What this tool does
Attempts a real TCP connection to the host and port you specify from our server, and reports whether it succeeded, was actively refused, or timed out — the same signal `telnet` or `nc -zv` gives you, without needing shell access.
Open vs filtered vs closed
Open means something is actively listening and accepted the connection. Closed means the host responded but nothing is listening on that port (a TCP reset was returned). Filtered/timeout usually means a firewall is silently dropping the packets rather than rejecting them outright — this is the hardest state to diagnose without a check like this one, since your own connection attempt just seems to hang.
When you'd use this
Confirming a firewall rule actually took effect after a change, verifying a newly deployed service is reachable from the public internet before pointing DNS at it, or ruling out network-level blocking as the cause of a connection issue before digging into application logs.
Scope and limits
For safety, this tool only connects to publicly routable addresses — it won't attempt connections to private, loopback, or link-local ranges (including cloud metadata endpoints), since that could be used to probe internal infrastructure rather than test your own public-facing services.