You can set up the DNS forwarder:
Click “DNS” on the Left the “Forwarders” at the top.
Plug in domain and server pairs.
I use the domain ANY and OpenDNS servers at 18.104.22.168 and 22.214.171.124 but you can use whatever you want. This will tell Zeroshell which DNS servers to use. And if you set up DHCP and your clients are pointing to your Zeroshell box then they’ll use Zeroshell for DNS which will in turn use what you specify here.