Example Nginx configuration for adding cross-origin resource sharing (CORS) support to reverse proxied APIs

nginx.conf

#
# CORS header support
#
# One way to use this is by placing it into a file called "cors_support"
# under your Nginx configuration directory and placing the following
# statement inside your **location** block(s):
#
#   include cors_support;
#
# As of Nginx 1.7.5, add_header supports an "always" parameter which
# allows CORS to work if the backend returns 4xx or 5xx status code.
#
# For more information on CORS, please see: http://enable-cors.org/
# Forked from this Gist: https://gist.github.com/michiel/1064640
#

set $cors '';
if ($http_origin ~ '^https?://(localhost|www\.yourdomain\.com|www\.yourotherdomain\.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;
        # required to be able to read Authorization header in frontend
        #add_header 'Access-Control-Expose-Headers' 'Authorization' always;
}

if ($request_method = 'OPTIONS') {
        # 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;
}

one silly question: I have nginx ingress which I installed using helm charts. Any idea where to put this annotation or snippet???

Curtly facing issue "Access to XMLHttpRequest at 'https://xxxxx/tenant/dashboard/ml/project?startDate=2021-11-23T09%3A37%3A16-06%3A00&endDate=2022-11-23T09%3A37%3A16-06%3A00&filterby=InProgress&page=1&offset=5' from origin 'https://xxxxxxxxx.io' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource."

Justin Fortier

I am trying to do the same as you.
My BE is api.defaultseo.com, and FE defaultseo.com
When I log in from the FE, the refreshToken and Token are set on the BE only
I want to send the tokens to the FE on the response header
so when the FE requests access to a restricted BE route, FE sends the Token & Refresh Token to the BE.

Would you be able to help me fix the above? I also use MyVesta control panel to host the two websites on a droplet with Digital Ocean.

+1 for @mPanasiewicz 👍🏼

Justin Fortier

I am trying to do the same as you. My BE is api.defaultseo.com, and FE defaultseo.com When I log in from the FE, the refreshToken and Token are set on the BE only I want to send the tokens to the FE on the response header so when the FE requests access to a restricted BE route, FE sends the Token & Refresh Token to the BE.

Would you be able to help me fix the above? I also use MyVesta control panel to host the two websites on a droplet with Digital Ocean.

I figure this out. It was not related to Nginx

It was on the cookies I just added the cookie attribute: domain: 'defaultseo.com' for example that was my issue.

I'm just familiarizing myself with nginx configurations, but apparently

• if You use an add_header directive in an if statement, no other add_header from outside the if seems to apply
• empty strings used in add_header values aren't added/applied to the response

With these in mind, i only could made my case work (different methods allowed for different origins) was defining only variables inside the if statements.

set $allowed_headers '';
set $allowed_methods '';
set $allowed_credentials '';
set $allowed_origin '';
set $type '';
set $length '';

if ( $http_origin ~ "https://put-enabled" ) {
    set $allowed_origin $http_origin;
    set $allowed_methods 'OPTIONS, PUT';
    set $allowed_credentials false;
}

if ( $http_origin ~ "https://delete-enabled" ) {
    set $allowed_headers 'content-type, X-Requested-With';
    set $allowed_origin $http_origin;
    set $allowed_methods 'OPTIONS, DELETE';
    set $allowed_credentials true;
}

if ($request_method = OPTIONS) {
    set $allowed_headers 'content-type';
    set $type 'text/plain';
    set $length 0;
}

add_header Access-Control-Allow-Origin $allowed_origin always;
add_header Access-Control-Allow-Methods $allowed_methods always;
add_header Access-Control-Allow-Headers $allowed_headers always;
add_header Access-Control-Allow-Credentials $allowed_credentials always;
add_header Content-Type $type;
add_header Content-Length $length;

if ($request_method = OPTIONS) {
    return 204;
}

if ($request_method = POST) {
    return 404;
}

If You wonder why the POST returns a 404, it's because the navigator (chrome at least) ignores the Access-Control-Allow-Methods for POST requests. 🤷

I'm just familiarizing myself with nginx configurations, but apparently

* if You use an `add_header` directive in an `if` statement, no other `add_header` from outside the `if` seems to apply

That's because IF is evil