Slightly tighter CORS config for nginx

cors-nginx.conf

#
# Slightly tighter CORS config for nginx
#
# A modification of https://gist.github.com/1064640/ to include a white-list of URLs
#
# Despite the W3C guidance suggesting that a list of origins can be passed as part of
# Access-Control-Allow-Origin headers, several browsers (well, at least Firefox) 
# don't seem to play nicely with this.
#
# To avoid the use of 'Access-Control-Allow-Origin: *', use a simple-ish whitelisting 
# method to control access instead.
#
# NB: This relies on the use of the 'Origin' HTTP Header.

# Fork by @FGRibreau - 2018-07-16
# - match all sub-domain
# - `add_header` with `always` param to specify CORS headers even in 5xx & 4xx responses

location / {

    # will be interpreted as a case-insensitive regular expression match.
    if ($http_origin ~ ^(.*)\.domain-name\.tld) {
     set $cors "true";
    }

   # Nginx doesn't support nested If statements. This is where things get slightly nasty.
   # Determine the HTTP request method used

   if ($request_method = 'OPTIONS') {
       set $cors "${cors}options";  
   }
   if ($request_method = 'GET') {
       set $cors "${cors}get";  
   }
   if ($request_method = 'POST') {
       set $cors "${cors}post";
   }

   if ($cors = "true") {
       # Catch all incase there's a request method we're not dealing with properly
       add_header 'Access-Control-Allow-Origin' "$http_origin" always;
   }

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

   if ($cors = "trueoptions") {
       add_header 'Access-Control-Allow-Origin' "$http_origin" always;

       #
       # Om nom nom cookies
       #

       add_header 'Access-Control-Allow-Credentials' 'true' always;
       add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS' always;
       
       #
       # Custom headers and headers various browsers *should* be OK with but aren't
       #

       add_header 'Access-Control-Allow-Headers' 'DNT,X-Mx-ReqToken,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type' always;
       
       #
       # Tell client that this pre-flight info is valid for 20 days
       #

       add_header 'Access-Control-Max-Age' 1728000 always;
       add_header 'Content-Type' 'text/plain charset=UTF-8' always;
       add_header 'Content-Length' 0 always;
       return 204;
   }

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

}