If your WAN interface(s) are set to Auto (DHCP), the ISP device (or next device upstream) will provide one or two DNS servers to NG Firewall. These servers will be used for all DNS lookups sent by NGFW itself, including those sent to it by downstream devices (if NGFW is acting as a DHCP server).
These can be overridden if you prefer to use different DNS servers. You can specify different DNS overrides for each WAN if necessary.
Configuring DNS servers in Interfaces
First, go to Config > Network > Interfaces and Edit a WAN interface. You'll find the current settings displayed in the IPv4 Configuration tab:
The field marked Primary DNS Override will display the current DNS server received from the upstream device. If that device is providing two DNS servers, you'll see the second in Secondary DNS Override. Just type in the IP address(s) of the DNS server you would prefer to use and click Done, then Save, to apply the change.
To revert the change and use the DNS server provided by the DHCP lease, just edit the interface again and delete your override. The auto-populated option will appear and take effect once you click Done and Save again.
For some suggestions on free public DNS servers, please see Public DNS Servers.Follow