nginx_bad_gateway_proxy_redirect

upstream upstream_server {
  server 127.0.0.1;
  keepalive 512;
}

server {
  listen *:80;

  location / {

    error_page 502 @proxy_error;
    proxy_intercept_errors on;
    proxy_pass http://upstream_server;
  }

  location @proxy_error {
    rewrite ^ https://my.externalsite.com/app redirect;
  }
}