Sorry for not responding sooner Phil :'(
The thing is, i was trying 1001 things to solve my problem myself and didn't want to bother you with it because you've already devoted so much time in helping me - for which thank you once again ;D
But I keep on fighting with the Synology going to WAN2 when WAN1 has connection problems, which I don't want it to do.
As a recap:
1. WAN1 is VDSL with unlimited traffic; Synology downloads here.
2. WAN2 is Cable, metered 100 GB monthly traffic, so purely meant as fall back. Synology shall never go there.
3. Given your remarks before I put z_nas (= synology) not on the failover group but on the Gateway 'WAN1'.
4. Occassionally, I wake up in the morning discovering WAN1 was down and Synology hopped over on WAN2 and downloaded too much there (metered).
I know you said this shouldn't be possible, but it does do it( :-). I double checked to make sure the Synology always on WAN1 rule is before the 'LAN to any' which goes through Failover1. So, if I understand correctly the Synology traffic is covered by the more specific rule, and therefor should never be hit by the more general 'LAN to any' rule that comes later.
Would you happen to know how I might fix this mess?
Also, I don't quite understand this (screenshots). I have a rule that says Synology should never go to WAN2 (you wrote in the above that won't work, but I am still playing with it to see). But now in the firewall log there are blocks caused by that rule, preventing the Synology to go DNS on the WAN1-interface.
I suspected before already that copying a rule and adjusting it turns out buggy (I get the wrong descriptions in the logs, for example a block on LAN shows up in the logs as a block triggered by a rule for OPT3, which is a VLAN).
As ever I am in big debt towards you Phil; thank you for your help ;D