Nat traversal port selection

[whyrusleeping]

I have an ipfs node that is behind a NAT, but I have port forwarding set up.
Despite this, my peer does not advertise any reachable address (it doesn't know about the port forwarding).

I could manually add the right address to my advertised addresses, but forcing users to do that is kindof obnoxious UX.

We should probably have nodes discover their gateways, attach their listen ports to it, and then ask some nat service to try dialing us through it. If that succeeds, then we should add it to our list of known good addresses.

[Stebalien]

I believe the issue here is that the node is running on freebsd, which doesn't support reuseport. Therefore, it's using random ports when dialing. Unfortunately, that means that we're advertising these random ports.

But yeah, we should do as @whyrusleeping says: figure out which external addresses are reasonable and ask some node to try dialing them. Unfortunately, the current libp2p construction makes this a bit tricky (connection deduplication).

[Stebalien]

NATs may also be messing with these ports.

[Stebalien]

So, my attempted fix was wrong. We already expose the addresses on Host.Addrs.

There are two issues here:

1. The nat we were testing on was broken. The router reported its internal address as its external address. Fixing this will require some work.
2. We don't re-announce addresses to the network. That is, we gossip them when connecting to our DHT neighbors but we don't then send these neighbors updated addresses.

[marten-seemann]

This should (partially) be solved by AutoNAT, and more elegantly by AutoNAT v2.