Troubleshooting with Telnet

Kyle Rankin — Fri, 01 Mar 2013 16:58:05 +0000

Poor telnet, it used to be the cool kid on the block. It was the program all sysadmins turned to when they needed to connect to a remote server. Telnet just wasn't that good at keeping a secret—all communication went over plain text—so administrators started switching to SSH for encrypted remote shell sessions. Of course, along with the switch came a huge stigma against administrators who still used telnet. Eventually, telnet became an outcast—the program you used if you were an out-of-touch old-timer who didn't care about security.

I for one think telnet isn't all bad. Sure, it can't keep a secret, but it still can do a lot of useful things around the server room. Really, telnet just provides you a convenient way to connect to a network port and send commands. Telnet can work well to diagnose problems with one of the many services out there that still accept plain-text commands in their protocol. In fact, it's one of my go-to command-line programs when I'm troubleshooting. In this column, I'm going to give telnet a second chance and describe how to use it to perform some common troubleshooting tasks.

Test Remote Ports

There are many different ways to test whether a network port is listening on a system, including GUI port scanners, Nmap and nc. Although all of those can work well, and even I find myself using Nmap more often than not, not all machines end up having Nmap installed. Just about every system includes telnet though, including a lot of embedded systems with BusyBox environments. So if I wanted to test whether the SMTP port (port 25) was listening on a server with the IP 192.168.5.5, I could type:


$ telnet 192.168.5.5 25
Trying 192.168.5.5...
telnet: Unable to connect to remote host: Connection refused

In this case, the remote port is unavailable, so I would fall back to some other troubleshooting methods to figure out why. If the port were open and available though, I could just start typing SMTP commands (more on that later).

As you can see from the above example, the syntax is to type the command telnet, the IP or hostname to connect to, and the remote port (otherwise it will default to port 23—the default port for telnet). So if I wanted to test a Web server instead, I would connect to the HTTP port (port 80):


$ telnet www.example.net 80

Troubleshoot Web Servers

While you are connecting to port 80, you might as well actually throw some HTTP commands at it and test that it works. For starters, you want to make sure you actually are connected:


$ telnet www.example.net 80
Trying 192.168.5.5...
Connected to www.example.net.
Escape character is '^]'.

Once you are connected, you can pass a basic HTTP GET request to ask for the default index page followed by the host you want to connect to:

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