DNS Client under linux
How-to Flush the DNS Cache on Linux
To flush DNS cache root access is required.
sudo systemd-resolve --flush-caches
You can then check the statistics in order to make sure that your cache size is now zero, thus cleared.
If systemd is not available on your system try
sudo service dns-clean restart
or (this should be exactly the same)
sudo /etc/init.d/dns-clean restart
How-to see DNS you use