[Standards] Proposed changes to XEP-0233

Wed, 24 Jun 2015 04:42:08 -0700 

Hi,
I've proposed some changes to XEP-0233, mainly about how to construct the Kerberos Principal Name for an XMPP server on Windows. 

Could I please have some feedback on the attached xep before I submit it?

Cheers!
Mili

Title: XEP-0233: XMPP Server Registration for use with Kerberos V5





XEP-0233: XMPP Server Registration for use with Kerberos V5
Abstract:This specification defines the Kerberos principal name of an XMPP server. It also details a method by which a connecting client can determine this Kerberos principal name when authenticating using the "GSSAPI" SASL mechanism.
Authors:Matthew Miller, Peter Saint-Andre, Joe Hildebrand
Copyright:© 1999 - 2014 XMPP Standards Foundation. SEE LEGAL NOTICES.
Status:Deferred
Type:Standards Track
Version:0.5
Last Updated:2015-05-06
WARNING: Consideration of this document has been Deferred by the XMPP Standards Foundation. Implementation of the protocol described herein is not recommended.
Table of Contents

1.  Introduction
2.  Client Determination of Hostname
3.  Kerberos Principal Name of the XMPP Server
4.  Kerberos Principal Name in the Microsoft Windows environment
5.  Examples
6.  Security Considerations
7.  IANA Considerations
8.  XMPP Registrar Considerations
   
      8.1.  Protocol Namespaces
   
      8.2.  Protocol Versioning
9.  XML Schema
10.  Acknowledgements
Appendices
    A: Document Information
    B: Author Information
    C: Legal Notices
    D: Relation to XMPP
    E: Discussion Venue
    F: Requirements Conformance
    G: Notes
    H: Revision History
1.
       Introduction

  
The Kerberos Network Authentication Service (V5) is described in RFC 4120  [1]. An application can call the Kerberos library through the Generic Security Services Application Programming Interface or GSS-API (RFC 4121  [2]).

  
The Simple Authentication and Security Layer or SASL (RFC 4422  [3]) is a framework for adding authentication support to connection-based protocols. The SASL mechanism used in environments that make use of Kerberos V5 is called "GSSAPI" and is described in RFC 4752  [4]. Note that the API has a hyphen and the SASL mechanism does not.

  
Before using the "GSSAPI" SASL mechanism to authenticate to an XMPP server (which is referred to as the "acceptor" in Kerberos terminology), a connecting client needs to obtain a Kerberos ticket. For this the client needs to determine the Kerberos principal name of the XMPP server. This scenario was not addressed in RFC 3920  [5] or RFC 6120  [6].

  
This specification sets out the rules that must be followed when registering the Kerberos principal name of an XMPP server. It also details how a client can determine the hostname of the XMPP server which can then be used to construct the Kerberos principal name.


2.
       Client Determination of Hostname

  
An XMPP client will initiate a connection to the XMPP server.

  
The XMPP server will communicate its hostname in a child element of the <mechanisms/> element during SASL negotation, as allowed by RFC 6120 (see Section 6.3.5 and the schema for the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace in Appendix A.4).

  
This child element is <hostname/> qualified by the 'urn:xmpp:domain-based-name:1' namespace. (see Protocol Namespaces regarding issuance of one or more permanent namespaces).

  
The XML character data of the <hostname/> element specifies the fully-qualified name of the XMPP server. This should be used for constructing the Kerberos principal name and is independent of the usual rules that an XMPP client uses for establishing a network connection to the XMPP server.


3.
       Kerberos Principal Name of the XMPP Server

  
When using GSS-API, the domain-based service name (RFC 5178  [7], RFC 5179  [8]) is used as the Kerberos principal name. Domain-based service names contain a domain name in addition to a hostname. This allows naming clustered servers after the domain which they service.

  
The domain-based service name is mapped to the Kerberos principal name following the format specified in RFC 5179 (i.e., "service/hostname/domain@REALM") and setting the values as follows:

  
    
    
  • The service string MUST be "xmpp".
    • 
    
  • The hostname string MUST be the hostname of the XMPP server, as provided by the server in the XML character data of the <hostname/> element during SASL negotiation.
    • 
    
  • The domain string MUST be the canonical name of the service. This is typically communicated by the client in the 'to' address of the initial stream header.
    • 
    
  • The REALM string SHOULD be determined according to the network policies in effect (usually the domain name, in an uppercase mapping).
    • 
  


4.
       Kerberos Principal Name in the Microsoft Windows environment

  
Microsoft Windows implements the proprietary Security Service Provider Interface (SSPI) and not GSS-API to support the "GSSAPI" SASL mechanism. This section describes the Windows equivalent of the domain-based service name and so is necessary for developers of applications in the Windows environment.

