TCP Port Check
Check whether a TCP port is open on any host — domains are resolved first, results show open, closed or filtered with response time.
Frequently asked questions
The server performs a real TCP handshake with the target host and port — exactly what your browser or SSH client does when connecting. Nothing needs to be installed locally.
Open means the TCP handshake succeeded. Closed means the host actively refused the connection (RST) — nothing is listening on that port. "No response" means no reply arrived within the timeout: the port may be filtered by a firewall, or the host may be down. A probe cannot always tell these two apart.
Any TCP port from 1 to 65535. Common ones: 22 (SSH), 25 (SMTP), 80 (HTTP), 443 (HTTPS), 3306 (MySQL), 6379 (Redis).
The probe waits up to 5 seconds for the TCP handshake. Firewalls that silently drop packets make the check run the full timeout before reporting "no response".
Yes. The domain is resolved first (A and AAAA records) and the check runs against the resolved IP, preferring IPv4 and falling back to IPv6. Private and reserved addresses are never probed.
The probe runs from our server, so firewalls, geo-blocking and routing can make the same port reachable from one place and unreachable from another. Verifying that difference is often the point of a port check.
No. Query contents are never stored.