sudo cp /etc/network/interfaces /etc/network/interfaces.bak

adding net for 2nd interface eth1

add the following information to /etc/network/interfaces

# Set up the internal wired network
#
# Don't forget to change eth1 to the proper name of the internal
# wired network interface if applicable.
#
auto eth1
iface eth1 inet static
    address 192.168.0.1
    network 192.168.0.0
    netmask 255.255.255.0
    broadcast 192.168.0.255

restart network with

sudo /etc/init.d/networking restart

Setup dhcp server for local network

sudo apt-get install dhcp3-server

inte the file /etc/dhcp/dhcpd.conf insert the following

authoritative;
option domain-name "mydomain";
option domain-name-servers 8.8.8.8, 8.8.4.4, 192.168.0.1;

default-lease-time 600;
max-lease-time 7200;

option subnet-mask 255.255.255.0;
option broadcast-address 192.168.0.255;
option routers 192.168.0.1;

subnet 192.168.0.0 netmask 255.255.255.0 {
        range 192.168.0.32 192.168.0.128 ;
        option routers 192.168.0.1 ;
}

then edit /etc/default/isc-dhcp-server

INTERFACES="eth1"

then restart dhcp server

sudo service isc-dhcp-server restart

Setting up the firewall

Be sure that ufw is installed on the system

sudo apt-get install ufw

edit the file /etc/default/ufw and change the line

DEFAULT_FORWARD_POLICY="DROP"

so it reads

DEFAULT_FORWARD_POLICY="ACCEPT"

then we need to enable packet forward between the interfaces

edit the file /etc/ufw/sysctl.conf and remove the # infront of the line below so it reads

net/ipv4/ip_forward=1

One final file to change before we are upp and running

change the file /etc/ufw/before.rules add these line in the top before any commands

# nat rules
*nat
  OSTROUTING ACCEPT [0:0]

# Forward all packes through eth0
-A POSTROUTING -s 192.168.100.0/24 -o eth0 -j MASQUERADE

# WARNING, do not remove COMMIT line. This breaks the loading
COMMIT

Before starting the firewall, if you want to be able to access it with ssh you need to enable a rule accepting connections on port 22

ufw allow 22

Then start it up

ufw disable && sudo ufw enable