Update kubelet systemd service default allowed IP addresses for cluster hardening

[bmelbourne]

What type of PR is this?

/kind bug

What this PR does / why we need it:

Adds localhost link-local {{ kube_pods_subnet }} (default 10.233.64.0/18) to the list of allowed IP addresses IPAddressAllow in the kubelet.service configuration to resolve an issue preventing kube-proxy pods from starting correctly when kubelet_systemd_hardening: true is applied to Kubernetes clusters.

Which issue(s) this PR fixes:

Fixes #10744

Special notes for your reviewer:

None.

Does this PR introduce a user-facing change?:

None.

[k8s-ci-robot]

Hi @bmelbourne. Thanks for your PR.

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[yankay]

/ok-to-test

[bmelbourne]

@yankay
Is it possible for me to retry the failed CI jobs? I can't seem to find a retry button anywhere in the GitLab UI.

[yankay]

Thanks @bmelbourne
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: bmelbourne, mzaian

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[VannTen]

@bmelbourne @hadi2f244
Could you explain two things (I'm looking at that commit following git blame)
Why is etcd needed in kube_node_addresses ?
In #10744 you mention kube_service_subnet, but this isn't in kubelet_secure_addresse currently ?
Finally, it's not clear why the addresses of all the node is supposed to be there exactly. I'm guessing this is related to the problem of kube-proxy, but what is that problem exactly ?

The above is not clear from the commit message.

Thanks.

[bmelbourne]

@VannTen
The specific fix was applied to resolve the issue outlined in the PR description...

Adds localhost link-local {{ kube_pods_subnet }} (default 10.233.64.0/18) to the list of allowed IP addresses IPAddressAllow in the kubelet.service configuration to resolve an issue preventing kube-proxy pods from starting correctly when kubelet_systemd_hardening: true is applied to Kubernetes clusters.

[VannTen]

an issue preventing kube-proxy pods from starting correctly

Ok, but what is this issue ? I can't find a description.

[bmelbourne]

@VannTen
I didn't capture the specific error from the kube-proxy pods that were failing to start, but it should be fairly easy to re-produce the issue by removing {{ kube_pods_subnet }} from the kubelet_secure_addresses setting.