telnetd

telnetd is the daemon process that implements the Telnet server on Unix-like operating systems. Telnet, a network protocol, is used for interactive text communication over a network. When a client connects to a telnetd server, it is typically presented with a login prompt, allowing the user to authenticate and gain access to a remote shell. Once authenticated, the client can execute commands on the server and see their output as if they were physically present at the machine.

The telnetd daemon listens for incoming connections on TCP port 23. It handles the negotiation of Telnet options, such as echoing characters locally or on the server, and manages the communication stream between the client and the server's shell. Historically, telnetd was widely used for remote administration and access.

However, Telnet, and by extension telnetd, is considered insecure. The protocol transmits all data, including usernames and passwords, in plaintext over the network. This makes it vulnerable to eavesdropping and man-in-the-middle attacks. Consequently, for secure remote access, protocols like SSH (Secure Shell) have largely replaced Telnet and telnetd. Many modern systems have telnetd disabled or removed by default due to its security risks.