IP problem in backend of reverse proxy

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 :
https://help.ubuntu.com/community/Nginx/ReverseProxy but I doesn’t work.
In the logs and on the server it display the IP of the reverse proxy
( or Test if you want :
Thanks for the help,