io uso questo mettendo come ip alle schede
192.168.0.1 192.168.0.2

echo '0'> /proc/sys/net/ipv4/ip_forward
iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE
iptables -I FORWARD -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -A FORWARD -s 192.168.0.0/24 -j ACCEPT

echo '1'> /proc/sys/net/ipv4/ip_forward