UDP hole-punched Multiplayer from behind NAT

[GoodClover]

Problem

Jack is playing a world (possibly with "Host Server" toggled on), and Jill wants to join, but Jack (and possibly Jill) is behind a NAT.

Currently to connect a port-forward or tunnel would be required.

Solutions

Do UDP hole-punching either via a public service fully negotiating, or just to get the external port and have them manually exchange ip:port pairs with each-other.

This makes it much easier for people to host & join private games.

Alternatives

Everyone must use an existing tunnel solution (s.a. VPN) or port-forwarding.
Tunnelling is often technical to set up or costs money.
Port-forwarding is easier but requires permission and is impossible on non-home networks.

A kid or layperson just wanting to join a game with their friends is unlikely to do either.

(There's also the possibility of UPnP, PCP, or NAT-PMP, but I think those're almost always disabled. At the least I know UPnP is very prone to security issues. This should possibly be a separate feature request.)

Additional context

This is NOT distributed servers or anything of the sort - there is still a single host (Jack). This is just an alternative way to initiate a connection between the host & clients.

Note that mobile networks used by phones often do NAT in a way that makes hole-punching unreliable at best.
For home NAT hole-punching has become a feature so should always work afaik.

[HybridDog]

Port-forwarding is easier but requires permission and is impossible on non-home networks.

A kid or layperson just wanting to join a game with their friends is unlikely to do either.

Network traffic is still unauthenticated and unencrypted, so it's a privacy and security risk if laypersons can easily play together without a VPN, tunnel or something similar. (#10206)

Tunnelling is often technical to set up or costs money.

Tunnelling and using a VPN is usually free. I don't know if it's too technical to set up for laypersons nowadays.

[sfan5]

Is there any precedent of other games doing this?
I imagine it's too much implementation work for such an unreliable solution in practice.

Other ways can be explored however. Multicraft has(?) "multiplayer share code"-solution, where you get a short code
that other people can enter in the menu to join you. I assume the connection is proxied over their servers.
Money wise I think it would be within our resources to run this (though it should better be encrypted then).

so it's a privacy and security risk if laypersons can easily play together

That's no different from multiplayer on "normal" servers, in fact less third parties are involved.
So this shouldn't be a concern.

I don't know if it's too technical to set up for laypersons nowadays.

Back in the day I used Hamachi to play Minecraft with my friends. Not everyone would be able to figure this out and also I'm really not sure if any of these solutions work on mobile.

[rubenwardy]

Is there any precedent of other games doing this?

Project Zomboid does this

[Soupborsh]

Hello, I think proxying will result in high ping for players far away from server, I think it requires many servers distributed around the globe for good ping for everyone.

If these user friendly P2P connections will be implemented I think it is good idea to prioritize IPv6 connection, if it is not possible fallback to this. Also I believe that IPv6 should be enabled by default.