xforward-for doesnt work

The challenge isn't so much if it works or doesn't, but how that data is presented to BIG-IP, and how you choose to manage it. You can receive multiple X-Forwarded-For headers with a single IP, or a single X-Forwarded-For header with multiple IPs, or some combination of the two. It could also be that there is another similar header name used for XFF that your backend server is reading, but you'd need a dump of the headers to know for sure.