Open port checker
Enter a host and a port. Uplapse tries to open a TCP connection to it from its own servers and tells you whether the port is open, closed or filtered.
The check runs once from Uplapse’s servers when you press the button. Nothing is stored and nothing keeps checking.
Open, closed and filtered
Open means something accepted the connection: a service is listening and the network lets you reach it. Closed means the host answered and refused: it is reachable, but nothing listens on that port. Filtered means nothing answered within the time limit, which is what a firewall that drops packets looks like: the port may be open behind it, but not from the internet.
IPv4 and IPv6
When a name has both A and AAAA records, the checker tries one address of each. A port that is open on IPv4 but filtered on IPv6 is a common mistake: the firewall rule was only added for one of them, and visitors whose networks prefer IPv6 cannot connect. If the checker itself has no route to an address family, that address is shown as not tested rather than as a problem with your server.
One port at a time
The checker tests a single port per request and only on public addresses, so it cannot be used to scan a network. It opens the connection and closes it straight away without sending anything, so it does not log in to or talk to the service.
Common ports
22 is SSH, 25 SMTP between mail servers, 587 mail submission, 80 and 443 HTTP and HTTPS, 3306 MySQL, 5432 PostgreSQL, 6379 Redis and 3389 Remote Desktop. Databases and remote administration ports should usually be filtered from the internet: if this checker reports one of them open, check that it is meant to be.
Watch a port instead of checking it
A port that is open today can be closed tomorrow by a crashed service or a firewall change. Port monitoring connects on a schedule and alerts when the answer changes, in either direction.
Check it every few minutes instead of once
A one-off check tells you about now. Uplapse runs checks like this on a schedule and alerts the right person when the answer changes.