I have come across this same issue. From reading through various posts this is a limitation of DNSMASQ, but it appears there is a way around this by entering something like CharAznable's set of 'rev-server=' lines rather than using the web GUI. I think it would be good to have a post that details actually what needs to be done to get conditional forwarding working on non standard subnets.
Anyone want to contribute to this with corrections or ideas?
WORK IN PROGRESS, NOT GUARANTEED TO WORK - MOST LIKELY TO BROK YOUR SETUP!
As DNSMASQ doesn't allow conditional forwarding for subnets other than /8,/16,/24 or /32. If you have a network with (for example) /23 for its subnet, this can be configured manually within the PiHole server by following the steps below.
ssh root@pihole.local
cd /etc/dnsmasq.d/
nano 02-condfor.conf
Add one entry per line in the format :
rev-server=MySubNetwork/24,IpOfUpstreamServer
For example, I use a /23 network on 192.168.4.0/23 and want to forward requests to a server at 192.168.4.248
rev-server=192.168.4.0/24,192.168.4.248
rev-server=192.168.5.0/24,192.168.4.248
Once these entries have been made, restart (from ssh session) with
pihole restartdns
Unfortunately I am unable to test the above at present as my only PiHole server with a non /24 network is in use 24/7 so hoping some other users can contribute to this.
One thing I am unsure of is having a forward to a server that is not within the subnet (192.168.5.0/24 -> 192.168.4.248) does this work ok?
Is anything else required to make this work? Do any other PiHole or DNSMASQ files need to be edited? Or anything changed within the PiHole GUI?