Set up a gateway with NAT

Q I have been trying to help a friend set up a very basic gateway - I just need to NAT everything for him, but I've had no luck so far. I realise a complete script might be a lot to fit in, but could you get me started?

A OK Vikram, here's a quick and dirty guide. I am assuming only that the mangle table is cleared and does not affect things. Let's do it:

# iptables -F INPUT
# iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
# iptables -A INPUT -i lo -j ACCEPT
# iptables -P INPUT DROP
# iptables -F FORWARD
# iptables -P FORWARD ACCEPT
# iptables -t nat -F
# iptables -t nat -A POSTROUTING -j MASQUERADE

I don't like the last rule that much: it is the quickest way to do it but it's too dirty. Let's replace it with:

# iptables -t nat -A POSTROUTING -o <externalinterface> -j SNAT -to <externalIP>

If you have a static IP on the gateway, or if you have a dynamic one, just run

# iptables -t nat -A POSTROUTING -o <externalinterface> -j MASQUERADE

You should replace <externalInterface> with the interface name facing the internet, ie eth0, eth1 or whatever.

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