"Attempted redirect to a restricted host." occurs when redirect to external url

Hi experts,
I want to invoke getFacesContext().getExternalContext().redirect(url); to redirect to a external url when finishing some work, but mywebmethods give me a hint as follow:
Restricted Redirection Page
Attempted redirect to a restricted host.

Could anyone tell me how to figure it out?

I want to know Where to modify the Server Redirect WhiteList to allow redirects to this foreign URL?
thanks a lot.

I know it.
1.As system administrator, click Administration Dashboard >Configuration > Redirection Whitelist Administration.
2.Click Add Server, type in the hostname or IP address of a trusted server, click Apply, and then click Save.