How can I achieve port forwarding with IPTables?

Question

I need to achieve the following scenario using IPTables:

In this scenario we have a front end server with only one NIC with IP 1.2.3.4, and there are three other servers with IPs set to 172.20.20.20, 192.168.20.10 and 10.10.10.8.

The goal is to configure IP tables so that clients only connect to the front end server (1.2.3.4) and the request get forwarded based on the port, for example when the green client requests 1.2.3.4:8080, the request is sent to 10.10.10.8:8080, or if he sends a request to 1.2.3.4:443, his request is forwarded to 172.20.20.20:443 while preserving the original client's IP.

I need this IP as a part of user identification - based on the client's IPm different contents will be served.

I have already read these questions but it didn't help:

• IP forwarding with iptables
• Port forwarding with IPTABLES and VBox

Answer 1

On the firewall/router - issue the following commands (assuming you want this to work on Port 80. The first provides the path to the webserver, the second corrects the outbound leg.

IPTABLES -t nat A PREROUTING -p tcp  -d REAL.WORLD.IP --dport 80  -j DNAT --to-destination INTERNAL.IP
IPTABLES -t nat -A POSTROUTING -s INTERAL.IP -p tcp --dport 80 -j SNAT --to-source EXTERNAL.IP