Telnet

In short: An old protocol for controlling a computer through a text console that transmits all input unencrypted.

In more detail: Telnet (Teletype Network, since 1969, RFC 854) works on port 23 over TCP and belongs to the application layer. You connect to a server, log in and type commands as on its own console. Because user name, password and all data cross the network in plain text, Telnet was replaced by SSH.

In Depth

Usage

telnet 192.168.1.1          (port 23, default)
telnet mail.example.com 25  (another port)

On Windows the Telnet client is an optional feature that has to be enabled in the Windows features.

What Telnet is still known for

• Network devices: Old switches, routers and printers can still be configured via Telnet; in practice you should switch it off and use SSH.
• Testing ports: telnet host 80 shows whether a port is open. If an empty window opens the connection worked; “connection failed” means closed or filtered.
• Learning protocols by hand: With Telnet you can try SMTP, HTTP or POP3 character by character because these protocols use text commands.
• Exams and simulations: Telnet is standard material in networking, for example in Packet Tracer exercises.

Why it is insecure

An attacker in the same network (or on the path) can read Telnet sessions and capture credentials. There is no encryption and no verification of the remote side. Therefore: never use it on the internet, switch it off in internal networks or use it only in isolated test environments.

Telnet versus SSH

	Telnet	SSH
Port	23	22
Encryption	no	yes
Login	password in plain text	password or key, encrypted
Today	obsolete	standard

See also: SSH, TCP, TCP/IP Model, Remote Maintenance, SMTP, Encryption, Standard Port, Network Commands in the CMD