Nat desde Wlan a ETH

echo "1" > /proc/sys/net/ipv4/ip_forward  
/sbin/iptables -t nat -A POSTROUTING -o wlan0 -j MASQUERADE
/sbin/iptables -A FORWARD -i wlan0 -o eth0 -m state --state RELATED,ESTABLISHED -j ACCEPT          
/sbin/iptables -A FORWARD -i eth1 -o wlan0 -j ACCEPT

Nat con una sola interfaz

echo "1" > /proc/sys/net/ipv4/ip_forward  
iptables -A FORWARD -i eth0 -o eth0:1 -m state --state ESTABLISHED,RELATED -j ACCEPT  
iptables -A FORWARD -i eth0:1 -o eth0 -j ACCEPT  
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE