Service to tell a user their IP & whether their server port is open.

Server: Run the binary. It listens on port :6460 UDP.

Consider a user attempting to start a server on port 8000. Their IP is 1.2.3.4.

The first UDP packet sent to napi.?????.com:6460 must be 400 bytes.

It must contain the UTF-8 string net64ipc0000, followed by 2 bytes encoding the server port (typically 8000), and then the remaining 386 bytes can be anything.

The packet must be sent from the server port encoded in the message.

Wrong version scenario

1.2.3.4:8080 -> napi.?????.com:6460 (sends garbage that's 400 bytes long)

napi.?????.com:6460 -> 1.2.3.4:8080 BADVER

Wrong port scenario

1.2.3.4:8123 -> napi.?????.com:6460 net64ipc0000(binary short representing 5678)

napi.?????.com refuses to respond & ignores the request.

Correct version but port closed

1.2.3.4:8080 -> napi.?????.com:6460 net64ipc0000(binary short representing 8000)

napi.?????.com:6460 -> 1.2.3.4:8000 OK 1.2.3.4

(... packet never gets through to 1.2.3.4:8000 ...)

Session ends. Client should rely on timeout to detect failure. Service will wait 2 seconds for a reply.

The client should consider sending 3 attempts 500ms apart for every "test" to deal with network flakiness lost packets etc.

Correct version and port open

1.2.3.4:8080 -> napi.?????.com:6460 net64ipc0000(binary short representing 8000)

napi.?????.com:6460 -> 1.2.3.4:8080 OK 1.2.3.4

Session ends.

As UDP is lossy, the client should rely on timeout to detect failure but should probably send out 3 requests spaced 500ms apart or so & if any OK reply comes back then consider the port open.

(If 3 (or 5, whatever) attempts aren't enough or packet takes >2s, the user's connection is so shit we may as well say "hey, this ain't gonna work".)