How to prevent an iframe to display external websites?

Let's say I display an iFrame on iframe.domain.com. I want this iframe to be able to display content from iframe.domain.com or sub.domain.com. But those websites contain link to external websites, for example google.com. If the users clicks on a link redirecting him to google.com, I want the iFrame to be redirected to sub.domain.com

How can I do that?

via Chebli Mohamed