Linux Ipchains passthru

I want to send a http request coming to the public IP on my firewall to a http server on my private network.

IE someone requested www.reqeust.com it goes to 63.1.1.25 then the firewall sends it to 10.1.1.75.

I know it can be done but having problems finding info on it.

I am using ip forwarding version of IPchains.