If your Internet connection uses the PPPoE protocol, then your firewall should be configured with interface ppp0.
With PPPoE, the connection is established using the PPP protocol that works on top of the usual Ethernet. As the result, the firewall gets interface ppp0 in addition to the interfaces eth0 and eth1 that correspond to its "normal" physical network adapters. Here is how you can use Firewall Builder to configure such a firewall (assuming interface eth0 is connected to the DSL link and eth1 is connected to internal LAN):
Create a firewall object in the GUI.
Add interfaces ppp0 and eth1. You can simply skip eth0 as it does not have an IP address and never sees IP packets.
If you have a static IP address with your Internet connection, mark ppp0 as "static" and add an address object to it. Configure the address object with the IP address.
If your Internet connection uses dynamic IP address, mark ppp0 as "dynamic" and do not add an address object to it. Create a script /etc/ppp/ip-up to restart the firewall every time IP address of ppp0 changes.
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