auto lo

iface lo inet loopback
iface eth0 inet dhcp

iface eth1 inet static
  address 192.168.1.1
  netmask 255.255.255.0

iface eth2 inet static
  address 192.168.2.1
  netmask 255.255.255.0

iface eth3 inet static
  address 192.168.3.1
  netmask 255.255.255.0

*nat
:PREROUTING ACCEPT [1799:327587]
:POSTROUTING ACCEPT [0:0]
:OUTPUT ACCEPT [23:2190]
-A POSTROUTING -j MASQUERADE
COMMIT
*filter
:INPUT ACCEPT [3474:500657]
:FORWARD ACCEPT [24:1613]
:OUTPUT ACCEPT [857:128814]
-A FORWARD -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -s 192.168.1.101 -i eth1 -o eth0 -m conntrack --ctstate NEW -j ACCEPT
-A FORWARD -s 192.168.2.102 -i eth2 -o eth0 -m conntrack --ctstate NEW -j ACCEPT
-A FORWARD -s 192.168.3.103 -i eth3 -o eth0 -m conntrack --ctstate NEW -j ACCEPT
COMMIT