I’ve been running Pi-hole off and on for a few years. It mostly works great, but it would randomly stop working and I’d see ads again. I could never track down what was happening — until recently.
I’ve been redesigning my home network the last couple of weeks. This included revamping my Pi-hole setup, and I finally set about fixing that problem.
Like many of you, when I setup my computers’ DNS servers to use Pi-hole, I saw that secondary server option and plugged in Google or CloudFlare. I thought that configuration would behave such that the Pi-hole was always used unless it was offline, and then the computer would fallback to the secondary DNS server.
That is not what happens!
To test this, I setup two separate Pi-holes and configured my computers to use them. Hourly, I saw random PTR requests hitting that second Pi-hole.
As it turns out, your computer can use any DNS server at any time. In an effort to add redundancy, I’d inadvertantly set my computers to randomly choose a non-Pi-hole DNS server.
I ended up leaving that 2nd Pi-hole on my network and have set all my computers to use them. Solid ad-blocking, finally!