To force piHole to push a defined IP address as DNS server to DHCP clients:
Edit /etc/dnsmasq.d/02-pihole-dhcp.conf file:
#> sudo nano /etc/dnsmasq.d/02-pihole-dhcp.conf
Add the IP address to be push as DNS server:
dhcp-option=6,<Pi-holeIP>,<SecondaryDNSServerIP>
Save file modification.
Restart pihole-FTL server:
sudo service pihole-FTL restart
⚠️ You would have to release/renew DHCP lease on each host after that to pull the new settings.
⚠️ Change is not persistant over time as this file can be overwritten by piHole.