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 manager.

This is especially true in environments that make use of Kerberos V and negotiation of Simple Authentication and Security Layer (SASL) 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 &rfc3920; or &rfc6120;. However, the problem can now 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 V service principal names as described in &rfc5179;.

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

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 &rfc6120;. In the case of the Kerberos V SASL mechanism, the child element is a <hostname/> element qualified by the 'urn:xmpp:domain-based-name:0' namespace &NSNOTE;. 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"):

• 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.

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

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

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; &rfc5246;) as described for XMPP in RFC 6120 and RFC 6120. For additional security considerations, refer to RFC5178 and RFC 5179.

This document requires no interaction with &IANA;.

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