coredns pods have CrashLoopBackOff or Error state

Question

I\'m trying to set up the Kubernetes master, by issuing:

kubeadm init --pod-network-cidr=192.168.0.0/16

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Answer 1

This error

[FATAL] plugin/loop: Seen "HINFO IN 6900627972087569316.7905576541070882081." more than twice, loop detected

is caused when CoreDNS detects a loop in the resolve configuration, and it is the intended behavior. You are hitting this issue:

https://github.com/kubernetes/kubeadm/issues/1162

https://github.com/coredns/coredns/issues/2087

Hacky solution: Disable the CoreDNS loop detection

Edit the CoreDNS configmap:

kubectl -n kube-system edit configmap coredns

Remove or comment out the line with loop, save and exit.

Then remove the CoreDNS pods, so new ones can be created with new config:

kubectl -n kube-system delete pod -l k8s-app=kube-dns

All should be fine after that.

Preferred Solution: Remove the loop in the DNS configuration

First, check if you are using systemd-resolved. If you are running Ubuntu 18.04, it is probably the case.

systemctl list-unit-files | grep enabled | grep systemd-resolved

If it is, check which resolv.conf file your cluster is using as reference:

ps auxww | grep kubelet

You might see a line like:

/usr/bin/kubelet ... --resolv-conf=/run/systemd/resolve/resolv.conf

The important part is --resolv-conf - we figure out if systemd resolv.conf is used, or not.

If it is the resolv.conf of systemd, do the following:

Check the content of /run/systemd/resolve/resolv.conf to see if there is a record like:

nameserver 127.0.0.1

If there is 127.0.0.1, it is the one causing the loop.

To get rid of it, you should not edit that file, but check other places to make it properly generated.

Check all files under /etc/systemd/network and if you find a record like

DNS=127.0.0.1

delete that record. Also check /etc/systemd/resolved.conf and do the same if needed. Make sure you have at least one or two DNS servers configured, such as

DNS=1.1.1.1 1.0.0.1

After doing all that, restart the systemd services to put your changes into effect: systemctl restart systemd-networkd systemd-resolved

After that, verify that DNS=127.0.0.1 is no more in the resolv.conf file:

cat /run/systemd/resolve/resolv.conf

Finally, trigger re-creation of the DNS pods

kubectl -n kube-system delete pod -l k8s-app=kube-dns

Summary: The solution involves getting rid of what looks like a DNS lookup loop from the host DNS configuration. Steps vary between different resolv.conf managers/implementations.