Passing SSL protocol info to backend via HTTP header
Solution 1:
Nginx use many variables which can be used in config. This page provided the complete list of the variables. The variable that holds the protocol in a HTTPS request is ssl_protocol. Citation:
$ssl_protocolreturns the protocol of an established SSL connection;
So your proxy_set_header configuration would be
proxy_set_header X-HTTPS-Protocol $ssl_protocol;
Another reference: here