I'm trying to understand UDP hole punching and I just don't quite get it. In concept it seems simple but when I put it into practice I can't pull it off. From what I understand there's a public server we call the hole-punch server. A client makes a request to hole-punch server (this is public). The hole-punch server spits out a public ip and port of the client that just made the request. So long as that port is open then essentially any random client can make a request to that client using that specific port and ip ?
The issue I guess I'm having is, the client is able to make a request to the server. The server is able to send data back to the client on that public port and ip however when another client tries to send a request to that client using that same port and ip it just doesn't go through and that's what's confusing me. If the server can make the request why can't another random client make that request?
The thing to know about UDP hole-punching is that many consumer-grade Internet routers/NAT-firewalls have a policy along the lines of "block any incoming UDP packets, except for UDP packets coming from an IP address that the user's local computer has recently sent a UDP packet to"; the idea being that if the local user is sending packets to a particular IP address, then the packets coming back from that same IP address are probably legitimate/desirable.
So in order to get UDP packets flowing between two firewalled/NAT'd computers, you have to get each of the two computers to first send a UDP packet to the other one; which is a bit of a chicken-and-egg problem since they can't know where to send the UDP packet without being able to communicate; the public server is what solves that problem. Since that server is public, both clients can communicate with the server (via UDP or TCP or HTTP or whatever), and that server can tell each client the IP address and port to send its UDP packets to. Once each client has sent some initial packets to the other, it should also (in most cases) then be able to receive UDP packets from the other client as well, at which point the server is no longer necessary as a go-between.