X-Forwarded-Host (XFH) header is a de-facto standard
header for identifying the original host requested by the client in the
Host HTTP request header.
Host names and ports of reverse proxies (load balancers, CDNs) may differ from the
origin server handling the request, in that case the
header is useful to determine which Host was originally used.
The domain name of the forwarded server.
Not part of any current specification.