p2p/nat: handle responses with alternative port in NAT-PMP

[dbadoy]

For NAT-PMP, returns success with an available port if the requested port is already preempted. If this situation occurs, the current implementation considers it a success and performs the remaining tasks, but in practice, communication with external network nodes is impossible, and it is difficult to track down this issue.
This PR adds check logic for preempted ports and define this situation as an error.

ref: rfc6886 p.12, p.29

[fjl]

Good catch! I've changed the wording from 'preempted' to 'already mapped'.

[dbadoy]

Thanks, that's more neat.
However, this error occurred because the request's external port was already mapped. How about including the external port in the error instead of the internal port?

[fjl]

You're right, it's better to mention the external port. I got confused.

[fjl]

Another thing, while this is a good fix, it'd be so much nicer to actually handle the alternative port returned by the router and use it, instead of abandoning the mapping. Same for UPnP.

At the time when p2p/nat was introduced we had no way of doing that, but things have changed. Nowadays the announced endpoint is managed by enode.LocalNode, and we could absolutely set the external port to a different one on that. Let me know if you are interested in implementing this functionality.

[dbadoy]

Nice. I would like to implement that. Shall I close this PR and make a new one?

[fjl]

Nice that you want to look into it. Let's just merge this fix now, and then see how it goes with the other idea.