IP problem in backend of reverse proxy

Hi,
I’ve installed a reverse proxy with nginx. And I have 2 backends
webservers (one nginx and one apache), i’m hosting MediaWiki on it.
I want the backends servers to have the IP of the “real” client in their
logs. I tried “proxy_set_header” like here :
Nginx/ReverseProxy - Community Help Wiki but I doesn’t work.
In the logs and on the server it display the IP of the reverse proxy
(127.0.0.1 or 87.106.165.190) Test if you want :
http://wiki.okira.net/w/Accueil
Thanks for the help,
Regards.