What is a full specification of X-Forwarded-Proto HTTP header?

Question

What is a full specification of X-Forwarded-Proto HTTP header values?

Answer 1

I'm not aware of a "full specification".

The IETF APPSArea Working Group recently decided to define a new header field "Forwarded" which is supposed to replace the "X-Forwarded-*" header fields that you mentioned.

See http://tools.ietf.org/html/draft-ietf-appsawg-http-forwarded for details.