Lock down access of the API server proxy through iptables

[simonswine]

Is this a BUG REPORT or FEATURE REQUEST?:

/kind feature

What happened:

API server is quite vulnerable to attacks that setup arbitrary IP addresses (cf https://github.com/kubernetes/kubernetes/pull/71980 / #670) on status.podIP / hostIP. We should lock down access of the API server

What you expected to happen:

I expect us to limit all outgoing connection of the apiserver by using a custom kubernetes-apiserver UID and limit it's processes through iptables to lock down access to certain destinations only. These destinations should be allowed:

• APIserver - etcd ports + vpc IPs
• All protocols/ports pod IPs
• OIDC servers (?!)
• more things I am not thinking about (?!)

I would suggest to do a reject instead of a drop

Anything else we need to know?:

Maybe help full to setup a logging iptables rule before rejecting packages