Install and Enable Telnet server in Ubuntu Linux

1.Install telnet use this command in terminal(Applications/Accessories/Terminal):

sudo apt-get install xinetd telnetd

2.Edit /etc/inetd.conf using your favorite file editor with root permission,add this line:

telnet stream tcp nowait telnetd /usr/sbin/tcpd /usr/sbin/in.telnetd

3.Edit /etc/xinetd.conf, make its content look like following:

# Simple configuration file for xinetd
#
# Some defaults, and include /etc/xinetd.d/
defaults
{
# Please note that you need a log_type line to be able to use log_on_success
# and log_on_failure. The default is the following :
# log_type = SYSLOG daemon info
instances = 60
log_type = SYSLOG authpriv
log_on_success = HOST PID
log_on_failure = HOST
cps = 25 30
}

4.You can change telnet port number by edit /etc/services with this line:

telnet        8100/tcp

5.If you’re not satisfied with default configuration.Edit etc/xinetd.d/telnet, add following:

# default: on
# description: The telnet server serves telnet sessions; it uses
# unencrypted username/password pairs for authentication.
service telnet
{
disable = no
flags = REUSE
socket_type = stream
wait = no
user = root
server = /usr/sbin/in.telnetd
log_on_failure += USERID
}

add these lines as you like:

only_from = 192.168.120.0/24 #Only users in 192.168.120.0 can access to
only_from = .bob.com #allow access from bob.com
no_access = 192.168.120.{101,105} #not allow access from the two IP.
access_times = 8:00-9:00 20:00-21:00 #allow access in the two times
......

6.Use this command to start telnet server:

sudo /etc/init.d/xinetd start
sudo /etc/init.d/xinetd stop
sudo /etc/init.d/xinetd restart

Reference Link