There is possibility you get command not found if you execute below command. In that case you have to install nscd on your system.
To install nscd use below command:
sudo apt-get install nscd
Use the below commands as per your need to flush dns:
# To flush DNS cache run this command sudo /etc/init.d/dns-clean restart or sudo /etc/init.d/networking force-reload # To Flush nscd dns cache execute below command sudo /etc/init.d/nscd restart # If you wan to refresh your settings you can disable and then execute below command sudo service network-manager restart