▷ Telnet what it is and what it is for 【the most complete】

Telnet is a tool that has been used for decades by computer system administrators in the field of networks. Remote connections are not exactly new, since the first networks and with systems without a desktop tools such as Telnet were used to connect to servers and equipment connected to a network, both remotely and internally. Here we will know better what Telnet is and what it was used for.

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Although it has now been replaced by more secure tools, such as SSH, Telnet still has some use in secure environments for system administrators.

What is Telnet

The name of Telnet comes from the acronym Telecomunication Network, and basically a TCP / IP network protocol that has been used since 1960 to establish remote connections with other computers, servers, and devices with a compatible system in access through this communication system. By default connection port 23 is used.

In addition to the protocol itself, the program that uses it to establish the connection itself also receives this name. To access the other machine remotely, we must use a terminal, for example, Windows Command Prompt, or the Linux Terminal. In this way we can interact on the remote machine browsing its files, executing other internal commands, if we have permission, and monitoring the state of the machine itself without the need to physically go to the place where it is located.

In addition to being used on MSDOS and Windows systems, it is also compatible with UNIX-based systems, such as Mac and Linux, and FreeBSD. With this protocol, we will also be able to check the connectivity of other machines and check if they have certain ports open to the outside. This is something we will see in a moment.

How Telnet Works

As we say, this protocol and program can only be used in command mode. In order for us to establish a connection between two computers with Telnet, we will first need to have a client at the end that we are, and a server on the machine that we intend to access. If we also do it outside an intranet, or LAN, we will need to have port 23 open on the destination machine.

The next thing we are going to need is to open a session on the destination machine in which there is one or more user accounts that are allowed access. In short, to access a target machine with a client, it must contain a user account enabled for access, and we will need to know both the user's name and password to establish communication.

Security issues with Telnet and SSH as a substitute

Currently the use of Telnet is practically limited to internal networks where there is a security shield that isolates the network from the outside. In all other cases, the SSH protocol is always used.

The big problem with telnet is that the information from one terminal to another travels without any encryption, only in plain text. For a hacker, this information is extremely easy to get, considering that both the username and the password to establish the communication also come as plain text, the security breach is brutal.

In response to these problems, the use of another encrypted communication protocol used in UNIX systems, called SSH (Secure Shell), became popular. Today it is perfectly compatible with Windows environments, where we can install both the client and the server to use it. SSH uses RSA keys as encryption so that the information it travels cannot be easily decrypted. There are also other applications such as OPENSSH, Putty, Shell or SSH-Agent, where this communication protocol can be used. In this case the connection travels through TCP port 22.

How to use Telnet

To use Telnet we need a command terminal, either command prompt or Windows PowerShell. Once opened, all we have to write is

telnet

And we will access the application. If once inside we put:

help

We can see the different options we have for using the program.

But if we want to establish a direct connection, we will only have to put:

telnet

Can I still use Telnet?

Currently, on operating systems prior to Windows 10 and Windows 8, they still have a Telnet server implemented in the system as an installable feature. So we can connect to them from a client without problem. It is also available in Windows Server versions. In other operating systems like Linux, we will need to install it through the repositories.

In the case of Windows 10, to avoid security problems, we will only have the Telnet client available in the features. To use a server, we will need an external program like Putty.

Activate Telnet client in Windows 10

We have a tutorial already made where we teach how to activate the telnet client in Windows 10. It is extremely simple.

How to activate Telnet client in Windows 10

How to activate Telnet server in Windows and Linux

Similarly, we have taken the trouble to fully show how we can activate a Telnet server on Windows systems other than Windows 10 and also on Linux operating systems such as Ubuntu. It is a very interesting topic if you are thinking of using this communication protocol.

How to use Telnet server in Linux

How to use Telnet server in Windows

Test open ports with Telnet

As we saw before, with Telnet we can easily test if a host has open or closed ports. For this, we only need to know the external IP address or the domain name. The syntax will be as follows:

telnet

When we make the connection, if the port is indeed open we will get an empty black screen. If, on the other hand, it is closed, an error will be shown after making a connection attempt. Let's see it with our Professional Review page, on a web server both port 80 and 443 of http and https should be open, along with a port like 22 for remote connections via SSH.

We put:

telnet www.profesionalreview.com 80 telnet www.profesionalreview.com 443 telnet www.profesionalreview.com 22

Even with port 22 the host gives us a clue that it works with OpenSSH under the Debian system.

Let's try for example with Telnet port 23:

We see that we have not been able to establish a connection, so it is closed.

As you can see, using Telnet is quite simple and does not have much complication. But if you want to use it for remote connections, better opt for SSH which is much more secure. Telnet is only recommended for connections on internal networks.

You may also find these tutorials interesting:

What do you plan to use Telnet for? If you have any questions or want to clarify something, write us in the comments.