Selfhosted

You can do SSH tunneling over DNS, so everything is possible.

Acronyms, initialisms, abbreviations, contractions, and other phrases which expand to something larger, that I've seen in this thread:

6 acronyms in this thread; the most compressed thread commented on today has 15 acronyms.

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In theory, you could make a XMPP bot that can do all these things. Status information and executing simple commands shouldn't be that hard but interactive commands might need a custom wrapper.

https://github.com/msantos/xmppipe

Would be one example, but there are actually many such projects.

Especially if you are on a really bad or congested connection it actually works better than regular SSH.

Obviously security trade-offs regarding this need to be carefully considered.

https://github.com/Shura0/mastaj

For ActivityPub access.

Like the other guy said, they're probably not doing DPI to actually check for XMPP, so if something like portquiz.net:5222 loads, then you could host a VPN on the same ports as XMPP and have unrestricted internet.

XMPP can indeed be used for some other things

For example, Google uses it to this day for Google Cloud Messaging - the push notifications service in Android.

Just the how is something I cannot answer :/

What was your XMPP client connecting to? Was it a well-known public endpoint (that they could be whitelisting) or was it a private server? If the latter then that indicates that they are allowing arbitrary IP connections which in theory means that you should be able to proxy any traffic you want. I doubt they are doing DPI, since TLS makes this very difficult these days when you don't control the certificate stores on the clients.

I'd imagine they're relying on some combination of DNS whitelisting and port blocking which should be trivial to circumvent if you know ahead of time what traffic they allow through.

It is possible. As long as the protocol allows sending bytes of data controlled by the user which XMPP allows.

You would basically wrap http with XMPP. You need a server that would understand XMPP, read the payload, create the http request, do the request for you, wrap the response with XMPP and send it back to you.

You can do that with DNS as well which would bypass probably everything. However, your bandwidth wouldn't be great.

The project is dead now, but have a look at Archipel, it's a whole orchestrator (like Proxmox) based around XMPP

https://github.com/ArchipelProject/Archipel

There used to be web, ftp, and gopher relays that would work through email, so I'm sure it's possible.