Using Telnet to Test Open Ports
The Setup on Digitalcrunch.com:
- 5555 is blocked via a firewall
- 80 is open and running http service
- 22 is open and running ssh
How to Manually Use Telnet
Typing anything and then hitting enter is like sending protocol information to the remote service. If you know the protocol and it accepts ascii, you can manually interact. If not, it will give you an error message. You can’t type binary data this way though.
Example of Telnet Error Message
C:\>telnet digitalcrunch.com 22
Example of Manually Typing ASCII protocol
C:\>telnet digitalcrunch.com 80 GET /index.php HTTP/1.1 host: digitalcrunch.com <enter>
Example of Blocked Port:
C:\>telnet digitalcrunch.com 5555
No Service exists on server:
For example, the database team wants to test if they can get to digitalcrunch.com on port 8888, however, the server isn’t provisioned yet – but they still want you to test! In this example, I have opened the firewall to port 8888, but there is nothing listening on that port. Compare to the “blocked port” above to see the difference.
C:\>telnet digitalcrunch 8888 Connecting To digitalcrunch.com...Could not open connection to the host, on port 8888: Connect failed