pi.hole/admin should take me to the web interface. This does not work on the host machine (works on others). For the host machine, I have to use IP/admin to get to the web interface.
-Host machine is Intel NUC running Ubuntu 20.04.3 LTS (dual booted alongside Windows)
-clients that can reach pi.hole/admin: Windows 10 PC, android phones, android tablet
Actual Behaviour:
On the host machine, pi.hole/admin returns "Hmm. We're having trouble finding that site. We can't connect to the server at pi.hole."] I can access it via the IP/admin.
nslookup does not find pi.hole but it does find the IP from the host machine
nslookup on client Windows 10 can find the pi.hole and the IP
The host machine is not using Pi-hole for DNS, it is using something else. Only the Pi-hole itself can resolve the domain pi.hole to the Pi-hole IP, and other DNS servers cannot. This is why the pi.hole domain does not work, but the IP does.
Change the nameserver assignment on this machine to the loopback IP (127.0.0.1).
File /etc/resolv.conf will show you the nameserver currently in use, and it will not be the loopback.
The file will also tell you the source of the data that populates this file. I do not run Ubuntu so I can't give you further advice on how to permanently change the nameserver on that device.
Thank you this was the problem. There seemed to be many solutions to changing or de-activating the /etc/resolve.conf file (or pseudofile whatever it is in linux) so that it is not automatically updated with the wrong nameserver every start-up. I think I followed a guide to change permissions on this file so it can't be overwritten, and it works. Thank you for the help.