now , we were upgrade our website to windows 2012, but i cannot found iis advanced logging at iis 8 support platform, so i don't know how setup iis log can use client ip, thanks
reference link:
http://blog.loadbalancer.org/iis-and-x-forwarded-for-header/
http://www.microsoft.com/en-gb/download/details.aspx?id=7211