  
In the Microsoft Windows environment, the concept of Service Principal Name (SPN) is used, which is specified in https://msdn.microsoft.com/en-us/library/ms677601%28v=vs.85%29.aspx. This format ("service class/host:port/service name") is similar to the one specified in RFC 5179. The SPN can be generated by setting the values as follows:

  
    
    
  • The service class string MUST be "xmpp".
    • 
    
  • The host string MUST be the hostname of the XMPP server, as provided by the server in the XML character data of the <hostname/> element during SASL negotiation.
    • 
    
  • The port is optional. It can be used to differentiate between multiple XMPP servers on a single host computer and should be omitted if the XMPP server uses the default port of 5222 for accepting client connections.
    • 
    
  • The service name string MUST be the canonical name of the service. This is typically communicated by the client in the 'to' address of the initial stream header.
    • 
  



5.
       Examples

  
Consider the example of an XMPP service "example.com" offered by the XMPP server located on the host "auth42.us.example.com". When a client connects to the XMPP server, the server communicates its hostname along with supported SASL mechanisms as follows:

  
Example 1. Communicating the hostname
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
  <mechanism>GSSAPI</mechanism>
  <mechanism>DIGEST-MD5</mechanism>
  <hostname xmlns='urn:xmpp:domain-based-name:1'>auth42.us.example.com</hostname>
</mechanisms> 
  

  
To use the "GSSAPI" SASL mechanism, the client needs to determine the Kerberos principal name of the XMPP server, which will be:

  
    
    
  • the domain-based service name "xmpp/auth42.us.example.com/[email protected]"
    • 
    
  • or if using in the Microsoft Windows environment, and assuming that the default port is used, the SPN "xmpp/auth42.us.example.com/example.com".
    • 
  


6.
       Security Considerations

  
The communication of the XMPP server's hostname during SASL negotiation is not known to introduce new security vulnerabilities, as long as it is done after the underlying channel has been secured using Transport Layer Security (TLS; RFC 5246  [9]) as described for XMPP in RFC 6120. For additional security considerations, refer to RFC 5178 and RFC 5179.


7.
       IANA Considerations

  
This document requires no interaction with the Internet Assigned Numbers Authority (IANA)  [10].
 

8.
       XMPP Registrar Considerations

  
8.1 Protocol Namespaces

    
This specification defines the following XML namespace:

    
    
      
  • urn:xmpp:domain-based-name:1
    • 
    

    
Upon advancement of this specification from a status of Experimental to a status of Draft, the XMPP Registrar  [11] shall add the foregoing namespace to the registry located at <http://xmpp.org/registrar/namespaces.html>, as described in Section 4 of XMPP Registrar Function (XEP-0053)  [12].

  

  
8.2 Protocol Versioning

    
If the protocol defined in this specification undergoes a revision that is not fully backwards-compatible with an older version, the XMPP Registrar shall increment the protocol version number found at the end of the XML namespaces defined herein, as described in Section 4 of XEP-0053.

  


9.
       XML Schema

  
<?xml version='1.0' encoding='UTF-8'?>

<xs:schema
    xmlns:xs='http://www.w3.org/2001/XMLSchema'
    targetNamespace='urn:xmpp:domain-based-name:1'
    xmlns='urn:xmpp:domain-based-name:1'
    elementFormDefault='qualified'>

  <xs:element name='hostname' type='xs:string'/>

</xs:schema>
  


10.
       Acknowledgements

  
Thanks to Owen Friel, Shane Hannon, Seamus Kerrigan, Eliot Lear, Alexey Melnikov, and Klaas Wierenga for their comments.


Appendices
Appendix A: Document Information

            Series: XEP

            Number: 0233

            Publisher: XMPP Standards Foundation

            Status: 
            Deferred

            Type:
            Standards Track

            Version: 0.5

            Last Updated: 2015-05-06

                Approving Body: XMPP Council
Dependencies: XMPP Core, RFC 5178, RFC 5179

                Supersedes: None

                Superseded By: None

            Short Name: NOT YET ASSIGNED

              Source Control: 
                HTML

            This document in other formats: 
                XML 
                PDF
Appendix B: Author Information
Matthew Miller

        Email:
        [email protected]

        JabberID: 
        [email protected]
Peter Saint-Andre

        Email:
        [email protected]

        JabberID: 
        [email protected]

        URI: 
        https://stpeter.im/
Joe Hildebrand

        Email:
        [email protected]

        JabberID: 
        [email protected]
Appendix C: Legal Notices
Copyright
This XMPP Extension Protocol is copyright © 1999 - 2014 by the XMPP Standards Foundation (XSF).
Permissions
Permission is hereby granted, free of charge, to any person obtaining a copy of this specification (the "Specification"), to make use of the Specification without restriction, including without limitation the rights to implement the Specification in a software program, deploy the Specification in a network service, and copy, modify, merge, publish, translate, distribute, sublicense, or sell copies of the Specification, and to permit persons to whom the Specification is furnished to do so, subject to the condition that the foregoing copyright notice and this permission notice shall be included in all copies or substantial portions of the Specification. Unless separate permission is granted, modified works that are redistributed shall not contain misleading information regarding the authors, title, number, or publisher of the Specification, and shall not claim endorsement of the modified works by the authors, any organization or project to which the authors belong, or the XMPP Standards Foundation.
Disclaimer of Warranty
## NOTE WELL: This Specification is provided on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, express or implied, including, without limitation, any warranties or conditions of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A PARTICULAR PURPOSE. ##
Limitation of Liability
In no event and under no legal theory, whether in tort (including negligence), contract, or otherwise, unless required by applicable law (such as deliberate and grossly negligent acts) or agreed to in writing, shall the XMPP Standards Foundation or any author of this Specification be liable for damages, including any direct, indirect, special, incidental, or consequential damages of any character arising from, out of, or in connection with the Specification or the implementation, deployment, or other use of the Specification (including but not limited to damages for loss of goodwill, work stoppage, computer failure or malfunction, or any and all other commercial damages or losses), even if the XMPP Standards Foundation or such author has been advised of the possibility of such damages.
IPR Conformance
This XMPP Extension Protocol has been contributed in full conformance with the XSF's Intellectual Property Rights Policy (a copy of which can be found at <http://xmpp.org/about-xmpp/xsf/xsf-ipr-policy/> or obtained by writing to XMPP Standards Foundation, 1899 Wynkoop Street, Suite 600, Denver, CO 80202 USA).
Appendix D: Relation to XMPP
The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) specifications contributed by the XMPP Standards Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.
Appendix E: Discussion Venue
The primary venue for discussion of XMPP Extension Protocols is the <[email protected]> discussion list.
Discussion on other xmpp.org discussion lists might also be appropriate; see <http://xmpp.org/about/discuss.shtml> for a complete list.
Given that this XMPP Extension Protocol normatively references IETF technologies, discussion on the <[email protected]> list might also be appropriate.
Errata can be sent to <[email protected]>.
Appendix F: Requirements Conformance
The following requirements keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".
Appendix G: Notes
1. RFC 4120: The Kerberos Network Authentication Service (V5) <http://tools.ietf.org/html/rfc4120>.
2. RFC 4121: The Kerberos Version 5 Generic Security Service Application Program Interface (GSS-API) Mechanism: Version 2 <http://tools.ietf.org/html/rfc4121>.
3. RFC 4422: Simple Authentication and Security Layer (SASL) <http://tools.ietf.org/html/rfc4422>.
4. RFC 4752: The Kerberos V5 ("GSSAPI") Simple Authentication and Security Layer (SASL) Mechanism  <http://tools.ietf.org/html/rfc4752>.
5. RFC 3920: Extensible Messaging and Presence Protocol (XMPP): Core <http://tools.ietf.org/html/rfc3920>.
6. RFC 6120: Extensible Messaging and Presence Protocol (XMPP): Core <http://tools.ietf.org/html/rfc6120>.
7. RFC 5178: Generic Security Service Application Program Interface (GSS-API) Internationalization and Domain-Based Service Names and Name Type <http://tools.ietf.org/html/rfc5178>.
8. RFC 5179: Generic Security Service Application Program Interface (GSS-API) Domain-Based Service Names Mapping for the Kerberos V GSS Mechanism <http://tools.ietf.org/html/rfc5179>.
9. RFC 5246: The Transport Layer Security (TLS) Protocol Version 1.2 <http://tools.ietf.org/html/rfc5246>.
10. The Internet Assigned Numbers Authority (IANA) is the central coordinator for the assignment of unique parameter values for Internet protocols, such as port numbers and URI schemes. For further information, see <http://www.iana.org/>.
11. The XMPP Registrar maintains a list of reserved protocol namespaces as well as registries of parameters used in the context of XMPP extension protocols approved by the XMPP Standards Foundation. For further information, see <http://xmpp.org/registrar/>.
12. XEP-0053: XMPP Registrar Function <http://xmpp.org/extensions/xep-0053.html>.
Appendix H: Revision History
Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/
Version 0.5 (2015-05-06)
Included support for Microsoft Windows Service Principal Name.
 (mv)
    
Version 0.4 (2011-08-26)
Focused the document purely on the Kerberos use case, with the intent of handling the non-Kerberos use case in a separate specification; incremented the protocol version number to prevent confusion with deployed uses of the non-Kerberos functionality.
 (psa)
    
Version 0.3 (2011-06-22)
Tightened the conformance terminology in several places; updated references.
 (psa)
    
Version 0.2 (2010-06-10)
Expanded use beyond Kerberos; updated namespace; corrected schema; updated references.
 (psa)
    
Version 0.1 (2008-01-30)
Initial published version.
 (psa)
    
Version 0.0.2 (2007-12-11)
Corrected syntax.
 (mm/psa)
    
Version 0.0.1 (2007-12-05)
First draft.
 (mm/psa)
    
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