I installed pi-hole yesterday using google as the DNS. It worked well for about 12 hours. Then all Internet traffic appeared to get blocked. By which I mean I get things like: "www.facebook.com’s server DNS address could not be found." I uninstalled completely, reinstalled. It worked again for a while. Then the same thing. Tried using OpenDNS. Same thing.
What things should I be looking at to help resolve this?
When you get in a situation that is showing that you can't resolve a domain, try running a pihole -d and get us the debug information so that we can take a look at the configuration at that time.
I ran pihole -d with my router pointing to it as the DNS. It ran. Then I tried to hit LinkedIn and Facebook when it said to. At the end it said:
::: There was an error uploading your debug log.
::: Please try again or contact the Pi-hole team for assistance.
So I uploaded it here:
https://docs.google.com/document/d/18BVKLdDvofMmuA9Tf5RXNgynjQsfZ0inhRh14G8M2dc/edit?usp=sharing
less squiffy now...
Looking at it, I see no reference to either LinkedIn, nor facebook.. should those have appeared in this file?
Your markdown is a little squiffy 
try [description](link)
That's what we would expect to show, and the internal tests do show dnsmasq is running, and doubleclick.net is resolving to 192.168.1.100. So the Pi-hole does appear to be functional in itself for name resolution, but it's unable to contact the Google server for it's test. The ping tests failed, so your Pi-hole is not able to communicate with 192.168.1.1, which is what the device sees as it's gateway to the internet.
How do you have the Pi-hole connected to your router?
Interesting. It's hardwired with a short bit of Cat 5 cable. When I SSH into it, it cannot ping the router. Of course I can SSH into it, so it is on the network. But it cannot ping the router. What could that be?
That's the thought that was going through my mind, you were able to SSH in, but it wasn't able to see the router. Lets take a look at the routing tables:
ip route
That should let us know if there is an issue with packet routing, but I suspect that will come out okay. Has the Pi-hole been rebooted since the software was installed?
pi@raspberrypi:~ $ ip route
default via 192.168.1.1 dev eth0 metric 202
192.168.1.0/24 dev eth0 proto kernel scope link src 192.168.1.100 metric 202
Yes, it has been rebooted since the install.
I'm really not sure why you can't ping the 192.168.1.1 device, the route is indeed correct, and being able to SSH in shows routing is working correctly. Are you able to ping that address from a client machine?
Yes, I am able to ping the router 192.168.1.1 from other machines on the same network. I might have to refresh and readup on how to read a routing table.
The other thing to try on the Pi-hole device is a
traceroute 192.168.1.1
And see if there are any helpful errors that return and point in the direction we should be looking.
pi@raspberrypi:~ $ traceroute 192.168.1.1
traceroute to 192.168.1.1 (192.168.1.1), 30 hops max, 60 byte packets
1 * * *
2 * * *
etc...
I could reinstall everything from scratch tonight. Then run these same tests to see how they should respond. That might give me something to compare against when it fails again. Clearly it's not a pi-hole problem. It's some networking problem. Something is timing out over a long, several hour, period. I wonder if I have to disable wlan0 or something.
That's showing that there is a route to the host, so the routing tables are configured correctly. The only thing I can think of is to flash a new image on to the uSD card and give it another try. It's not the best option, but I'm not sure why you can't ping the router or get names from the internet. There's sometimes an option in router firmware that is called DNS rebind, and toggling that sometimes works if it's there. But I don't see how that could relate to the IP stack.
If they both have the same IP then there may be something there to take a look at.
I'll report back when I've taken those steps, late tonight or early tomorrow. Thanks for thinking this through with me.
So, I'm fairly sure this was a problem with my 12 year old router. I just installed a new/modern router, and the problem no longer appears.
I came across the same problem, using rPI 4B board and an ASUS RT-AC68U router with the latest PiHole available at the time writing this. It seems that the "internet detection" function on the router is responsible for the problem, if it's configured for DNS probe. If you select PPP Echo instead of DNS probe than everything works well. Note, that PPP Echo is the default, but DNS probe in my experience is more reliable, that's why I have used that before.