Discovery server connection test

[calmh]

The discovery server could make connection tests to the announced addresses/ports and return the result to the client when it does the announcement. This could provide one small part of the "connection doctor" like functionality, in that Syncthing could display an indicator for success/failure.

In case of failure it could say something like Incoming connections on port 22000 blocked, check port forwards or firewall settings.

In case of success it can instead report the actual port used and that port forwarding works.

The test would have to be fairly "shallow" and use short timeouts, in order to not put too much extra work on the discovery servers. We need to plan on doing several hundred connection tests / second if we do this. We can skip checks on addresses that are reannounced and have succeeded within the last 24 hours or something.

[canton7]

Alternatively, have a big "Troubleshoot connection issues" button, and just do the connection test when clicked. That should reduce the load a lot.

It also means you could hive the connection test off to another server if necessary.

[calmh]

True

[seniorm0ment]

this would be nice on more complex networks to easily see what ports syncthing has access to and what it doesn't