You'll get client IP in headers.
"forwardfor" option which will add an "X-Forwarded-For" header with the original client's IP address. You must also use "httpclose" to ensure that you will rewrite every requests and not only the first one of each session :
option httpclose
option forwardfor
Line 122 http://haproxy.1wt.eu/download/1.2/doc/architecture.txt