In environments that make use of Kerberos V5 (&rfc4120;) and negotiation of Simple Authentication and Security Layer or SASL (&rfc4422;) over XMPP, a connecting client often needs to know the identity of the Kerberos acceptor principal so that it can obtain a proper ticket for authentication. This scenario was not addressed in &rfc3920; or &rfc6120;. However, the problem can be solved using the concept of domain-based service names (&rfc5178;). In particular, when an XMPP server uses the Kerberos V5 ("GSSAPI") SASL mechanism (&rfc4752;), it can communicate the identity of the acceptor principal as a Kerberos V5 service principal name (&rfc5179;). This document defines an XMPP method for such communication.

The acceptor principal's hostname is communicated 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:1' namespace &NSNOTE;. The XML character data of the <hostname/> element specifies the fully-qualified name of the acceptor principal. 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, such as typically communicated 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).

Consider the example of an XMPP service whose canonical name is "example.com". A user might make use of an acceptor principal located at "auth42.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/auth42.us.example.com/example.com@EXAMPLE.COM".

The communication of acceptor principal 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; &rfc5246;) as described for XMPP in 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, Eliot Lear, Alexey Melnikov, and Klaas Wierenga for their comments.