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

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 (GSS-API) described in RFC 4121 [2] or the proprietary Microsoft Windows Security Service Provider Interface (SSPI).

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 RFC 4121 [2] 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 from the Key Distribution Centre (KDC). 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 [6] (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 which may use insecure DNS queries (also see RFC 4120 [1]).

3. Kerberos Principal Name in the GSS-API environment¶

When the XMPP server is implemented 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 [8] (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 SSPI environment¶

Microsoft Windows provides the proprietary SSPI to support the "GSSAPI" SASL mechanism. This section describes the Windows equivalent of the domain-based service name for an XMPP server.

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 [8]. 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. Interoperability between GSS-API and SSPI¶

The goal of this section is to help developers of applications so that clients and servers implemented over SSPI can interoperate with servers and clients implemented over GSS-API.

Interoperability is achieved by the GSS-API system joining the Windows Active Directory domain or by having a cross-realm trust between the KDCs of the GSS-API and SSPI systems.

The SPN of the SSPI server does not specify a realm. A GSS-API client constucts the Kerberos principal name according to the rules in the GSS-API environment and adds a realm to the Kerberos principal name, but the Kerberos principal name is mapped to the correct XMPP server on SSPI.

When the server uses GSS-API, the SPN for the server needs to be created in the SSPI environment. The SPN constructed by the SSPI client according to the rules in the SSPI environment is then mapped to the correct GSS-API XMPP server.

The domain-based service name of GSS-API does not specify a port, so the port option of the SPN in SSPI should only be used in testing scenarios when both the XMPP client and the XMPP server are implemented using SSPI. The port SHOULD NOT be used in any other scenarios.

So in effect, whether an endpoint uses SSPI or GSS-API does not affect interoperability as long as the port in SSPI is not used.

6. Examples¶

Consider the example of an XMPP service "example.com" offered by the XMPP server located on the host "auth42.us.example.com", using the default port of 5222 for accepting client connections. When a client connects to the XMPP server, the server communicates its hostname along with supported SASL mechanisms as follows:

<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/example.com@EXAMPLE.COM" if the client is using GSS-API.
• the SPN "xmpp/auth42.us.example.com/example.com" if the client is using SSPI.

7. 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 [6]. For additional security considerations, refer to RFC 5178 and RFC 5179.

8. IANA Considerations¶

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

9. XMPP Registrar Considerations¶

10. 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:annotation>
    <xs:documentation>
      The protocol documented by this schema is defined in
      XEP-0233: http://www.xmpp.org/extensions/xep-0233.html
    </xs:documentation>
  </xs:annotation>

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

</xs:schema>

11. Acknowledgements¶

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

Appendices

Appendix A: Document Information¶

Series

XEP

Number

0233

Publisher

XMPP Standards Foundation

Status

Stable

Type

Standards Track

Version

1.0.0

Last Updated

2017-03-16

Approving Body

XMPP Council

Dependencies

XMPP Core, RFC 5178, RFC 5179

Supersedes

None

Superseded By

None

Short Name

kerberos5

Source Control

HTML

This document in other formats: XML  PDF

Appendix B: Author Information¶

Matthew Miller

Email

linuxwolf@outer-planes.net

JabberID

linuxwolf@outer-planes.net

Peter Saint-Andre

Email

stpeter@stpeter.im

JabberID

stpeter@jabber.org

URI

https://stpeter.im/

Joe Hildebrand

Email

jhildebr@cisco.com

JabberID

hildjj@jabber.org

Mili Verma

Email

mili.verma@isode.com

JabberID

mili.verma@isode.com

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 <https://xmpp.org/community/> 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 https://xmpp.org/extensions/attic/

1. Version 1.0.0 (2017-03-16)¶

   Move to draft by order of the XMPP Council.

   XEP Editor (ssw)
2. Version 0.5.1 (2017-03-03)¶

   Clarified regarding insecure DNS queries.

   mv
3. Version 0.5 (2015-10-20)¶

   Included support for Microsoft SSPI (Mili Verma).

   mv
4. 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
5. Version 0.3 (2011-06-22)¶

   Tightened the conformance terminology in several places; updated references.

   psa
6. Version 0.2 (2010-06-10)¶

   Expanded use beyond Kerberos; updated namespace; corrected schema; updated references.

   psa
7. Version 0.1 (2008-01-30)¶

   Initial published version.

   psa
8. Version 0.0.2 (2007-12-11)¶

   Corrected syntax.

   mm/psa
9. Version 0.0.1 (2007-12-05)¶

   First draft.

   mm/psa

Appendix I: Bib(La)TeX Entry

@report{miller2007kerberos5,
  title = {XMPP Server Registration for use with Kerberos V5},
  author = {Miller, Matthew and Saint-Andre, Peter and Hildebrand, Joe and Verma, Mili},
  type = {XEP},
  number = {0233},
  version = {1.0.0},
  institution = {XMPP Standards Foundation},
  url = {https://xmpp.org/extensions/xep-0233.html},
  date = {2007-12-05/2017-03-16},
}

END