When the telnet service is enabled on your Linux system, users who connect will generally be presented with a login: prompt requesting the name of a user on the host system. This is confusing, and often unnecessary since shell users are probably using ssh to log in. So ideally you want your telnet service to connect directly to the Citadel text client.
To achieve this effect, you must configure your telnetd to run the Citadel client instead of /bin/login.
If your system is running inetd, edit /etc/inetd.conf and search for the telnet lines. Uncomment them if they are commented out. Change them to look like this:
telnet stream tcp4 nowait root /usr/sbin/tcpd /usr/sbin/in.telnetd -h -L /usr/local/citadel/citadel telnet stream tcp6 nowait root /usr/sbin/tcpd /usr/sbin/in.telnetd -h -L /usr/local/citadel/citadel
If your system is runnint xinetd, the configuration will be found in /etc/xinetd.d/telnet.