Pi-hole takes over the local DNS service and may conflict with existing DNS services on your server. Ubuntu 18.04 currently uses systemd-resolv to server DNS and needs to be configured to either give up port 53 or be disabled.
On Ubuntu 18.04 and newer you will have
netplan controlling your network and should see https://netplan.io/ for examples on how to configure it. You need to set your nameserver to use your LAN's DNS or a public DNS such as
126.96.36.199 before proceeding with any instructions below.
On Ubuntu 18.04, resolvconf was removed as the default means to control DNS. In addition to the settings mentioned regarding netplan, we recommend setting up resolvconf.
To install run
sudo apt install resolvconf
/etc/resolvconf/resolv.conf.d/head using sudo and enter
nameserver 188.8.131.52 on the first uncommented line.
Restart the service
sudo service resolvconf restart
Stop systemd-resolv from listening on port 53
/etc/systemd/resolved.conf and set
DNSStubListener=no (make sure it is not commented out with a
# at the beginning of the line) and then run
sudo systemctl restart systemd-resolved
If that does not work you can try the following:
Stop and disable systemd-resolv (only if the above does not work)
sudo systemctl stop systemd-resolv.service sudo systemctl disable systemd-resolv.service
Name resolution for localhost
In most cases it might be required to set your localhost name in
127.0.0.1 machinename.localhost machinename 127.0.0.1 domain.com