I recently updated Pi-hole from AUR to version 4.1 on my Pi running Arch. After restart my pihole-FTL.service wouldn't start and systemctl status pihole-FTL.service gave me this :
> pihole-FTL.service - Pi-hole FTLDNS engine
> Loaded: loaded (/usr/lib/systemd/system/pihole-FTL.service; enabled; vendor preset: disabled)
> Active: failed (Result: exit-code) since Mon 2019-01-21 19:17:22 UTC; 1s ago
> Process: 546 ExecStart=/usr/bin/pihole-FTL no-daemon (code=exited, status=2)
> Main PID: 546 (code=exited, status=2)
>
> Jan 21 19:17:21 alarmpi pihole-FTL[546]: [2019-01-21 19:17:21.831] -> Known forward destinations: 0
> Jan 21 19:17:21 alarmpi pihole-FTL[546]: [2019-01-21 19:17:21.832] Successfully accessed setupVars.conf
> Jan 21 19:17:21 alarmpi dnsmasq[546]: FAILED to start up
> Jan 21 19:17:21 alarmpi systemd[1]: pihole-FTL.service: Failed with result 'exit-code'.
After some digging around and an fuser 53\tcp and ps -p X -o comm= where X is the PID given by fuser I realised that systemd-resolve was listening on port 53. After stopping and disabling systemd-resolved.service everything is back up and running. Hopefully this helps someone else in my situation.