Quad9 is a free security solution that uses DNS to protect your systems against the most common cyber threats. Please follow the steps below to configure Ubuntu 16.04 LTS to use Quad9.
From the Ubuntu desktop, select the Network icon (up and down arrows) at top right corner of the screen, and click Edit Connections...
Select the connection that you wish to modify and click Edit
Select the IPv6 Settings tab, set the Method to Automatic, addresses only, enter the appropriate Quad9 IP addresses for your needs, then click Save. For more information on which server is right for you, please visit our Quad9 IPs and Other Settings page. We strongly suggest entering both a primary and a secondary address as a precaution. Multiple IP addresses can be entered in the list by using comas.