How to handle a pi-hole failure / crash?

I run pi-hole within my home network, which is relatively small, and contains the normal kind of devices you'd expect to see in a home devices, tablets, smart TVs, Xbox etc. etc.

As my ISP router does not allow for custom DNS servers, I use pihole as my DHCP perfect.

But, what is the best practice when it comes to fail safe...for instance if Pi-Hole fails / stops for some reason....when this happens I lose all internet connectivity.

I know I could run a second Pi-hole, and name this as the secondary upstream DNS (I think that's right?) but I'm curious as to other people solutions.
As my ISP provided router is locked down, I have no option to use customer router firmware or hardware.

It actually happened last night at around midnight...I don't know what happened, but pi-hole stopped resolving queries for about 1 hour and I couldn't access any web site / service...and I saw a huge spike in the number of times Google home was sending panic connectivity check queries (10,000+), after about 1am they stopped and I could access the web again.

There are a few things you can do for improved reliability:

  1. Put your Pi's and network equipment on UPS (battery backup). This protects them against short term power loss (the most common type of power loss) and surges. Pi's can misbehave after a power loss (lost time sync, corrupted SD card, etc.)

  2. Run a second Pi-Hole in parallel. It doesn't have to be on a Pi, it could be on an always on PC. The second Pi-Hole can also be a DHCP server, just put in on a different IP range.

Case in point - my Pi's, routers and modem are on UPS. Lost power this morning for about a minute. I was on the web on an iPad and the only way I knew it was that the lamp on the table went out. Power came back and everything was still running normally.

There have been some threads on this topic here and on Reddit/pihole. Here are a few examples:

Thanks for that @jfb
I'll have a read of those posts
But I was more thinking towards a failure or crash of Pi-Hole.
I've never actually had a power cut (although clearly it could and inevitably will).
But I often get times where pihole seems to be down for a while and DNS resolution does not function?

In reality, given the (small) size of my network, it'll likely never be an issue, I'd just like a fallback / backup.
I don't have an always on PC, so a second Pi-Hole is, I think my best option.

I'm running a second copy of Pi-hole on an old Pi version 1 and it is serving up about half my DNS requests with no issues. I just have the DHCP server offer up both Pi addresses when it is queried.

Having a backup DHCP server isn't really necessary for brief outages as the clients already have DHCP assigned addresses that have lifetimes that will keep them working thorough a several hour outage.

A good power supply seems to be essential to keeping my Pis happy, tossed several that had decent specs but were a possible problem source for the connectred PIs and replaced them with 2.5 Amp CanaKit ones that have proven electrically problem free in my use. The CanaKit ones are a bit wide for most power strips and a bit of sanding of the grip lugs is needed to fit them side by side.

Since the power draw is so low I did hang both Pis off my UPS which has proven helpful a couple times during the summer storm season.