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On my Ubuntu, I can run the following commands, and get HTTP response

telnet www.ietf.org 80
GET /rfc.html HTTP/1.1
Host: www.ietf.org

I heard that Telnet protocol is insecure, so is it correct that the remote host doesn't run Telnet server?

What does my telnet client connect to, if no telnet server running on the remote host?

Does telnet client connect to any open port on any host, without the host running telnet server?

Thanks.

  • You see that telnetting to port 80 got an HTTP response -- in fact, you sent an HTTP request, so I'm a bit confused by your current subject line. – Jeff Schaller Aug 21 '18 at 2:12
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Telnet is a clear-text protocol, hence insecure.

In your example you are using a telnet client to connect to port 80 on another box. This has nothing to do with telnet on the remote end; its just the program you choose to use on the client end to make the connection.

Running the telent client does not require a telnet server on the client end. If you aren't connecting to port 23 (telnet), a telnet server is not required on the remote end either.

People often use a telnet client to test connectivity to services on remote servers to verify they are not blocked by firewalls or not responding at all. All you are doing is making a TCP connection from a client to a server... it will work on any open TCP port which is not blocked.

  • Thanks. can a telnet client connect to any remote server for any protocol? what kind of protocols whose servers can a telnet client connect to? – Tim Aug 21 '18 at 2:38
  • If the server is listening on a TCP port, yes. However some protocols are not clear-text. While you can connect, you won't get much out of it other than seeing you were connected and then disconnected. In your example, you connected to a webserver on port 80. Do the same test to a server listening on port 443 and see the difference. – Inetquestion Aug 21 '18 at 2:42
  • if a server uses a protocol which is not clear text, can telnet client still send a message to the server and receive a response from it? – Tim Aug 21 '18 at 2:45
  • It would vary by protocol of the server you're connecting to. Some will disconnect you immediately if they dont get what they're expecting in the connection setup. You'll never be able to mimic an encrypted protocol using telnet, but you can use it to verify you can make a TCP connection from A==>B. Now that you know this works, you can use a proper client with the assurance its not a network problem if the connection fails. – Inetquestion Aug 21 '18 at 2:48

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