Which takedown defenses are banned in UFC?

- Kneeing the opponent while he's attempting the takedown
- Any form of stand up kicks against a takedown
- Bruce Lee's backflip in ETD against O'hara (Just kidding)
- If you and your opponent are laying, it's illegal to still kick while you're on the ground?

I'm curious, I've seen lots of MMA matches but I've never seen anyone get kicked in the face when both challengers were on the ground. I've also never seen any sort of kneeing or front kicks to a takedown.

Is it against the rules?

So...

Which takedown defenses are banned in UFC?