Module: Configuring upstream params (eg fastcgi_param) per request

[davidjb]

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!

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,266934,266934#msg-266934