====== NAT on linux with iptables ======
''eth0'' is our external network and ''eth1'' is our intenral one: 
<code>
echo 1 > /proc/sys/net/ipv4/ip_forward
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
iptables -A FORWARD -i eth0 -o eth1 -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -A FORWARD -i eth1 -o eth0 -j ACCEPT
</code>
to make all this permanent change the following parameter in ''/etc/sysctl.conf'' to ''1'': 
  net.ipv4.ip_forward = 1
then save the current iptables permanetnly. in **ubuntu** you do: 
<code>
apt install iptables-persistent
</code>
choose ''yes'' at the end to save the current config and you're done! if you change other iptables rules, you can run the following command to update the saved settings: 
<code>
iptables-save > /etc/iptables/rules.v4
</code>