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