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By default block service proxy to external IP addresses.
**What this PR does / why we need it**:
Currently, the Service Proxy on the APIServer allows unrestricted access to any IP address that the APIServer machine can reach. This is likely undesirable in many cases.
Update the service proxy so that it filters Endpoints to only those that have a TargetRef that matches a known Pod.
Fixes https://github.com/kubernetes/kubernetes/issues/58761
**Release note**:
```release-note
By default disable access to external IP addresses from the apiserver service proxy.
```
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