/etc/network/interfaces file

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo eth1 eth3 eth4 eth5
 
iface lo inet loopback

# The primary network interface
allow-hotplug eth1
iface eth1 inet dhcp
#
iface eth3 inet static
address 192.168.3.1
netmask 255.255.255.0
#
iface eth4 inet static
address 192.168.4.1
netmask 255.255.255.0
#
iface eth5 inet static
address 192.168.5.1
netmask 255.255.255.0


> ifconfig -a
eth1      Link encap:Ethernet  HWaddr 00:50:8D:5A:77:B5  
          inet addr:12.219.61.253  Bcast:255.255.255.255  Mask:255.255.248.0
          UP BROADCAST RUNNING MULTICAST  MTU:576  Metric:1
          RX packets:6102 errors:0 dropped:0 overruns:0 frame:0
          TX packets:260 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:511967 (499.9 KiB)  TX bytes:45976 (44.8 KiB)
          Interrupt:17 Base address:0xa000 

eth3      Link encap:Ethernet  HWaddr 00:10:B5:5F:9D:E2  
          inet addr:192.168.3.1  Bcast:192.168.3.255  Mask:255.255.255.0
          UP BROADCAST MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:0 (0.0 b)  TX bytes:0 (0.0 b)
          Interrupt:19 Base address:0x9000 

eth4      Link encap:Ethernet  HWaddr 00:04:5A:72:9E:6E  
          inet addr:192.168.4.1  Bcast:192.168.4.255  Mask:255.255.255.0
          inet6 addr: fe80::204:5aff:fe72:9e6e/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:734 errors:0 dropped:0 overruns:0 frame:0
          TX packets:915 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:116657 (113.9 KiB)  TX bytes:686136 (670.0 KiB)
          Interrupt:20 Base address:0x9400 

eth5      Link encap:Ethernet  HWaddr 00:04:5A:72:9E:51  
          inet addr:192.168.5.1  Bcast:192.168.5.255  Mask:255.255.255.0
          inet6 addr: fe80::204:5aff:fe72:9e51/64 Scope:Link
          UP BROADCAST MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:3 dropped:0 overruns:0 carrier:6
          collisions:0 txqueuelen:1000 
          RX bytes:0 (0.0 b)  TX bytes:0 (0.0 b)
          Interrupt:21 Base address:0x9800 

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:9 errors:0 dropped:0 overruns:0 frame:0
          TX packets:9 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:636 (636.0 b)  TX bytes:636 (636.0 b)


/etc/network/firewall file

# assuming eth1 = ext net and eth0 = int net
#
iptables -F; iptables -t nat -F; iptables -t mangle -F
iptables -t nat -A POSTROUTING -o eth1 -j SNAT --to 12.219.61.253
echo 1 > /proc/sys/net/ipv4/ip_forward
#
# to secure it
#
iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A INPUT -m state --state NEW -i ! eth1 -j ACCEPT
iptables -P INPUT DROP
iptables -A FORWARD -i eth1 -o eth1 -j REJECT


/etc/rc.local file

# rc.local
#
# This script is executed at the end of each multiuser runlevel.
# Make sure that the script will "exit 0" on success or any other
# value on error.
#
# In order to enable or disable this script just change the execution
# bits.
#
# By default this script does nothing.
/etc/network/firewall
exit 0