Problem with Nginx and "header contains multiple values"

Question

I needs to enable cors policy to reach to my api, i did the following configuration on my nginx server file:

server {
    listen 80 default_server;
    listen [::]:80 default_server;

    server_name api.domain.com;

    location / {

       proxy_set_header X-Real-IP $remote_addr;
       proxy_set_header Host $host;
       proxy_pass http://my_ip:6869/;

        set $ref "*";
        if ($http_referer ~* ^(http?\:\/\/)(.*?)\/(.*)$) {
          set $ref $1$2;
        }
        add_header 'Access-Control-Allow-Origin' $ref always;
        add_header 'Access-Control-Allow-Credentials' 'true' always;
        add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS, PUT, DELETE' always;
        add_header 'Access-Control-Allow-Headers' 'DNT,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Range,signature,timestamp' always;
        add_header 'Access-Control-Expose-Headers' 'Content-Length,Content-Range' always;

    }

    listen [::]:443 ssl ipv6only=on; # managed by Certbot
    listen 443 ssl; # managed by Certbot
    ssl_certificate /etc/letsencrypt/live/api.domain.com/fullchain.pem; # managed by Certbot
    ssl_certificate_key /etc/letsencrypt/live/api.domain.com/privkey.pem; # managed by Certbot
    include /etc/letsencrypt/options-ssl-nginx.conf; # managed by Certbot
    ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem; # managed by Certbot

}

But i keep getting the following error:

Access to fetch at 'https://api.domain.com/data/key?matches=^art(.*)' from origin 'http://localhost:3500' has been blocked by CORS policy: The 'Access-Control-Allow-Origin' header contains multiple values '*, http://localhost:3500', but only one is allowed. Have the server send the header with a valid value, or, if an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.

It seem that the add_header is adding on top of an already set header for Access-Control-Allow-Origin * but i only have this config file and don't see anywhere else where it could come from.

Is there a way to figure out what is setting the initial header cors policy or simply override it instead of adding to it?

Thank you in advance.

Answer 1

This forum is only for Webinoly users!

By the way, you should not use that code in production environment. For your particular question, see the "ref" variable you are setting.