Telnet Protocol Explained: Key Characteristics

Telnet is a network protocol used on the Internet or local area networks to provide a bidirectional interactive text-oriented communication facility using a virtual terminal connection. It enables a user on one host computer to log into another host computer that supports the Telnet protocol, and then execute commands on the remote host as if operating locally. The term “Telnet” can also refer to the client application program that implements this protocol.

Core Characteristics

Unencrypted TransmissionTelnet transmits all data (including usernames, passwords, and command content) in plaintext over the network. This means attackers can easily intercept and read the transmitted information through packet capture tools, leading to significant security risks.
Client-Server Architecture
- Telnet Client: The end that initiates the connection request, responsible for sending user input commands to the remote server and displaying the server’s response results.
- Telnet Server: The end that accepts the connection request, runs a Telnet service program, and executes the commands sent by the client, then returns the execution results.
Port NumberThe default well-known port number for Telnet is TCP port 23. The client establishes a TCP connection with the server through this port to realize data interaction.

Working Principle

The user runs the Telnet client program and enters the command to connect to the remote server (e.g., telnet remote_host_ip).
The client initiates a TCP connection request to the Telnet server on port 23.
After the server accepts the connection, it authenticates the user (usually requiring input of a username and password).
Once authentication is successful, the user can enter various commands through the client, which are transmitted to the server for execution.
The server returns the command execution results to the client, and the client displays them to the user in text form.

Limitations and Alternatives

Security Defect: Due to plaintext transmission, Telnet is rarely used in modern network environments, especially in public networks or scenarios with high security requirements.
Common Alternatives:
- SSH (Secure Shell): A secure remote login protocol that encrypts all transmitted data, effectively preventing information interception and tampering. It has become the mainstream alternative to Telnet, with the default port number 22.