I haven't heard "slug" before, is that your own phrase? It sounds like you've implemented what's known as a stateless transparent firewall if I'm understanding what you've said correctly. Stateless because it's just plain ACLs, transparent because it's not a routed hop/isn't detectable by the client (outside traffic being dropped), and firewall because it's filtering unauthorized traffic. Most people just call it "ACLs" for short though since "stateless transparent firewall" is a bit of a mouthful and "firewall" by itself often implies a routed mode stateful firewall.