Various methods exist to connect to XMPP servers over direct TLS, SRV records for XMPP over TLS (XEP-0368)  defines SRV records, RFC 7395  defines websocket, XMPP Over BOSH (XEP-0206)  defines BOSH. HACX defines a method to retrieve all of those along with optional extra information such as SNI/ALPN requirements and pinned public keys from a RFC 5785  well-known URI over HTTPS.
Several alternatives were considered to avoid writing yet-another-spec but they were all deficient in various ways:
HACX servers MUST use HTTPS. This means that the HACX client MUST verify the certificate of the HTTPS service at the source domain in order to securely "bootstrap" into the use of HACX; specifically, the rules of RFC 2818  apply to this "bootstrapping" step to provide a secure basis for all subsequent HACX operations. HACX clients and servers MUST support HPKP (RFC 7469 ), HSTS (RFC 6797 ), and DANE (RFC 6698 ), and SHOULD support any future standard that serves to increase the security of this HTTPS request.
A HACX document is retrieved over secure HTTP in the following way:
HACX provides a simple way to look up exactly how to connect to any given XMPP server in a simple, censorship resistant, distributed way. An attacker would have to block all A and AAAA record lookups and/or HTTPS to completely block this lookup, and even then it's easily done over something like Tor which otherwise does not support things like DNS SRV or TXT lookups. Additionally, DNS SRV records can not provide values for SNI or ALPN extensions, or a relative priority for other connections methods like BOSH and Websockets.
This should be trivial to implement for any program that already implements SRV records for XMPP over TLS (XEP-0368)  and POSH , much of the same code can be reused.
Server operators might want to prioritize connection methods that maximize privacy, like no ALPN, domain fronting (different SNI value), or no SNI. Consider looking at everything available at an HTTP level to potentially lie to attackers, while providing correct info to users. Consider providing different sets of servers to different IP blocks or regions. Consider using any of the multitude of free or cheap HTTP hosts or CDNs to forward traffic to your real server. The possibilities are endless.
POSH (POSH ) already provides the ability of anyone with control of an HTTPS server to override trust of TLS certificates for the XMPP server matching that domain, HACX is slightly different in that it pins public keys like HPKP (RFC 7469 ) and not certificates like POSH, but the delegation is the same.
HACX additionally delegates what were previously DNS lookups to the HTTPS server, since this is protected by TLS certificate validation, this should be even more secure than plain DNS, equivalent to something like DNS-over-HTTPS or DNS-over-TLS, but without trusting a 3rd party DNS server. DNSSEC delegation is preserved as well since it protects the A/AAAA record for the HTTPS host, and can also validate it with DANE (RFC 6698 ).
This does allow anyone with write access to the /.well-known/ directory on an HTTPS host to host a XMPP server for that domain even if they don't have access to create DNS records or listen on port 5222, but this was already true due to Discovering Alternative XMPP Connection Methods (XEP-0156)  (HTTP Lookup Method) and RFC 7395 
Well-known URIs (RFC 5785 ) requires registration of new URI suffixes. This document specifies two URI suffixes:
URI Suffix: xmpp-client.xml
Change Controller: XSF
Reference: [TODO: HACX LINK HERE]
Related Information: XMPP jabber:client namespace
URI Suffix: xmpp-server.xml
Change Controller: XSF
Reference: [TODO: HACX LINK HERE]
Related Information: XMPP jabber:server namespace
The Well-known URI registry is currently located here.
This document requires no interaction with the XMPP Registrar .
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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.
The primary venue for discussion of XMPP Extension Protocols is the <email@example.com> 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 <firstname.lastname@example.org> list might also be appropriate.
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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".
6. PKIX Over Secure HTTP (POSH) <http://datatracker.ietf.org/doc/draft-miller-posh/>.
14. The XMPP Registrar maintains a list of reserved protocol namespaces as well as registries of parameters used in the context of XMPP extension protocols approved by the XMPP Standards Foundation. For further information, see <https://xmpp.org/registrar/>.
Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/
Fix requirements, editing, add alternatives.