I'm working on an authentication module for nginx, namely the Shibboleth auth module (https://github.com/nginx-shib/nginx-http-shibboleth). This module is based off the core nginx auth_request module (https://github.com/nginx/nginx/blob/master/src/http/modules/ngx_http_auth_request_module.c). The original module allows a sub-request to determine whether to grant or deny access to a nginx location; the Shibboleth module works in the same way, but copies specific headers (in the format "Variable-[Field-name]") from the auth sub-response into the original parent request so that they are sent to the upstream application.

As it stands, the Shibboleth module can copy these variables automatically as headers, by iterating across all `headers_out` in the auth sub-response, testing for the `Variable-` string prefix, and then copying the values into the parent request's headers_in (see https://github.com/nginx-shib/nginx-http-shibboleth/blob/development/ngx_http_shibboleth_module.c#L299). This works well but care needs to be exercised to avoid spoofing.

What I'd like to do is allow the same automated copying of `Variable-[Field-name]` headers from the sub-response's `headers_out` into relevant environment parameters (eg fastcgi_param, uwsgi_param etc) for upstreams that support this.

I can achieve the desired result with manual use of `shib_request_set` (identical to auth_request_set, which sets nginx variables from the auth request response), like so:

location / {
shib_request_set $shib_auth_type $upstream_http_variable_auth_type;
fastcgi_param Auth-Type $shib_auth_type;
fastcgi_pass localhost:8000;
}

which sets `$shib_auth_type` to the value of header `Variable-Auth-Type`, and then sets the FastCGI param `Auth-Type` to the given value. The drawback is that this requires manual configuration for all potential `Variable-` prefixed headers (dozens or more are possible at times), and also different directives for each type of upstream (fastcgi_param, uwsgi_param, scgi_param etc).

So, is it possible to set an upstream's parameters dynamically from my module's request handler (eg in the ngx_http_auth_request_handler function) or another part of the module (eg a filter)?

Looking at the fastcgi, scgi and uwsgi modules in nginx, they have different (but similar) implementations and upstreams such as proxy_pass don't support environment parameters. So if this is possible, I'd envisage that my module would need to be aware of how the different upstreams' params are configured. My manual config solution might already be best, but I wanted to ask the question all the same.

Thanks!