nginx as a CORS-enabled HTTPS proxy

So you need a CORS frontend to your HTTPS target server that is completely unaware of CORS.

I tried doing this with Apache but it couldn't support the creation of a response to the "preflight" HTTP OPTIONS request that is made by CORS-compliant frameworks like jQuery.

Nginx turned out to be just what I needed, and furthermore it felt better too - none of that module-enabling stuff required, plus the configuration file feels more programmer-friendly with indenting, curly-braces for scope and if statements allowing a certain feeling of control flow.

So without any further ado (on your Debian/Ubuntu box, natch) :

Get Nginx:

sudo apt-get install nginx

Get the Nginx HttpHeadersMore module which allows the CORS headers to be applied whether the request was successful or not (important!)

sudo apt-get install nginx-extras

Now for the all-important config (in /etc/nginx/sites-available/default ) - We'll go through the details after this:

#
# Act as a CORS proxy for the given HTTPS server(s)
#
server {
  listen 443 default_server ssl;
  server_name localhost;

  # Fake certs - fine for development purposes :-)
  ssl_certificate /etc/ssl/certs/ssl-cert-snakeoil.pem;
  ssl_certificate_key /etc/ssl/private/ssl-cert-snakeoil.key;

  ssl_session_timeout 5m;

  # Make sure you specify all the methods and Headers 
  # you send with any request!
  more_set_headers 'Access-Control-Allow-Origin: *';
  more_set_headers 'Access-Control-Allow-Methods: GET, POST, OPTIONS, PUT, DELETE';
  more_set_headers 'Access-Control-Allow-Credentials: true';
  more_set_headers 'Access-Control-Allow-Headers: Origin,Content-Type,Accept';

  location /server1/  {
    include sites-available/cors-options.conf;
    proxy_pass https://<actual server1 url>/;
  }

  location /server2/  {
    include sites-available/cors-options.conf;
    proxy_pass https://<actual server2 url>/;
  }
}

And alongside it, in /etc/nginx/sites-available/cors-options.conf:

    # Handle a CORS preflight OPTIONS request 
    # without passing it through to the proxied server 
    if ($request_method = OPTIONS ) {
      add_header Content-Length 0;
      add_header Content-Type text/plain;
      return 204;
    }

What I like about the Nginx config file format is how it almost feels like a (primitive, low-level, but powerful) controller definition in a typical web MVC framework. We start with some "globals" to indicate we are using SSL. Note we are only listening on port 443 so you can have some other server running on port 80. Then we specify the standard CORS headers, which will be applied to EVERY request, whether handled locally or proxied through to the target server, and even if the proxied request results in a 404:

  more_set_headers 'Access-Control-Allow-Origin: *';
  more_set_headers 'Access-Control-Allow-Methods: GET, POST, OPTIONS, PUT, DELETE';
  more_set_headers 'Access-Control-Allow-Credentials: true';
  more_set_headers 'Access-Control-Allow-Headers: Origin,Content-Type,Accept';

This last point can be important - your JavaScript client might need to inspect the body of an error response to work out what to do next - but if it doesn't have the CORS headers applied, the client is not actually permitted to see it!
The little if statement that is included for each location provides functionality that I simply couldn't find in Apache. This is the explicit response to a preflighted OPTIONS:

  
    if ($request_method = OPTIONS ) {
      add_header Content-Length 0;
      add_header Content-Type text/plain;
      return 204;
    }

The target server remains blissfully unaware of the OPTIONS request, so you can safely firewall them out from this point onwards if you want. A 204 No Content is the technically-correct response code for a 200 OK with no body ("content") if you were wondering.
The last part(s) can be repeated for as many target servers as you want, and you can use whatever naming scheme you like:

 location /server1/  {
    include sites-available/cors-options.conf;
    proxy_pass https://server1.example.com;
  }

This translates requests for:

    https://my.devbox.example.com/server1/some/api/url

to:

    https://server1.example.com/some/api/url

This config has proven useful for running some Jasmine-based Javascript integration tests - hopefully it'll be useful for someone else out there too.