Checking for Open Ports

Here are different ways to check if a port is open. Not all of these commands may be available on every system.

nc (netcat) #

nc -z <host> <port>
nc -z <host> <port-range-lowerbound>-<port-range-upperbound>

Warning: nc will always show UDP ports as open.

telnet #

telnet <host> <port>

Telnet is not script-friendly. It opens an interactive terminal that you must quit by pressing the escape characters and sending the quit command.

$ telnet github.com 80
Trying 20.205.243.166...
Connected to github.com.
Escape character is '^]'.
^]
telnet> quit
Connection closed.

curl #

curl <host>:<port>

It’s also handy to pass -v so that you’ll see a confirmation if the port is open when the command shows no output and appears to hang up.

$ curl -v localhost:53
*   Trying [::1]:53...
* Connected to localhost (::1) port 53
> GET / HTTP/1.1
> Host: localhost:53
> User-Agent: curl/8.4.0
> Accept: */*
>
^C