**Ubuntu 18.04**

Install the packages

 $ sudo apt-get update
 $ sudo apt install xinetd telnetd telnet -y

Create a new file `/etc/xinetd.d/telnet` with following entries:

 service telnet
 {
 disable = no
 flags = REUSE
 socket_type = stream
 wait = no
 user = root
 server = /usr/sbin/in.telnetd
 log_on_failure += USERID
 }

Then restart xinetd service

 $ sudo /etc/init.d/xinetd restart

Time to test from server side:

 $ telnet localhost

**CentOS-7**

Install the packages

 $ sudo yum clean all && yum repolsit
 $ sudo yum install xinetd telnet-server telnet -y

Enable and start the services

 $ sudo systemctl enable telnet.socket
 $ sudo systemctl start telnet.socket

Allow telnet (TCP/23) through the firewall

 $ sudo firewall-cmd --permanent --add-port=23/tcp
 $ sudo firewall-cmd --reload

Test from server side

 $ telnet localhost