First of all: thank you very much for creating pi-hole!
I started with pi-hole on an ARM-eva-board (iMX53QSB), which was running with Debian-8 (Jessie). Pi-hole worked like a charm.
After updating the system to Debian-9 (Strech) I got the error 'FTL inactive' in the dash-board. Searching your forum brought up many threads about this, but no direct solution for my issue.
The basic issue I found out, was that the DNS-port 53 obviously was occupied by a different service. Using
sudo ss -tulpn
I could identify 'connman' blocking port 53. In Debian-9 'conmann' handles all services concerning network. Completely blocking it via 'systemctl' blocks the whole network services.
Fortunately I could find the solution under this link:
https://wiki.archlinux.org/index.php/ConnMan#Avoiding_conflicts_with_local_DNS_server
Here a configuration file is given to deactivate the DNS-proxy function of 'connman' only. In that way port 53 is free for pihole-FTL. After a restart of the board pi-hole and pihole-FTL were working again as expected.
One note to the config-file mentioned above:
On one line it is said:
ExecStart=/usr/bin/connmand -n –nodnsproxy
In my system the 'conmann'-daemon is located at:
/usr/sbin/connmand
Hope this might help other users to get pi-hole running when using Debian-9.
With best regards from Germany
Thomas