server {


   # We check if $subdomain is not empty and if the X-subdomain header is not present in the request 
   # ($http_x_subdomain = ""). 
   # If both conditions are met, we set the X-subdomain header using proxy_set_header. 
   # Otherwise, we do nothing, and the existing X-subdomain header (if any) will be preserved.

    listen 80;
    server_name ~^(?<subdomain>\w*)\.${DOMAIN}.${ZONE}${DOLLAR};

    location / {
        if ($subdomain != "" && $http_x_subdomain = "") {
            proxy_set_header x-subdomain $subdomain;
        }
        proxy_pass http://${SERVER}:${SERVER_PORT}/;
    }
}