Connection Debugger & Transport Finder
“Why can’t I connect?” bisected layer by layer, and “polling, SSE, WebSockets or HTTP/3?” — answer the questions and get a diagnosis or a recommendation with reasons.
Walk the layers in the order a request crosses them — DNS, route, reachability, port, handshake, TLS, HTTP. Each question is one command; stop at the first one that fails, because every layer above it will fail too and tell you nothing.
Step 1
Does the name resolve? Run `dig +short <host>` (and `dig AAAA` if you use IPv6).
A hostname that does not resolve cannot fail at any later layer — there is no address to send to.