Testing if a port is open

TCP

Netcat

nc -v ADDRESS PORT

• Works on: Linux and macOS.
• On Linux, nc is the netcat command.

cURL

curl ADDRESS:PORT

• Connection refused: port is closed.
• Connection timed out: port is not reachable.
• cURL exits immediately if the port is open. May get an “empty reply” if the server does not send any data.

UDP

socat

socat STDIO UDP-CONNECT:ADDRESS:PORT

• Connection refused: port is closed.
• No response within a few seconds: port is (likely) not reachable.
• socat exits immediately if the port is open.