xeps/xep-0233.xml

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  <!ENTITY % ents SYSTEM 'xep.ent'>
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<xep>
<header>
  <title>Domain-Based Service Names in XMPP SASL Negotiation</title>
  <abstract>This specification defines a method by which a connection manager associated with an XMPP server can inform a connecting client about its domain-based service name.</abstract>
  &LEGALNOTICE;
  <number>0233</number>
  <status>Experimental</status>
  <type>Standards Track</type>
  <sig>Standards</sig>
  <approver>Council</approver>
  <dependencies>
    <spec>XMPP Core</spec>
    <spec>RFC 5178</spec>
    <spec>RFC 5179</spec>
  </dependencies>
  <supersedes/>
  <supersededby/>
  <shortname>NOT YET ASSIGNED</shortname>
  &linuxwolf;
  &stpeter;
  &hildjj;
  <revision>
    <version>0.3</version>
    <date>2011-06-22</date>
    <initials>psa</initials>
    <remark><p>Tightened the conformance terminology in several places; updated references.</p></remark>
  </revision>
  <revision>
    <version>0.2</version>
    <date>2010-06-10</date>
    <initials>psa</initials>
    <remark><p>Expanded use beyond Kerberos; updated namespace; corrected schema; updated references.</p></remark>
  </revision>
  <revision>
    <version>0.1</version>
    <date>2008-01-30</date>
    <initials>psa</initials>
    <remark><p>Initial published version.</p></remark>
  </revision>
  <revision>
    <version>0.0.2</version>
    <date>2007-12-11</date>
    <initials>mm/psa</initials>
    <remark><p>Corrected syntax.</p></remark>
  </revision>
  <revision>
    <version>0.0.1</version>
    <date>2007-12-05</date>
    <initials>mm/psa</initials>
    <remark><p>First draft.</p></remark>
  </revision>
</header>

<section1 topic='Introduction' anchor='intro'>
  <p>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.</p>
  <p>This is especially true in environments that make use of Kerberos V5 (&rfc4120;) and negotiation of Simple Authentication and Security Layer or SASL (&rfc4422;) over XMPP, because the client might need additional information about the Kerberos principal so that it can obtain a proper ticket for authentication.</p>
  <p>This scenario was not addressed in &rfc3920; or &rfc6120;. However, the problem can be solved using the concept of domain-based service names as described in &rfc5178;. In particular, because XMPP servers typically use the Kerberos V5 ("GSSAPI") SASL mechanism as described in &rfc4752;, they can communicate domain-based names as Kerberos V5 service principal names as described in &rfc5179;.</p>
  <p>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.</p>
</section1>

<section1 topic='Use with Kerberos' anchor='kerberos'>
  <p>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 &lt;mechanisms/&gt; element during SASL negotation, as allowed by <cite>RFC 6120</cite> (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 &lt;hostname/&gt; element qualified by the 'urn:xmpp:domain-based-name:0' namespace &NSNOTE;. In the context of Kerberos, the &lt;hostname/&gt; element MUST include a 'mechanism' attribute, where the value MUST be "GSSAPI". The XML character data of the &lt;hostname/&gt; 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 <cite>RFC 5179</cite> (i.e., "protocol/hostname/domainname@REALM") and setting the values as follows:</p>
  <ul>
    <li>The <strong>protocol</strong> string MUST be "xmpp"</li>
    <li>The <strong>hostname</strong> string MUST be the XML character data of the &lt;hostname/&gt; element</li>
    <li>The <strong>domainname</strong> string MUST be the canonical name of the service</li>
    <li>The <strong>REALM</strong> string SHOULD be determined according to the network policies in effect (usually the domain name, in an uppercase mapping)</li>
  </ul>
  <p>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.</p>
  <example caption="Communicating the hostname"><![CDATA[
<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> 
  ]]></example>
  <p>The client would then attempt to obtain a ticket for the domain-based principal "xmpp/cm7.us.example.com/example.com@EXAMPLE.COM".</p>
</section1>

<section1 topic='Non-Kerberos Use Cases' anchor='other'>
  <p>This protocol can also be used to communicate connection manager hostnames outside the context of Kerberos. In this case, the &lt;hostname/&gt; element MUST NOT include the 'mechanism' attribute, and multiple instances of the &lt;hostname/&gt; element MAY be included. An example follows.</p>
  <example caption="Communicating hostnames"><![CDATA[
<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> 
  ]]></example>
</section1>

<section1 topic='Security Considerations' anchor='security'>
  <p>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; &rfc5246;) as described for XMPP in <cite>RFC 6120</cite>. For additional security considerations, refer to <cite>RFC5178</cite> and <cite>RFC 5179</cite>.</p>
</section1>

<section1 topic='IANA Considerations' anchor='iana'>
  <p>This document requires no interaction with &IANA;.</p> 
</section1>

<section1 topic='XMPP Registrar Considerations' anchor='registrar'>
  <section2 topic='Protocol Namespaces' anchor='registrar-ns'>
    <p>This specification defines the following XML namespace:</p>
    <ul>
      <li>urn:xmpp:domain-based-name:0:0</li>
    </ul>
    <p>Upon advancement of this specification from a status of Experimental to a status of Draft, the &REGISTRAR; shall add the foregoing namespace to the registry located at &NAMESPACES;, as described in Section 4 of &xep0053;.</p>
  </section2>
  <section2 topic='Protocol Versioning' anchor='registrar-versioning'>
    &NSVER;
  </section2>
</section1>

<section1 topic='XML Schema' anchor='schema'>
  <code><![CDATA[
<?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>
  ]]></code>
</section1>

<section1 topic='Acknowledgements' anchor='ack'>
  <p>Thanks to Owen Friel, Shane Hannon, Seamus Kerrigan, and Alexey Melnikov for their comments.</p>
</section1>

</xep>