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

WARNING: This Standards-Track document is Experimental. Publication as an XMPP Extension Protocol does not imply approval of this proposal by the XMPP Standards Foundation. Implementation of the protocol described herein is encouraged in exploratory implementations, but production systems are advised to carefully consider whether it is appropriate to deploy implementations of this protocol before it advances to a status of Draft.

Table of Contents

1. Introduction

In certain kinds of XMPP deployments, multiple connection managers associated with the XMPP server can be used to handle requests from connecting clients. In such an architecture, the connection manager might need to communicate the hostname to which the client has connected, or information about alternative connection managers.

This is especially true 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, because the client might need additional information about the Kerberos 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 as described in RFC 5178 [5]. In particular, because XMPP servers typically use the Kerberos V5 ("GSSAPI") SASL mechanism as described in RFC 4752 [6], they can communicate domain-based names as Kerberos V5 service principal names as described in RFC 5179 [7].

Therefore this document defines a method for communication of authentication hostnames (especially Kerberos V5 domain-based service names) in the context of SASL negotiation by XMPP entities.

2. Use with Kerberos

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 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:0' namespace (see Protocol Namespaces regarding issuance of one or more permanent namespaces). In the context of Kerberos, 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 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
• 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 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:domain-based-name:0'
            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. Non-Kerberos Use Cases

This protocol can also be used to communicate connection manager hostnames outside the context of Kerberos. In this case, the <hostname/> element MUST NOT include the 'mechanism' attribute, and multiple instances of the <hostname/> element MAY be included. An example follows.

Example 2. Communicating hostnames

<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
  <mechanism>GSSAPI</mechanism>
  <mechanism>DIGEST-MD5</mechanism>
  <required/>
  <hostname xmlns='urn:xmpp:domain-based-name:0'>cm3.us.example.com</hostname>
  <hostname xmlns='urn:xmpp:domain-based-name:0'>cm5.us.example.com</hostname>
  <hostname xmlns='urn:xmpp:domain-based-name:0'>cm9.us.example.com</hostname>
</mechanisms> 
  

4. Security Considerations

The communication of hostnames 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.

5. IANA Considerations

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

6. XMPP Registrar Considerations

7. 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:0'
    xmlns='urn:xmpp:domain-based-name:0'
    elementFormDefault='qualified'>

  <xs:element name='hostname'>
    <xs:complexType>
      <xs:simpleContent>
        <xs:extension base='xs:string'>
          <xs:attribute name='mechanism' type='xs:NMTOKEN' use='optional'/>
        </xs:extension>
      </xs:simpleContent>
    </xs:complexType>
  </xs:element>

</xs:schema>
  

8. Acknowledgements

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

Appendices

Appendix A: Document Information

Series: XEP
Number: 0233
Publisher: XMPP Standards Foundation
Status: Experimental
Type: Standards Track
Version: 0.3
Last Updated: 2011-06-22
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 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.

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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