FTP on policy DMZ questions

Hi,

I have an FTP set-up on a DMZ and I use VIPS to forward the ports too it,  as we only have one public IP address, which is assigned to the FG. I have a policy setup to allow access from internet to it. This all works fine.  

My problem is the FTP server is seeing all source connections coming from the FG's local DMZ IP and not the original external source IP. Is there a way to preserve the original external source IP? Reasons being for logging purposes on the FTP and other services it's running. 

Thanks,