Long time, no see.
This morning I was trying to debug a faulty Nginx configuration, so I wanted to use the add_header directive to add some debug informations in my headers.
add_header x-nginx-rq-filename $request_filename;
But wierdly this never showed up in the response of the server... This is because, the add_header directive are correlated to the HTTP response Nginx will give back. Some errors are then stripping whatever you are trying to put in your headers. But a parameter always does exist so you can force Nginx to add this header unconditionally.
add_header x-nginx-rq-filename $request_filename always;