Tip for OP is to explain wanted outcome, not process to get there. Its hard to do, but gives better results

Besides being able to learn about all of this, this would be the ideal outcome:

• Send the Raspberry Pi to a friend who knows about this
• Have the friend plug in the Raspberry Pi to his Ethernet
• Use the friend’s network as a VPN/proxy/whatever to obscure my home IP address

I wouldn’t want to make the friend set up port forwarding or configure the Raspberry Pi himself, I want to do as much as I can on my own. And, if that friend ever relocates his residence or changes his ISP, setting it up again should be as easy as him plugging in the Raspberry Pi to the new network and I reconfigure it from my end.

So, you want the traffic to go other way around. Traffic from the HomeNet should go to the internet via FriendNet, right?

That is what I would like to achieve, yes. Since I want to avoid setting up port forwarding on FriendNet, I would need to configure port forwarding on HomeNet. The Raspberry Pi would have to act a client, and either the home server or the home computer would act as the server, and the Raspberry Pi would proxy traffic from the server to the internet.

I can’t think of how it would work in practice, though. I’m assuming in this case the home server would act as a proxy between the home computer and the Raspberry Pi, and the Raspberry Pi would act as a proxy between the home server and the internet. Unless there’s an easier way that would remove the home server entirely, that might be the best way to do it.

I’ll let you know if I figure it out! I don’t know much about proxies, but I’m still willing to try

So, you want a box which you can connect to any network around and then use some other device to connect to your raspberry box which redirects your traffic trough your home connection to the internet?

I think you may have misunderstood. I’ll try to clarify a bit:

I have a Raspberry Pi, which I’ll connect to, say, a friend’s network called “FriendNet”

I have a computer at home (which I’ll just call my “home computer”) connected to my home network called “HomeNet”

I also have a server connected to HomeNet, which is always active. Let’s call it my “home server”

I would like to proxy my home computer’s connection through the Raspberry Pi, so that my IP address will show up as the public IP address for FriendNet (i.e. tunneling my connection through FriendNet using the Raspberry Pi).

The Raspberry Pi will automatically send the proxy details to my home server, so that I can get the network details of the Raspberry Pi to connect my home computer to it without needing to figure it out manually. That would probably be achieved with a basic Python script.

I’m not trying to setup a home VPN server, but rather use an external network as a non-permanent proxy.