rlnclientipaddr re-write?

My engineers have given me that requirement that the http header field rlnclientipaddr always has to be populated with the real source IP of the http traffic.

 

 

I am not sure if I will be using SNAT yet for the pool these servers live in, so this will be the only way for the logging data to retain the real request IP.

 

 

I am guessing this should be set up on http_request action, but how exactly would I implement the rewrite itself?