Networking – How to forward traffic between Tun device and eth0

In summary, I would like to have Tun device traffic forwarded
back and forth through another network interface that has internet
connection. While I can see the traffic going out to internet and coming back, it is not routed back to my Tun device.

Here's my setup:

I have a very simple setup using Mint Linux 15 VM under VirtualBox with Win7 as host.

Within the VM, there are two network interfaces – eth0 and tun0.

• eth0 interface is connected to the internet, and is assigned as 192.168.1.115/24.

• tun0 interface is assigned as 10.0.5.1/24.

  ip tuntap add dev tun0 mode tun user askldjd
  ip link set tun0 up
  ip addr add 10.0.5.1/24 dev tun0
  

I set up my iptables rule to masquerade all traffic going out through eth0.

    iptables -I FORWARD -i tun0 -o eth0 -s 10.0.5.0/24 -m conntrack --ctstate NEW -j ACCEPT
    iptables -I FORWARD -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
    iptables -t nat -I POSTROUTING -o eth0 -j MASQUERADE

I tested the rule using a simple ping -I. So to some degree, my iptable rules are working.

    ping -I 10.0.5.1 google.com
    PING google.com (74.125.228.65) from 10.0.5.1 : 56(84) bytes of data.
    64 bytes from iad23s07-in-f1.1e100.net (74.125.228.65): icmp_req=1 ttl=55 time=7.46 ms

My next step is to create a ICMP Request packet with src = 10.0.5.1, and dst = 74.125.228.6 (google.com). This was done through a tcpdump capture, so I know the packet fields/checksum are all valid. And I am sending this packet out to a raw IP socket using a very simple Python script.

• Python send_packet.py : http://pastebin.com/Xh5YgsAx

Before I run the python script, I set up tshark to monitor eth0 and tun0.

    tshark -i eth0
    tshark -i tun0

Then I run the script. From the tshark console, I can see the ICMP Request going out, and ICMP Reply coming back from google.

    1811.947250 192.168.1.115 -> 74.125.228.6 ICMP 98 Echo (ping) request  id=0x0990, seq=1/256, ttl=64
    1811.955146 74.125.228.6 -> 192.168.1.115 ICMP 98 Echo (ping) reply    id=0x0990, seq=1/256, ttl=55

And from the tun0 tshark window, I see nothing.

In my mind, since the ICMP Request packet is set as source = 10.0.5.1, I would expect that the IPTable to un-nat the ICMP Reply when it comes back. This is not happening.

So either my IPTables setup is incorrect, or I am simply misunderstanding the concept of the Tun device. Any guidance would be greatly appreciated.

I apologize if I misuse any terminology. I am a novice when it comes with networking.