Hi Comex,
Is this for IIS only? If so, here's a possible solution:
http://devcentral.f5.com/Community/GroupDetails/tabid/1082223/aff/31/afv/topic/aft/1178815/afc/1227279/Default.aspx
Posted By rjordan on 05/06/2011 12:53 PM
Regarding something simple to attempt, try installing ARR Helper for IIS (http://blogs.iis.net/anilr/archive/2009/03/03/client-ip-not-logged-on-content-server-when-using-arr.aspx). This will rewrite the REMOTE_ADDR value with the X-Forwarded-For value. We've used this when we have to deal with some closed source applications that can't be updated to look at X-Forwarded-For. I don't think it rewrites the source IP in the web logs, though.
Another option would be to modify the servers to set the default gateway to an LTM self IP and then disable SNAT.
Aaron
Aaron