Nginx CORS-support for proxied Grape/Rails/Passenger APIs

nginx.conf

#
# CORS-header support example while nginx proxies Rails/Grape + Passenger
# ...not a complete config file
#
server {
        listen 443 ssl;
        root /foo/public;

        # Modify for API-specific
        try_files $uri/index.html $uri @passenger;

        location @passenger {
                passenger_enabled on;

                # Set must be inside the location{...} block
                set $cors '';
                # Terminate regex with `\Z` to prevent forged `Origin: http://badregex.main.com.evil.com` from being allowed
                if ($http_origin ~ '^https?://((?:\w)+\.main\.com|(?:\w)+\.secondary\.com)\Z') {
                        set $cors 'true';
                }
                # Forcing a strict CORS policy in case domain doesn't match regex
                if ($cors = '') {
                        set $http_origin 'https://www.main.com  ';
                        set $cors 'true';
                }

                if ($cors = 'true') {
                        add_header 'Access-Control-Allow-Origin' "$http_origin" always;
                        add_header 'Access-Control-Allow-Credentials' 'true' always;
                        add_header 'Access-Control-Allow-Methods' 'GET, POST, PUT, DELETE, OPTIONS' always;
                        add_header 'Access-Control-Allow-Headers' 'Accept,Authorization,Cache-Control,Content-Type,DNT,If-Modified-Since,Keep-Alive,Origin,User-Agent,X-Requested-With' always;
                }

                if ($request_method = 'OPTIONS') {
                        # NOTE: Chrome max is 10mins, Firefox max 24hrs
                        # ...setting a reasonable 1hr, though Chrome will just use 10mins
                        add_header 'Access-Control-Max-Age' 3600;
                        add_header 'Content-Type' 'text/plain charset=UTF-8';
                        add_header 'Content-Length' 0;
                        return 204;
                }
        }
}