I was attempting to setup NAT Masquarading with Firewalld using the same procedure most other people do (See this blog post for full details, or Arch’s wiki for a more compat info).
But it refused to work and forward between networks.
After banging my head quite a lot I managed to find the culprit:
I was managing networks through YaST using the older Wicked system instead through NetworkManager, and it turns out that in addition to the net.ipv4.ip_forward sysctl setting that every blog post tells you to check, in YaST’s Network Settings’ tab “Routing” there’s an additional forwarding checkbox that needs to be checked to allow network forwarding (That one was unchecked and was causing forwarding to not work).
Also on a related note: If you’re running a setup as old as mine, this is a good time to remember to upgrade your firewalld config to use nftables instead of iptables - this has been default for ages by now.