Unclear why you need WAN{a,b} for internal traffic.Instead of LAN, you may be better suited with VLANs ?Depending on the traffic, it would be fairly easy to say that users in VLAN1 can access say HTTPS:IP in VLAN2Alternatively, define an alias for a group of IPs in one VLAN that need to access a service in the other.End your rules with 'if nothing matches {route to internet | reject}'