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Telnet

Telnet is a network protocol and a command-line application for virtually every operating system out there. It’s used to access a virtual terminal (command line) on another computer, router, or device across the network. It’s not really used for connecting to secure servers anymore, since everybody uses the much more secure SSH protocol, but it’s a useful troubleshooting tool.

Because it’s extremely flexible, telnet can even be used to access services on almost any unencrypted TCP/IP port–for instance, you can request a web page by using telnet to connect to a web server and view the actual text that is returned through the HTTP protocol, like so:

telnet www.google.com 80

> GET / HTTP/1.1

(hit the Enter key twice after typing the line above to get the output below)

HTTP/1.1 200 OK
Date: Mon, 10 Sep 2012 01:53:24 GMT
Expires: -1
Cache-Control: private, max-age=0
Content-Type: text/html; charset=ISO-8859-1
Server: gws
X-XSS-Protection: 1; mode=block
Transfer-Encoding: chunked

1000
<!doctype html><html itemscope="itemscope"
 itemtype="http://schema.org/WebPage"><head>
----truncated----

Telnet can also be used to check other ports, like SMTP for email, or pretty much anything else you could think of. It’s not installed by default on modern versions of Windows, but you can install it easily by heading into Control Panel’s “Turn Windows Features on or off” panel (use the search box to find it).

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