The correct HTTP header is X-HTTP-FORWARDED-FOR ! Sometimes HTTP-FORWARDED-FOR.
On most webservers/Proxie existing environment variables are:
HTTP_CLIENT_IP
HTTP_X_FORWARDED_FOR
HTTP_X_FORWARDED
HTTP_FORWARDED
HTTP_VIA
HTTP_X_COMING_FROM
HTTP_COMING_FROM
The correspoding HTTP header in request has a - instead of a _.