Computer and server management is a tricky task, especially when you don't have direct access to the system itself. For this reason, IT administration and support tools that facilitate remote connections are incredibly popular. These tools allow you to connect to and manage a PC or server, in precisely the same way as you would if you sat at the machine itself.
One such tool is Telnet, which was developed back in 1969. This tool is a type of network protocol that is baked into the command line interface (CLI) of the Windows operating system. Telnet facilitates communication with remote devices and servers, allowing you to remotely manage servers and even go through the initial device set-up phase. In short, Telnet is an invaluable tool that facilitates both the provision and administration of client and server hardware.
Telnet is a shortened way of saying Teletype Network. Due to the age of Telnet, it is considered insecure when compared to newer CLI protocols like the Secure Shell (SSH) protocol. This is because Telnet protocol data is not encrypted, posing a threat when operating in public-facing domains.
Considering this information, Telnet should be reserved for use on trusted domains only. The only real reason to use Telnet nowadays is when interfacing with legacy server hardware. This hardware lacks support for newer protocols and encryption technologies, forcing administrators to either use Telnet or replace the outdated equipment.
Securing your Telnet session
While Telnet does not natively support encryption, there is one way to implement encryption. The authentication protocol Kerberos can be used to wrap Telnet data packets in-transit, encrypting the data with either symmetrical or asymmetrical encryption methods.
With symmetric-key encryption, both the client and the server use the same encryption key to decrypt incoming data.
With an asymmetric key, the client and server have different encryption keys: typically a public key and a private key. Both the client and server have a public and private key, with the public key being used to encrypt, and the private key being used to decrypt.
You can find details on setting up Kerberos on the MIT website.
Please note that Windows 10 v2004 was used to create this guide.
To get started with Telnet, you need to install the Windows service. This has been disabled by default since Windows Vista, as the feature has been deprecated but retained for legacy use.
To install the service, press Windows + R on your keyboard to open the run command.
In the run box, type optional features and press Enter.
Scroll down until you find the Telnet Client. Tick the checkbox and press Okay to start the installation. Once finished, Windows will say Windows completed the requested changes.
Since Telnet is a command line tool, you will need to open the Command Prompt.
Press Windows + R again to open run. Type cmd and press Enter.
Type Telnet into the Command Prompt window to check that the service is working. If the service is working, you will see the window change with Welcome to Microsoft Telnet Client at the top. If Telnet did not install, you would get the message Telnet is not recognized as an operable program or batch file. In that case, restart your machine and start from the beginning of this section, following the instructions again.
Now you are ready to start using Telnet.
After following the previous section, you should have a Telnet session running in the Command Prompt.
If you do not have a Telnet session running, open Command Prompt and type telnet.
Here is the basic syntax for using Telnet:
o [hostname] [port]
By typing o, you open a session which connects to a hostname. The session will connect to your chosen IP address, which you put in place of [hostname]. To specify a connection port, input a number in the place of [port].
There is a list of Telnet servers at Chipkin, allowing you to test Telnet connections over the internet. In this case, we are going to connect to a basic text communication server called Telehack via Telnet.
The communication server URL is telehack.com The port is 23.
o telehack.com 23
The above command would open a session, connecting to telehack.com as the hostname, with the port used to connect being 23.
You should see a window open saying Connected to TELEHACK port  (the port rotates frequently, and is not set in stone).
There is a list of commands to use, which you simply type in the Command Prompt window.
One interesting command is zork which opens the classic text-based story game from the 1990s.
Welcome to ZORK.
Release 13 / Serial number 040826 / Inform v6.14 Library 6/7
West of House
This is an open field west of a white house, with a boarded front door.
There is a small mailbox here.
A rubber mat saying 'Welcome to Zork!' lies by the door.
You can try typing things like Open Mailbox or Knock On Door to progress through the story from here.
Testing email server connections with Telnet
One way to use Telnet productively is by testing your email server connection.
In this example, we try connecting to the Gmail SMTP server, with the address being smtp.gmail.com. In this example, the port to use is 25.
NOTE – SMTP stands for the simple mail transfer protocol.
Type the following:
o smtp.gmail.com 25
You should see something similar to the following appear in your Command Prompt window:
220 smtp.gmail.com ESMTP b139sm1084510wmd.19 – gsmtp
If you receive an error relating to the connection port, first try the following:
Open Command Prompt
Type ping -4 smtp.gmail.com
In this case, the IPv4 address was 184.108.40.206. Use this IP address in place of smtp.gmail.com using the command at the start.
If you still get an error, this may indicate a problem with your DNS resolver service.
This may also indicate that the port 25 is not forwarded on your router or client device firewall, which you will need to fix. This demonstrates how you can also fix basic network connectivity problems with Telnet.
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