Q. Can you example how can I install telnet service or server under Linux or FreeBSD operating system?
A. TELNET (TELetype NETwork) is a network protocol used on the Internet or local area network LAN connections.
The telnetd program (telnet server) is a server which supports the DARPA telnet interactive communication protocol. Telnetd is normally invoked by the internet server inetd or xinetd for requests to connect to the telnet port as indicated by the /etc/services file. Usaually telnet listen on port TCP port 23.
Telnet in is insecure protocol and it is recommended that you use ssh server. But some time you really need telnet then first install telnet server as according to version of Linux distribution.
Q. How do I telnet as the root user?
A. This is really a bad idea as telnet is insecure, because passwords are sent as clear text format. Instead, you should use SSH for connections. I suggest that you use SSH to connect to a system instead of Telnet for security purposes. By default, for security purposes, the root user can not telnet. However, here is method in case you need root access via telnet:
Edit /etc/pam.d/login and /etc/pam.d/remote files using a text editor such as vi. Find the line that read as follows:
auth required pam_securetty.so
Comment it out by prefixing # symbol:
#auth required pam_securetty.so
Save and close the file. Now, you should be able to telnet to your system as the root user.