Help configuring Shorewall with IP masquerading-Mandriva

Hi,
I had a desktop with working Mandriva LE 2005, with ADSL modem (PPPoE)
connected to eth1, and a standrad 2 NICs shorewall (firewall)
configuration.

Originally my /etc/shorewall/interfaces was

#ZONE INTERFACE BROADCAST OPTIONS

net ppp0 detect
loc eth0 detect dhcp


Recently, I tried to share the Internet connection with another Mandriva
laptop connected to eth0 of the desktop.
Thus I:
1. Enabled ip forwarding

and
2. added the following /etc/shorewall/masq file
#INTERFACE SUBNET ADDRESS PROTO
#PORT(S)
eth1 eth0

Naturally shorewall didn't start, complaining eth1 was undefined. However,
since the net interface in my interfaces file was ppp0 (rather than eth1),
I have tried to replace eth1 with ppp0 in the masq file. Again shorewall
didn't start.

My question: how should I configure both shorewall files (interfaces and
masq) in my case.

TIA

Michael Badt