Authentication Service (V5),” July 2005.) 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 is a well-known principal name is used as the server principal name but not supported. 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] (Neuman, C., Yu, T., Hartman, S., and K. Raeburn, “The Kerberos Network Authentication Service (V5),” July 2005.) 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] (Neuman, C., Yu, T., Hartman, S., and K. Raeburn, “The Kerberos Network Authentication Service (V5),” July 2005.) 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 is 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, authentication MUST fail with the code 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] (Neuman, C., Yu, T., Hartman, S., and K. Raeburn, “The Kerberos Network Authentication Service (V5),” July 2005.), 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, a well-known Kerberos realm name MUST NOT be present in the transited field [RFC4120] (Neuman, C., Yu, T., Hartman, S., and K. Raeburn, “The Kerberos Network Authentication Service (V5),” July 2005.) of a ticket. Aside from the hierarchical meaning of a null subfield, the DOMAIN-X500-COMPRESS encoding for transited realms [RFC4120] (Neuman, C., Yu, T., Hartman, S., and K. Raeburn, “The Kerberos Network Authentication Service (V5),” July 2005.) 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] (Neuman, C., Yu, T., Hartman, S., and K. Raeburn, “The Kerberos Network Authentication Service (V5),” July 2005.) 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] (Neuman, C., Yu, T., Hartman, S., and K. Raeburn, “The Kerberos Network Authentication Service (V5),” July 2005.) 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

If a well-known name is not supported, authentication MUST fail. Otherwise, access can be granted unintentionally, resulting in a security weakness.

Care MUST be taken to avoid accidental reuse of names.

5.  Acknowledgements

The initial document was mostly based on the author's conversation with Clifford Newman and Sam Hartman.

Jeffery Hutzelman and Ken Raeburn provided helpful commments for improvements based on 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

7.1. Normative References

7.2. Informative References

Author's Address

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