iptables + PPPoE routing problem

Hi,

I would like to install a Linux firewall connecting to ISP using PPPoE. The following are the software installed on the Linux box :-

Linux kernel 2.4.21 (SuSE distribution)
iptables 1.3.1
Roaring Penguin PPPoE 3.7
pppd 2.4.1

I have successfully connect to the ISP, and I could NAT'd through the ppp0 interface. On the Linux box, I could ping anywhere. However, on the DMZ, I could only ping the ppp0 interface and failed to ping ISP side of the point-to-point connection. It still failed even I make all the iptables rules with "ACCEPT".

What went wrong? I hope someone could help me to how to setup the PPPoE properly to make the DMZ work.

Thanks a lot.

John Mok