Comment by lxgr
It's completely impossible if you simply don't have the necessary access. Not everybody can administer all firewalls upstream from them.
Nor can everyone control whether their connection supports v6, unfortunately.
It's completely impossible if you simply don't have the necessary access. Not everybody can administer all firewalls upstream from them.
Nor can everyone control whether their connection supports v6, unfortunately.
The problem isn’t that it doesn’t work (and it does often not work – one “symmetric NAT” in the old/deprecated terminology is enough), it’s that it’s orders of magnitude more complex than it needs to be.
I’ve also never seen it work for TCP in practice, and not everybody should have to roll their own UDP wrapper for their TCP-expecting application.
Hole punching is a thing. Ports are not normally completely blocked. They allow replies, which can be exploited to do make a connection. Obviously this requires an out of band signaling mechanism. Tailscale does this, so does WebRTC, iirc.
Yes, but I don't believe all firewalls support that, especially for TCP, and as you've mentioned, now you also need to maintain a handshaking mechanism.
The complexity makes sense if you need to transport a lot of data peer-to-peer or the lowest possible latency, but if you don't, you might as well use that coordination server (which outbound-only clients are connecting to) for payload communication as well.
Firewalls don't track connections as carefully as you might think. They don't delete the mapping when an RST is received, so you can keep using it and receive a SYN in the other direction and now you have a connection.
Hole punching actually works most of the time. A lot more often than you might think. But enterprise firewalls usually don't allow it. And some home routers fail when you check all the anti-intrusion options. But it's the same for other VPNs. In the residential and small-business space, it's pretty rare. You might need to point it out to the network guy. If the customer wants the service, they should be open to it.