From DSL Wiki
To configure network interfaces start the DSLPanel and click on Netcardconf.
Manual configuration
Edit /etc/network/interfaces and add each network interface like this:
iface eth0 inet static
address 192.168.0.111
netmask 255.255.255.0
gateway 192.168.0.1
iface eth1 inet dhcp
See also Debian Reference - Network configuration