I reinstalled everything again, after the problem was gone. This time I got FTL 1.5.1 from the start, no upgrade required. Initially the graph showed 6 resolvers, witch seamed normal, because you have to select some resolvers during installation. I chose OpenDNS and the resolvers were even listed by name.
I then removed the resolvers from /etc/dnsmasq.d/01-pihole.conf and /etc/pihole/setupVars.conf. The graph still showed 6 resolvers, but the name of the OpenDNS servers changed into undefined.
I ran pi-hole -f, no change, but after a reboot the undefined resolvers were gone.
How do you clear the FTL cached data?
Does pihole -f also clears the FTL cached data?
Yes, but only indirectly, it will reparse your log after flushing. But keep in mind that it will also scan through pihole.log.1 if you have FTL running on default settings (no config file), so flushing once is not sufficient to drag the data out of FTL's view.
New setup, so there is no pihole.log.1. Yet, the first time, the undefined resolvers did NOT go away with pihole -f.
I'm testing all of this on a test pi, powered down over night.
I've just started it up and ran pihole-FTL:
FATAL: Opening of FTL log (/var/log/pihole-FTL.log) failed!
Make sure it exists
however, that log exists, with yesterdays timestamp, I can't find any logrotate instructions, so I assume it isn't rotated (yet)?
What should (could) be the content of the FTL-pihole conf file (where and what)?
I'll wirte an FAQ for this soon, currently there is no clean documentation for this since the config file exists only since two days and there is a lot of changes still going on.
ls -lh /var/log/pihole-FTL.log
-rw-r--r-- 1 pihole pihole 13K Mar 16 08:52 /var/log/pihole-FTL.log
so this is correct
about pihole -f (= really logrotate)
I think both You and I forgot about this
Witch implies pihole -f doesn't do anything (if logrotate is available) unless it's really a new day.
There is a pihole.log.1 today, but there wasn't yesterday, despite all the pihole -f commands.
The pihole-FTL error remains, although everything seems to be working...
Have you tried starting pihole-FTL as some user or have you used the command sudo service pihole-FTL start? Only the latter one will take care that pihole-FTL is also started with UID of the user pihole.
Well, that should not happen. pihole -f forces the rotation, no matter how much time has passed. I already thought about removing it altogether because it was used to heal a sitation that you won't encounter with FTL anymore. We have had a test setup running on a small server with 4GB of memory and it worked quite fine until reaching about 50 millions DNS queries.
Not sure what you mean. pihole -f adds the --force flag to the command.
pihole-FTL
FATAL: Opening of FTL log (/var/log/pihole-FTL.log) failed!
Make sure it exists
sudo service pihole-FTL stop
sudo service pihole-FTL start
pihole-FTL
FATAL: Opening of FTL log (/var/log/pihole-FTL.log) failed!
Make sure it exists
sudo pihole-FTL
FATAL: Another FTL process is already running! Exiting...
I just wanted to list the output of all possible combinations, for you to evaluate.
By default, I don't do anything with pihole-FTL, the settings page indicated FTL was running,
I just tried pihole-FTL to see if there was any help available. (config-file, settings, ...) but apparently, there isn't.
Still wondering why the error message appears when simply running pihole-FTL...
In the process of setting up the system from scratch (again) to see if I find something new...
That is exactly what happens, you are trying to start pihole-FTL as your own process (whatever user you might be). However, I'm pretty sure you are not the user pihole. However, the log file /var/log/pihole-FTL.log can only be opened with write-access from the user pihole (because it owns it and all other users are only allowed to read, but not to write to this file).
As always: I'm not a native speaker and happily accept and discuss suggestions of improvements of any messages.
Yeah, the first check is if it can open the log file and it will fail miserably if this is not the case (because it will not even be able to log anything when started in the background). It won't even check for other running instances of FTL at this point.
I am testing the FTL as well, on a Debian 9 VM. Let us know if anything in particular is to be tested.
FTL version: v1.5.1
Process identifier (PID): 596
Time FTL started: 10:12:38
User / Group: pihole / pihole
Total CPU utilization: 0.1%
Memory utilization: 1.7%
Used memory: 4.02 MB
Pi-hole can very well be used in business environments with more than fifty concurrently active clients without any issues. But for getting up to this humongous number of queries, I used a small script that queries for random domains (obviously most of them didn't even exist) for several hours.
ah okay good to know well i'm trying it in out at my working place and getting bigger and bigger at the moment i'm trying with 10 clients working quiet fine
Just a big fat No on trying just any command with 
that is because you did 