Since some hostile clients (such as TVs) have hard coded DNS, it is necessary to forward all port 53 and 853 traffic to a pi hole. This is easy enough with NAT redirection rules in the router, even with two pis.
Not your parent poster, but I manually configured them as primary and secondary DNS servers (dockerized PiHole), and then just used pi-hole's "export configuration / restore configuration" tool to keep the DNSs mostly in sync.
I heard there's a tool called Gravity Sync that will sync them, but I have not tried it yet.
I just advertise both Pi-Holes via DHCP and when I configure manual DNS settings. The primary DNS gets the vast majority of the DNS traffic, about 150 requests/minute and the secondary gets about 10 requests/minute (mostly from a single device).
Both piholes are on different UPS power, different switches, in different locations. In theory the lights will go out on the main UPS and switch first (it draws more power), but this configuration did save me once.
The typical reason for running redundant pi holes is high availability. If you really want all DNS traffic to go through your filter then your internet ceases working as long as your DNS server stops running.
https://github.com/vmstan/gravity-sync
Since some hostile clients (such as TVs) have hard coded DNS, it is necessary to forward all port 53 and 853 traffic to a pi hole. This is easy enough with NAT redirection rules in the router, even with two pis.
https://www.myhelpfulguides.com/2018/07/30/redirect-hard-cod...