Iptables NAT

iptables-nat

# Linux IPTables NAT Setup
It is important to ensure that the test-server has the proper routes. Else it will 
receive the request, but be unable to respond. Thinking that the test-server works and
has proper routes when it in fact doesnt can lead to a world of troubles.

It can be beneficial to just disable the main iterface.

Test to make sure you can reach the internet from the internal test-machine 
before trying to map a listening port to it.

The following steps works

## Enable Linux port forwarding
echo 1 > /proc/sys/net/ipv4/ip_forward

* Change the setting in /etc/sysctl.conf to keep throught reboot

### Enable NAT
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

### Forward established and related connection back to the requestee
iptables -A FORWARD -i eth0 -o eth1 -m state --state RELATED,ESTABLISHED -j ACCEPT

### Forward requests from the INSIDE network to the outside without question
iptables -A FORWARD -i eth1 -o eth0 -j ACCEPT

### Forward everything to port 80 to internal server 10.0.0.5
iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 80 -j DNAT --to 10.0.0.5


### Forward port 1911 to internal port 22 on 10.0.0.9 
iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 1911 -j DNAT --to 10.0.0.9:22