[LUAU] help with iptables

[Whoever Whatever]

Hi,
    when I have a linux box with two NICs, one $EXTIF (external) and
one $INTIF (subnet=192.168.176.0/20).  With ip masq turn on:
iptables -t nat -A POSTROUTING -o $EXTIF -j MASQUERADE

how do I block $INTIF(192.168.176/20) from accessing ports(ie: 80) on
the linux box(192.168.176.1), but still allow port 80 access using ip
masq to the outside world?

I tried the filters below, but I still can access port 80 after
applying the rules:
iptables -A INPUT -i $INTIF -p tcp -m tcp --dport 80 --syn -j DROP
iptables -A FORWARD -i $INTIF -p tcp -m tcp --dport 80 --syn -j DROP
iptables -A INPUT -p tcp -s 192.168.176.0/20 -d 192.168.176.1 --dport 80 -j DROP

anyone know if it's possible?

Thanks