A few years ago I was on a Southwest flight and had OpenVPN running because I forgot to turn it off. I was able to access the Internet through my tunnel without paying for access. I guess at the time they were only port blocking common ports (80, 443, 53 etc) if you didn't pay. They have since closed that hole.
For this same reason you used to be able to send messages via platforms like whatsapp without internet as well! I don't remember the airline I just remember I hadn't paid for internet but I could message and do a few other things but I couldn't browse the internet.
That was probably deliberate. I flew United recently and they advertised free wifi for certain messaging apps, or you could pay to access more apps or the general internet.
I also flew united recently and, in addition to the free messaging access, they also provided free access to the inflight entertainment, in case you wanted to watch it on your device instead of on the screen.
I would have loved to take advantage of this since my wireless earbuds were significantly better than the wired pair I had. Unfortunately, a little pop-up warned me that this was not available on Android 13 devices. I was more than a little annoyed, but also curious as to why this might have been the case.
Here's my hack for United's free messaging. Works on iOS, and makes the flight more useful than before, but not as good as paid internet.
Messaging and Notifications basically follow the same protocol. Even though I usually have notifications disabled, I go and activate it for anything I care about - News, Weather, Slack, Whatsapp (yes I have that silenced). Every single message pops up as a notification. Could be bank alert, Ring alert, homekit alert, whatever ... it just shows. So you can keep tab on things you care about, and if you are really needed, well you can pay and get on the full Wifi. And anyways you can iMessage to communicate if needed.
I had a mosh connection open before I got on my last United flight and was able to use it the whole time. Seemed to me the free messaging/inflight entertainment tier doesn't block arbitrary UDP packets at all.
I flew United recently, and I was able to use the free messaging service for basically everything without any intervention from my part. It's just a tad slow. Not sure if it was intended or not.
There was a report in the early to mid-2000s where someone got iChat AV to work, partly because it was fairly obscure and likely the network engineers didn’t consider blocking it.
KLM and United for sure have free in-flight messaging (at least as of a few days ago, the last time I used them).
It's interesting what does and doesn't go through. e.g. Facebook notifications update, but not the content. I guess that's because they use the same channel as FB Messenger.
