script used to share internet on my lan for pppoe: 
IP=`ifconfig ppp0 |  grep inet | cut -f2 -d ';' | cut -f1 -d ' '`
export IP
route add default gw $IP

iptables -F
iptables -X
iptables -F -t nat

echo "Starting internet .... "$IP

iptables -A POSTROUTING -t nat -s 192.168.0.1 -j SNAT --to $IP

unset IP

script from rc.firewall(took it from my old slack 10.2):
IP=`ifconfig eth0 | grep inet | cut -b 21-32`
iptables -F
iptables -X
iptables -F -t nat
echo "Detecting IP .... "$IP