telnet

Telnet is a network protocol and accompanying client-server software that provides bidirectional, interactive text-based communication over a TCP/IP network. It was one of the earliest Internet standards and is defined in RFC 854, with the default connection typically using TCP port 23. A Telnet client connects to a Telnet server to present a terminal-like interface for remote administration of devices or services.

The Telnet protocol supports simple terminal emulation and negotiates session options using the Interpret as Command (IAC) sequence. Through IAC, the client and server use commands such as DO, DONT, WILL, and WONT to configure features like terminal type, window size, and echo handling. Data exchanged during a session is plain text, including any authentication credentials.

Because Telnet transmits information without encryption, it is insecure for use over untrusted networks. Eavesdropping, credential theft, and man-in-the-middle attacks are risks whenever passwords or sensitive data are transmitted. As a result, SSH (Secure Shell) has largely supplanted Telnet for remote administration. Telnet remains in use primarily for legacy systems, internal networks, or for testing connectivity, banners, or debugging in controlled environments. When necessary, Telnet can be wrapped in secure channels such as VPNs or, in some cases, through TLS-based wrappers, though such configurations are nonstandard.

Most operating systems provide a Telnet client, though it may be optional or disabled by default in modern installations. Telnet servers are less common on new systems due to security concerns, with SSH serving as the standard alternative for remote login.

See also: SSH, Remote login, TCP port 23.