when I make my dhcp configuration using (wireless laptop whitout adhoc) or
sudo apt-get install hostapd
sudo apt-get install dhcp3-server
/sbin/iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE
for ppp0 (modem)

/sbin/iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
for LAN

I run through the rc.local, so if i want to connect to the LAN / modem must be change the configuration

how to let it run the automatic configuration. so if i use eth0 automatic use eth0 configuration, if i use ppp0 automatic use ppp0 configuration.