XEP-0233: Domain-Based Service Names in XMPP SASL Negotiation

WARNING: This document has been automatically Deferred after 12 months of inactivity in its previous Experimental state. Implementation of the protocol described herein is not recommended for production systems. However, exploratory implementations are encouraged to resume the standards process.

Table of Contents

1. Introduction

In environments that make use of Kerberos V5 (RFC 4120 [1]) and negotiation of Simple Authentication and Security Layer or SASL (RFC 4422 [2]) over XMPP, a connecting client often needs to know the identity of the Kerberos acceptor principal so that it can obtain a proper ticket for authentication. This scenario was not addressed in RFC 3920 [3] or RFC 6120 [4]. However, the problem can be solved using the concept of domain-based service names (RFC 5178 [5]). In particular, when an XMPP server uses the Kerberos V5 ("GSSAPI") SASL mechanism (RFC 4752 [6]), it can communicate the identity of the acceptor principal as a Kerberos V5 service principal name (RFC 5179 [7]). This document defines an XMPP method for such communication.

2. Protocol

The acceptor principal's hostname is communicated by including 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). In the case of the Kerberos V5 SASL mechanism, the child element is a <hostname/> element 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 acceptor principal. The client then generates a domain-based service name from the provided hostname, following the format specified in RFC 5179 (i.e., "protocol/hostname/domainname@REALM") and setting the values as follows:

• The protocol string MUST be "xmpp".
• The hostname string MUST be the XML character data of the <hostname/> element.
• The domainname string MUST be the canonical name of the service, such as typically communicated 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).

Consider the example of an XMPP service whose canonical name is "example.com". A user might make use of an acceptor principal located at "auth42.us.example.com". The hostname would be communicated 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> 
  

The client would then attempt to obtain a ticket for the domain-based principal "xmpp/auth42.us.example.com/example.com@EXAMPLE.COM".

3. Security Considerations

The communication of acceptor principal 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 [8]) as described for XMPP in RFC 6120. For additional security considerations, refer to RFC5178 and RFC 5179.

4. IANA Considerations

This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [9].

5. XMPP Registrar Considerations

6. 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>
  

7. 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: 0.4
Last Updated: 2011-08-26
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

Appendix C: Legal Notices

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 <standards@xmpp.org> 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 <xsf-ietf@xmpp.org> list might also be appropriate.

Errata can be sent to <editor@xmpp.org>.

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

Appendix H: Revision History

Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/

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