I solved it!!!!!
It might be usefull for someone... so i'll explain how did i do it...
RH9 has default iptables rules, so iptables -F will flush those and let you use your own.
# Reglas para hacer NAT con iptables
echo 1 > /proc/sys/net/ipv4/ip_forward
insmod ip_masq_ftp > /dev/null 2> /dev/null
iptables -F
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
iptables -A INPUT -i eth0 -m state --state NEW,INVALID -j DROP
iptables -A FORWARD -i eth0 -m state --state NEW,INVALID -j DROP
just copy/paste it on etc/rc.local and you'll get it permanent
Thanks for your help guys!!!!