NAT for use with OpenVPN

[Morgan Wesström]

> # tcpdump -nvvi eth0 icmp
> tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 
> 65535 bytes
> 15:22:29.614953 IP (tos 0x0, ttl 62, id 5638, offset 0, flags [DF], 
> proto ICMP (1), length 84)
>      10.8.0.8 > 8.8.8.8 <http://8.8.8.8>: ICMP echo request, id 13, seq 
> 1, length 64
> Are you thinking that the ping should be coming from 192.168.1.200 (my 
> OpenVPN server machine)? If not, how else would you know whether the 
> address is being NATed?

The packet is NATed when your Netgear router exchange the source ip 
address 10.8.0.8 with its own public external ip address 67.175.144.37.

I you ping from any machine on your 192.168.1.0/24 subnet you will see 
those packets as "67.175.144.37 > 8.8.8.8" on your external interface 
regardless of what ip is the source on the LAN. This is what should've 
been the case also from 10.8.0.0/24 if the router was doing its job 
properly.

If you listen with tcpdump on the internal interface, before NAT takes 
place, you will still see the original private ip addresses as source 
addresses.

Private ip addresses (10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16) 
can't be routed on the Internet (RFC1918). NAT means that when your 
router receives a packet on its internal interface, destined for the 
Internet, with one of those private ip addresses in the source field, it 
exchanges it with its own external ip before forwarding it to your ISP. 
It then keeps a table of what internal ip communicates with what ip on 
the Internet. When a reply returns it's matched against this table and 
if the router finds that this packet is meant for a computer on your LAN 
it will now reverse the NAT procedure and exchange its external ip 
(which is now the destination address) with the correct internal ip and 
put the packet on your LAN.

The reason you can't see pings from your internal ips to your external 
ip on the external interface is simply because those packets are never 
actually put on that interface physically. When those pings reach the 
internal interface, the ip stack in the router realizes that the ping is 
meant for itself and immediately responds on the internal interface. 
tcpdump listens to what's actually put on the physical interface and 
won't see those packets while listening on eth0. Everything you have 
shown me so far is consistent with our suspicion that the Netgear router 
only provides NAT for 192.168.1.0/24.

I have only rudimentary knowledge of iptables but I'm convinced your 
problem will be solved if you can find a way to add a NAT rule for 
10.8.0.0/24 or better yet, for any subnet existing on your LAN.

/Morgan