CORS on Nginx

The following Nginx configuration enables CORS, with support for preflight requests.

Wide-open CORS config for nginx

location / {
    if ($request_method = 'OPTIONS') {
        add_header 'Access-Control-Allow-Origin' '*';
        add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
        #
        # Custom headers and headers various browsers *should* be OK with but aren't
        #
        add_header 'Access-Control-Allow-Headers' 'DNT,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Range';
        #
        # Tell client that this pre-flight info is valid for 20 days
        #
        add_header 'Access-Control-Max-Age' 1728000;
        add_header 'Content-Type' 'text/plain; charset=utf-8';
        add_header 'Content-Length' 0;
        return 204;
    }
    if ($request_method = 'POST') {
        add_header 'Access-Control-Allow-Origin' '*' always;
        add_header 'Access-Control-Expose-Headers' 'Content-Length,Content-Range' always;
    }
    if ($request_method = 'GET') {
        add_header 'Access-Control-Allow-Origin' '*' always;
        add_header 'Access-Control-Expose-Headers' 'Content-Length,Content-Range' always;
    }
}

Origin-specified CORS config for nginx

Replace all $http_origin with $scheme://$host if you need to compatibility with Edge Legacy or Firefox <69.

location / {
    if ($allow_cors = OPTIONS) {
        # https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Access-Control-Allow-Origin#cors_and_caching
        add_header 'Vary' 'Origin' always;
        add_header 'Access-Control-Allow-Origin' $http_origin;
        add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
        #
        # Custom headers and headers various browsers *should* be OK with but aren't
        #
        add_header 'Access-Control-Allow-Headers' 'DNT,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Range';
        #
        # Tell client that this pre-flight info is valid for 20 days
        #
        add_header 'Access-Control-Max-Age' 86400;
        add_header 'Content-Type' 'text/plain; charset=utf-8';
        add_header 'Content-Length' 0;
        return 204;
    }
    if ($allow_cors = GET|POST) {
        # https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Access-Control-Allow-Origin#cors_and_caching
        add_header 'Vary' 'Origin' always;
        add_header 'Access-Control-Allow-Origin' $http_origin always;
        add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS' always;
        add_header 'Access-Control-Allow-Headers' 'DNT,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Range' always;
        add_header 'Access-Control-Expose-Headers' 'Content-Length,Content-Range' always;
    }
}
    

The following directive must be placed under http {} level

map '$request_method $http_origin' $allow_cors {
    '~^OPTIONS https://(allow\.first\.domain|allow\.second\.domain)'    OPTIONS;
    '~^(GET|POST) https://(allow\.first\.domain|allow\.second\.domain)' GET|POST;
    default 0;
}