In each machine, remember to setup the dns server, for example, in /etc/resolv.conf:
nameserver 8.8.8.8
auto lo
iface lo inet loopback
auto eth0
iface eth0 inet static
address 10.0.2.20
netmask 255.255.255.0
gateway 10.0.2.15
auto lo
iface lo inet loopback
auto eth0
iface eth0 inet static
address 10.0.2.10
netmask 255.255.255.0
gateway 10.0.2.15
auto lo
iface lo inet loopback
auto eth0
iface eth0 inet dhcp
auto eth1
iface eth1 inet static
address 10.0.2.15
netmask 255.255.255.0
Clean IPTABLES:
iptables --flush
iptables --table nat --flush
Configure the Gateway machine as a router:
echo 1 > /proc/sys/net/ipv4/ip_forward
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
iptables -A FORWARD -i eth0 -o eth1 -j ACCEPT
iptables -A FORWARD -i eth1 -o eth0 -j ACCEPT