I'm running an internal network with my linux box as diallup gateway.
I used the yaST firewall configuration with the following:
external int ppp0
internal int = eth0
Allow traceroute = yes
forward traffic, do masq = yes
protect all running services = no
protect from internal network = no
Everything seems to work fine except when the modem connects.
Then the firewall drops all active connections from the internal network.
This is bad since i use VNC and ssh to work on it.