Steps to reproduce
1. Setup a proxy server or use an existent one.
2. Make sure you cannot access Internet without the proxy server.
Note: the easiest way to turn off your Internet connection is editing your machine network preferences. Go to your IP configuration, switch to manual configuration instead of DHCP, keeping all the current settings, and delete only the Gateway (sometimes named Router) configuration. This way you won't have Internet access, unless you add the proxy server settings to your network preferences.
3. Configure Liferay to connect through the proxy server. To do this, add the following properties to your system-ext.properties:
4. Start Liferay.
5. In Forms, add a Data Provider to retrieve data from an external online source.
6. Create a form.
7. Add a Select from List field and use the Data Provider created to populate it.
8. Publish the form and check it through the provided URL.
Select From List field is not populated.
Select From List field is populated.