It is not the good method. This article describes only to block port 53 but in fact you should block all
new input packets from the internet interface.
Note that this is already the default. When you have a problem you have either modified the default
firewall, or you have modified the internet port without adjusting the firewall accordingly.
(this happens because of sub-optimal configuration of the default firewall, which they don't want to fix)
When you use PPPoE make sure that the interface "ether1-gateway" in the firewall is changed to your
PPPoE interface in the rule that drops incoming traffic from that interface.