Question

I have two Systems A and B. A is a TCP Client and sends a message to TCP Server on B.

------------------                --------------------------

  System A                        System B
  192.168.0.5 wlan0               192.168.0.3 wlan0
  127.0.0.1   lo                  127.0.0.1 lo
  TCP Client    <------------>    TCP Server on 127.0.0.1
------------------                ----------------------------

The TCP Client sends message to 192.168.0.3.

This should be redirected to the local interface of B as the TCP Server is running on 127.0.0.1 at Port 8000 of System B.

Therefore, I wrote the following ip table rules, however my Server at B doesn’t receive any messages. Oh btw these two systems are Ubuntu linux systems.

Here is what I did on System B:

#Enable IP Forwarding for NAT
echo "1" < /proc/sys/net/ipv4/ip_forward

#Flush all iptable chains and start afresh
sudo iptables -F

#Forward incoming packets on 192.168.0.3 at wlan0 interface to 127.0.0.1
sudo iptables -t nat -A PREROUTING -p tcp -i wlan0 -d 192.168.0.3 --dport 8000 -j DNAT --to 127.0.0.1:8000

#Explicitly allow incoming connections on port 8000 
sudo iptables -A INPUT -i wlan0 -p tcp --dport 8000 -m state --state NEW,ESTABLISHED -j ACCEPT

#Explicitly allow outgoing messages from port 8000
sudo iptables -A OUTPUT -o wlan0 -p tcp --sport 8000 -m state --state ESTABLISHED -j ACCEPT

Then I start the Server on B and send a message from TCP Client on A. I can see the packets on wireshark from 192.168.0.5 on wlan0 but they never get forwarded 🙁

Please help.

Answer 1

It should be “>” instead of “<“:

echo "1" > /proc/sys/net/ipv4/ip_forward