There may come a time when you need to log into your Mac from another machine or log into another Mac (or Unix system) from your machine. For this, Mac OS X offers remote login services such as the Secure Shell , Telnet , and the remote shell.
The Secure Shell (SSH) is a protocol for using key-based encryption to allow secure communication between machines. As its name suggests, it is most commonly used for interactive sessions with shells on remote machines, so that you can use the ssh command.
Mac OS X ships with the OpenSSH (http://www.openssh.com) client and server software. This includes the ssh command, which you use to open SSH connections to other machines, and the sshd daemon program, which you run to allow other machines to SSH into your Mac.
As with FTP (see the earlier section "File Transfer Protocol (FTP)"), running an SSH service (the sshd daemon) on Mac OS X is easy: just activate the Remote Login checkbox in the Sharing pane.
Mac OS X versions prior to 10.1.0 shipped with telnetd , a daemon that runs the Telnet protocol, as its default remote login server. Telnet is a decades-old method for getting a virtual terminal on a remote machine through a network. However, it's inherently insecure, because all its transmissions are cleartext, lacking any sort of encryption, and hence easily readable by malevolent entities monitoring the traffic that enters and leaves your network. Use of Telnet has, in recent years, fallen out of favor ...