On my flight home yesterday a free, but limited, wifi option was available that allowed only for messaging (WhatsApp, Messenger, and I think the Google and Apple ones were specifically mentioned), but not web browsing. I checked and, sure enough, I couldn't get web browsing to work, but WhatsApp and Messenger worked fine. I decided to try my XMPP client and I was pleasantly surprised to that that worked fine as well.
I know it's a limited use case, where XMPP is one of the few unblocked protocols, but are there things I can do with it besides chatting? Could I use it to receive status updates from my server? Is there a way to use it for SSH somehow? I guess some sort of bot running on my server would be required. Seems like there are lots of possibilities, like bots that fetch websites or interact with ActivityPub. Has anyone found or tried anything like that?
You can basically connect any local port to any remote port normal or reversed. Reversed is -R, normal with -L. In this setting, correct me if im worng, you want to connect the open port on the airplane to one already prepared on a vps which allows you to do what you want e.g. proxy websites.
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.
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.
You could port scan portquiz.net to find other unblocked ports if you want to use the same IP, or get a VPS or something to do the VPNing (Oracle cloud have free ones, or a cheap one will do).
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.
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.