my WordPress site cannot access external connection (e.g. cannot browse/update plugins using dashboard - manual zip install only). The site is can be access from the outside world (via domain name) but from the inside it can't.

I have try to set the WP_HTTP_BLOCK_EXTERNAL in wp-config.php to false but the result still the same.

define('WP_HTTP_BLOCK_EXTERNAL', false);

The web server is running on Ubuntu 14.04.4 LTS using Ajenti V (I did not set the server up myself so I don't know is there any configuration that blocks the outside connection or not)

I want to know is there anywhere I should look for inside the WordPress or even the outside (the webserver configuration itself)