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TCP port reachability checker

Check whether selected TCP ports on an authorised public address can be reached from this test server.

Use Detect to identify an authorised public target; a local address is not a public target.

Compare before and after

Run the same test on the same device and save a baseline. Activate your VPN or change one relevant setting, rerun under the same conditions and save the next result.

Save a before result and an after result to compare them.

What this test tells you

Use this test to check whether a service is reachable through a public address, router or VPN exit. An open port means a TCP connection succeeded on this path. It does not mean the service is vulnerable, correctly configured or accessible from every network.

How to run the test

  1. Select an authorised public target and the small set of TCP ports relevant to a service you control.
  2. Check that the expected service is running, then start the connection test.
  3. Read each port's status separately, including refusal, timeout and local or routing errors.

How the measurement works

Our server attempts a TCP connection to each selected port. It reports whether that connection succeeded, was refused, timed out or could not be interpreted.

The measurement originates at the test server, not your browser. Target validation restricts which public addresses may be tested; no UDP traffic or application authentication is tested.

Understanding your results

Open
The target accepted a TCP connection from this server. The application behind the port may still require authentication or fail its own protocol checks.
Connection refused
The connection was explicitly rejected on this path. A closed service or a rejecting firewall may explain it.
No response or indeterminate
A timeout may involve filtering or an unreachable path. Local permission, routing or address-family errors remain indeterminate rather than being attributed to the target.

Compare before and after

  1. Record the target address, address family, ports and running service state.
  2. Change one relevant firewall or forwarding rule, then retest from the same test server.
  3. Compare individual statuses. If a VPN changes the target IP, record that change as well because you are testing a different public endpoint.

Limits of the test

  • The hosting network's outgoing filters and the route to the target affect the result.
  • Incoming reachability depends on the public endpoint and its routing and forwarding rules; VPN connection status alone cannot establish it.

What to change, if needed

A required service is refused
Check that it listens on the expected address and port, then inspect relevant firewall rules and rerun.
A required service times out
Check public address selection, router forwarding and upstream restrictions before opening additional ports.

Frequently asked questions

Does this test UDP?

No. The results apply only to TCP connection attempts.

Should every port be closed?

That depends on the services you intentionally expose. Judge each reachable port against its purpose and access controls.

Sources and technical references