Create an alias of private networks (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16), set that alias as the destination in the firewall then invert the destinationThe above seems clunky and is surely not the solution that the developers intended for us to use.
allow "NOT rfc1918" https
block rfc1918 allow any https
allow 192.168.123.0/24 https block rfc1918 allow any https