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Tools

Open Port Checker

Network and domains

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Is the port open: state, response, service.

We test the TCP connection from our server. “Filtered” means the port did not respond at all — usually a firewall is blocking it.

FAQ

What is the difference between “closed” and “filtered”?
They are different things, and the distinction matters when configuring a server. “Closed” means the server actively refused: the port is reachable but nothing is listening on it. “Filtered” means there was no reply at all — the packet vanished. That is how a firewall behaves when it silently drops connections. If you have just set up a service and see “filtered”, look at your firewall rules rather than the application.
Why is my port open locally but closed in this check?
There are three usual causes. First: the service listens only on 127.0.0.1 instead of 0.0.0.0 — check the configuration; in MySQL this is the bind-address setting. Second: the port is blocked by the server firewall or a cloud security group. Third: your provider blocks the port upstream — this is common with port 25 to limit spam.
Which ports are dangerous to leave open?
Database ports first of all: 3306 (MySQL), 5432 (PostgreSQL), 6379 (Redis), 27017 (MongoDB). They should not face the internet — database access belongs on the server itself or through an SSH tunnel. Redis and MongoDB historically shipped with no password at all, so an open port means anyone can read the data. It is also worth closing 3389 (RDP) and 5900 (VNC), or at least restricting them by IP.
Is port scanning legal?
Checking your own servers is normal administrative work. Mass scanning of infrastructure you do not own is treated in many countries as reconnaissance ahead of an attack and can have consequences. Our tool is deliberately limited to 15 ports per request and uses no aggressive techniques — it is for diagnosing your own server, not for scanning networks.
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