Keep source IP after NAT

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http://serverfault.com – Until today I used a cheapy router so I can share my internet connection and keep a webserver online too, while using NAT. Users IP ($_SERVER['REMOTE_ADDR']) was fine, I was seeing class A IPs of users. But as traffic grown up everyday, I had to install a Linux Server (Debian) to share my Internet Connection, because my old router couldn't keep the traffic anymore. I shared the internet via IPTABLES using NAT, but now, after forwarding port 80 to my webserver, now instead of seeing real users IP, I see my Gateway IP (Linux Internal IP) as any user IP Address. How to solve this issue? (HowTos)