Telnet facts for kids

Telnet is a way for computers to talk to each other over a network, like the internet. Think of it as a special language that lets you control a computer far away, almost like you're sitting right in front of it. It was one of the very first ways people could connect to remote computers.

Telnet works like a "client-server" system. This means you have a "client" program on your computer that connects to a "server" program on another computer. Once connected, you can type commands on your computer, and they get sent to the remote computer. The remote computer then sends back its responses, which you see on your screen.

Telnet was created way back in 1969, making it one of the oldest internet standards. It was designed to let people use virtual computer terminals from anywhere. However, Telnet sends all information, including usernames and passwords, as plain text. This means anyone watching the network can easily see what you're typing. Because of this big security risk, people don't use Telnet much anymore for important tasks. Instead, they use a safer option called SSH, which encrypts everything.

How Telnet Works

Telnet is made of two main parts:

• The Telnet protocol: This is the set of rules that tells computers how to communicate with each other. It's like the grammar and vocabulary of the Telnet language.
• The Telnet service (or application): This is the actual program that runs on computers, allowing them to send and receive Telnet messages.

When you use a Telnet client program, it connects to a specific "port" (like a special door) on the remote computer, usually port 23. On the other side, a Telnet server program is "listening" on that port, waiting for connections. Once connected, your computer pretends to be a "Network Virtual Terminal" (NVT), which is a standard way for Telnet to understand different types of terminals.

Telnet's Early Days

Telnet was developed even before the modern internet as we know it. It was first used on an early computer network called ARPANET. The idea was to let users with simple terminals control powerful computers from a distance.

The name "Telnet" comes from "Teletype Over Network Protocol." A teletype was an old machine like a typewriter that could send and receive messages. Telnet basically let you use a virtual teletype over the network. It used an 8-bit channel, mostly for 7-bit ASCII characters (the basic letters, numbers, and symbols we use). Any byte with the eighth bit set was a special Telnet command.

Adding New Features

Over time, many new features were added to Telnet. These were called "extensions" or "options." Because Telnet was designed to be flexible, it was easy to add new ways for the client and server to agree on how to communicate. Some of these extensions became official internet standards.

Why Telnet Isn't Safe

Telnet has some serious security problems, which is why it's not recommended for most uses today. Imagine sending a secret message by shouting it across a crowded room – anyone can hear it! That's a bit like how Telnet works.

Here are the main reasons Telnet is risky:

• No Encryption: Telnet does not scramble any of the information it sends. This means if someone is secretly watching the network traffic (this is called "packet sniffing"), they can see everything you type, including your usernames and passwords. It's like sending your diary entries in a clear, open envelope.
• Easy to Spy: If a hacker can get access to any device on the network between your computer and the remote computer (like a router or switch), they can easily intercept all your Telnet communications.
• Weak Authentication: Many Telnet systems don't have strong ways to check if you are who you say you are. Some older Telnet devices don't even ask for a password, giving anyone full access!
• Man-in-the-Middle Attacks: It's possible for a hacker to pretend to be the server you're trying to connect to, or pretend to be you. This is called a "man-in-the-middle" attack.

Because of these dangers, security experts strongly advise against using Telnet for logging into remote computers or managing important systems.

What Telnet Was Used For (and Still Is, Sometimes)

BusyBox running under the Microsoft Telnet Client from a router.

Old Uses

In the past, Telnet was the main way to get a command-line interface (a text-based way to control a computer) on a remote computer. But as we learned about its security flaws, people quickly switched to safer options like SSH. SSH does everything Telnet does but adds strong encryption, making sure your data is private and secure.

Modern Uses (Rarely)

Even though it's old and unsafe for general use, the Telnet client program can still be helpful for very specific tasks, especially for people who work with computer networks:

• Testing Network Services: Sometimes, network experts use the Telnet client to directly talk to other network services, like web servers (HTTP), email servers (SMTP, POP3), or chat servers (IRC). They can type commands directly to these services to see how they respond, which helps them fix problems. For example, you could use Telnet to send a simple request to a website to see if it's working.
• Old Equipment: Very rarely, you might find old industrial or scientific machines that only support Telnet for communication. In these special cases, Telnet might still be used because there's no other option.
• Amateur Radio: Some amateur radio operators use Telnet to share public information.

Despite the warnings, some older systems on the internet still use Telnet, which is a security risk.

Technical Bits (Simplified)

Telnet uses special codes to control the communication. These are like secret signals that tell the other computer what to do.

Control Codes

Telnet uses some standard codes, like:

• NULL (0): Does nothing.
• Line feed (10): Moves to the next line.
• Carriage return (13): Moves the cursor to the beginning of the line.
• Bell (7): Makes a "ding" sound.
• Backspace (8): Deletes the character before the cursor.

Telnet Commands

Telnet also has special commands, which always start with a special "Interpret As Command" (IAC) byte (which is 255). This tells the computer that the next byte is a command, not just regular text. Some commands include:

• WILL (251): "I will start using this feature."
• DO (253): "Please start using this feature."
• WON'T (252): "I will not use this feature."
• DON'T (254): "Please don't use this feature."

These commands are used to "negotiate" options between the client and server, like whether they should echo characters back to each other or suppress the "Go Ahead" signal (which was used in older "half-duplex" communication where only one side could talk at a time).

Telnet Programs (Clients)

Many programs can act as Telnet clients, allowing you to connect to Telnet servers. Some of these programs also support safer protocols like SSH.

• PuTTY: A popular free program for Windows, Linux, and Unix.
• AbsoluteTelnet: A Telnet client for Windows that also supports SSH.
• telnet.exe: A simple command-line tool often included with Microsoft Windows.
• Inetutils: Includes a Telnet client and server, common on Linux.

Images for kids

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  BusyBox running under the Microsoft Telnet Client from a router.

See also

In Spanish: Telnet para niños

• List of terminal emulators
• SSH (the safer alternative to Telnet)
• Virtual terminal