JDS Australia

When requesting that firewall ports be opened, it is useful to be able to check that they really have been opened using the simplest method possible. Using a telnet client is the perfect way to do this, as it comes standard on Windows and all Unix-like systems.

Telnet will create a raw TCP connection to a port you specify. If the port does not have a service listening on it, or the port is unreachable (due to a firewall), then you will not be able to establish a connection.

On Windows, typing “telnet www.jds.net.au 80” establishes a connection to the JDS web server and waits for you to make an HTTP request, presenting you with a blank screen. You can now end the session by pressing CTRL+], or you can type in an HTTP request (just for fun).

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HEAD / HTTP/1.1<press Enter>
host: www.jds.net.au<press Enter>
<press Enter>

This should return something like…

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HTTP/1.1 200 OK
Date: Thu, 11 Dec 2008 00:11:10 GMT
Server: Apache/2.2.10 (Unix) mod_ssl/2.2.10 OpenSSL/0.9.8i DAV/2 mod_auth_passth
rough/2.1 mod_bwlimited/1.4 FrontPage/5.0.2.2635
X-Powered-By: PHP/5.2.6
X-Pingback: http://www.jds.net.au/xmlrpc.php
Set-Cookie: PHPSESSID=688bb2de5e03458279f1bcab9cffc595; path=/
Content-Type: text/html; charset=UTF-8
 
 
 
Connection to host lost.
 
C:\>

If the port is not accessible, then you will see “Connecting to {hostname} before the connection eventually times out.

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C:\>telnet www.jds.net.au 81
Connecting To www.jds.net.au...

Note that this method will not work to check that a service which uses UDP can be connected to, as telnet only uses TCP.

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