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

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

In certain kinds of XMPP deployments, multiple connection managers associated with the XMPP server may be used to handle requests from connecting clients. When such an architecture is used in environments that make use of Kerberos V and negotiation of Simple Authentication and Security Layer (SASL) over XMPP, the connection manager may need to give the client additional information about its principal so that the client can obtain a proper ticket for authentication.

This scenario was not addressed in RFC 3920 [1]. However, the problem can now be solved using the concept of domain-based service names as described in draft-ietf-kitten-gssapi-domain-based-names [2]. In particular, because XMPP servers typically use the Kerberos V5 ("GSSAPI") SASL mechanism as described in RFC 4752 [3], they can communicate domain-based names as Kerberos V service principal names as described in draft-ietf-kitten-krb5-gssapi-domain-based-names [4].

Therefore this document defines a method for communication of Kerberos V domain-based service names in the context of SASL negotiation by XMPP entities.

2. Protocol

When a connection manager associated with an XMPP server needs to communicate additional information about its service principal name to a connecting client, it can do so by including a child element of the <mechanisms/> element during SASL negotation, as specified in rfc3920bis [5]. In the case of the Kerberos V SASL mechanism, the child element is a <hostname/> element qualified by the 'urn:xmpp:tmp:domain-based-name' namespace (see Protocol Namespaces regarding issuance of one or more permanent namespaces). The <hostname/> element MUST include a 'mechanism' attribute, where the value MUST be "GSSAPI". The XML character data of the <hostname/> element shall specify the fully-qualified name of the connection manager (known as the hostname). The client then generates a domain-based service name from the provided hostname, following the format specified in draft-ietf-kitten-krb5-gssapi-domain-based-names (i.e., "protocol/hostname/domainname@REALM"):

• 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
• 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 may in fact connect to the physical machine "cm7.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>
  <required/>
  <hostname xmlns='urn:xmpp:tmp:domain-based-name'
            mechanism='GSSAPI'>cm7.us.example.com</hostname>
</mechanisms> 
  

The client would then attempt to obtain a ticket for the domain-based principal "xmpp/cm7.us.example.com/example.com@EXAMPLE.COM".

3. Security Considerations

The communication of hostnames during SASL negotiation is not known to introduce new security vulnerabilities. Communication of hostnames SHOULD NOT occur until after the underlying channel has been secured using Transport Layer Security (TLS; RFC 4346 [6]) as described for XMPP in RFC 3920 and rfc3920bis. For additional security considerations, refer to draft-ietf-kitten-gssapi-domain-based-names and draft-ietf-kitten-krb5-gssapi-domain-based-names.

4. IANA Considerations

This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [7].

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:tmp:domain-based-name'
    xmlns='urn:xmpp:tmp:domain-based-name'
    elementFormDefault='qualified'>

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

</xs:schema>
  

7. Acknowledgements

Thanks to Alexey Melnikov for his comments.

Appendices

Appendix A: Document Information

Series: XEP
Number: 0233
Publisher: XMPP Standards Foundation
Status: Deferred
Type: Standards Track
Version: 0.1
Last Updated: 2008-01-30
Approving Body: XMPP Council
Dependencies: XMPP Core, draft-ietf-kitten-gssapi-domain-based-names, draft-ietf-kitten-krb5-gssapi-domain-based-names
Supersedes: None
Superseded By: None
Short Name: NOT YET ASSIGNED
Source Control: HTML  RSS
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 3920) and XMPP IM (RFC 3921) 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.

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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