Port 23 (Telnet)

TCP port 23 is historically associated with the Telnet protocol, a now largely obsolete protocol used for providing bidirectional interactive text-oriented communication over a network, typically between a client and a server. Telnet enables users to remotely access and control a computer or device as if they were directly connected to it. The protocol operates by establishing a TCP connection to the server on port 23. Once the connection is established, the client and server exchange data as ASCII text, including commands, responses, and user input. The Telnet protocol itself is extremely simple, lacking built-in encryption or authentication mechanisms beyond a basic username and password prompt. This simplicity made it easy to implement but also inherently insecure. The protocol uses Network Virtual Terminal (NVT) data stream which defines a standard character set and control functions to ensure compatibility between different operating systems and terminal types.

At a technical level, Telnet involves the client initiating a TCP handshake with the server on port 23. The server listens for incoming connections on this port. After the handshake, the server typically presents a login prompt. The client sends the username and password (in plain text), which the server validates. If authentication is successful, the server grants the client access to a command-line interface or other applications. The protocol then transmits all subsequent data, including commands and responses, as unencrypted text. Commands such as 'ls', 'cd', 'pwd', or application-specific commands are sent from the client to the server, and the server executes these commands and sends the output back to the client. The connection persists until either the client or server terminates it, usually by sending a specific command like 'exit' or by closing the TCP connection.

## Firewall Recommendations

Telnet should generally be blocked on firewalls, especially on public-facing interfaces. If Telnet is absolutely necessary for legacy systems or specific internal network devices, it should be restricted to a dedicated, isolated network segment with strict access control policies. Consider using SSH (Secure Shell) as a secure alternative to Telnet, as it provides encryption and strong authentication. If Telnet must be used, implement strong passwords, regularly monitor Telnet logs for suspicious activity, and consider wrapping Telnet within a VPN or other secure tunnel to encrypt the traffic. Regularly audit devices running Telnet to ensure they are properly secured and patched against known vulnerabilities. Disabling Telnet entirely is the most secure option if it is not required.