My pi-hole should be blocking websites from my exact block list, e.g. "t.co", on IPv4 and IPv6.
Ubuntu 20.04.2 LTS on a Dell OptiPlex 7050
Pi-hole is used as DHCP server.
Actual Behavior:
I noticed my pi-hole stopped blocking my blacklisted websites and checked my DNS settings on my phone and I noticed that for IPv6 I had some values that I was not sure if they where right. After I deleted the IPv6 values on the DNS settings and only left the IPv4 address Pi-hole started working as usual again.
I don't remember well when this problem began, but it used to work fine at all times.
This is likely because the IPv6 IP being used for DNS was not the Pi-hole. This is frequently caused by routers providing IPv6 settings that bypass Pi-hole.
To avoid IPv6 by-passes, you've got to configure your router to stop advertising its own IPv6 address and advertise Pi-hole's IPv6 address instead.
Please consult your router's documentation and support to find out how to do so.
If your router doesn't offer such a configuration option, consider disabling IPv6 altogether.
If your router doesn't support that either (EDIT: or you have to rely on IPv6 for other reasons), clients will have the ability to by-pass Pi-hole via IPv6.