nodeup: Check if /run/systemd/resolve/resolv.conf exists on the node and use it

[zak905]

fixes #16494
The PR is in draft because I have not tested yet on a Hetzner account (maybe worth checking also other cloud providers)

[k8s-ci-robot]

Hi @zak905. Thanks for your PR.

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[zak905]

@hakman I added a task to check whether /run/systemd/resolve/resolv.conf is present. In case it is there, the resolvConf is set directly in the kubelet.conf file, otherwise kubelet defaults to /etc/resolv.conf, so we do not need to explicitely set it. WDYT ?

[hakman]

@hakman I added a task to check whether /run/systemd/resolve/resolv.conf is present. In case it is there, the resolvConf is set directly in the kubelet.conf file, otherwise kubelet defaults to /etc/resolv.conf, so we do not need to explicitely set it. WDYT ?

@zak905 Is there a need for a task in this case? Wouldn't just checking the existence be enough?

[zak905]

I agree, but based on my current understanding, I did not find a way to access the node file system (when bootstrapping) except using a task. Can you please clarify ?

[hakman]

@zak905 I think the only change should be in HasLoopbackEtcResolvConf():

func (d *Distribution) HasLoopbackEtcResolvConf() bool {
	switch d.project {
	case "ubuntu":
		return d.version >= 18.04
	case "flatcar":
		return true
	default:
		if _, err := os.Stat("/run/systemd/resolve/resolv.conf"); err == nil {
 			return true
		}
		return false
	}
}

[zak905]

I see thanks. Done. It's much cleaner:) I will try to do an end to end test on Hetzner.

[hakman]

/ok-to-test

[hakman]

I see thanks. Done. It's much cleaner:) I will try to do an end to end test on Hetzner.

Thanks @zak905. If you don't mind, please squash the commits.
Probably tests need an adjustment to fix resolvConf to some value and running hack/update-expected.sh:

kops/nodeup/pkg/model/tests/kubelet/featuregates/cluster.yaml

Lines 23 to 27 in c670303

	kubelet:
	featureGates:
	ExperimentalCriticalPodAnnotation: "true"
	AllowExtTrafficLocalEndpoints: "false"
	podManifestPath: "/etc/kubernetes/manifests"

and

kops/nodeup/pkg/model/tests/kubelet/warmpool/cluster.yaml

Lines 23 to 24 in c670303

	kubelet:
	podManifestPath: "/etc/kubernetes/manifests"

[hakman]

Thanks @zak905! 😄
/lgtm
/approve

[k8s-ci-robot]

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[zak905]

Welcome! sorry I did not catch your last comment about testing until later.

[hakman]

Welcome! sorry I did not catch your last comment about testing until later.

No worries! 👍