How to use IRule to determine client ip after pass through Proxy

X-Forwarded-For is what F5 can add to the headers when it is the proxy. It sounds like you are talking about another device that is acting as a proxy. Check to see if that proxy adds a custom header that you can use to read the IP address. Standard practice is to do so, but the header name is no universal. Many use X-Forwarded-For and many use X-Remote-Addr. If the proxy does not add such a header, you must find something else besides IP address to identify your users.