Telnet Server

Install telnet server on centos 7

yum install telnet-server telnet
systemctl enable --now telnet.socket

Access to root from 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.