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Network Troubleshooting With Telnet

Using Telnet to Test Open Ports

The Setup on

  • 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 22

telnet protocol mismatch

Example of Manually Typing ASCII protocol

	C:\>telnet 80
	GET /index.php HTTP/1.1 

testing http with telnet

Example of Blocked Port:

	C:\>telnet 5555

telnet connect failed

No Service exists on server:

For example, the database team wants to test if they can get to 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 not open connection to the host, on port 8888: Connect failed

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