Applicable versions

All Longhorn versions.

Symptoms

The longhorn-driver-deployer or longhorn-csi-plugin or longhorn-ui Pods unable to access the longhorn manager backend http://longhorn-backend:9500/v1.

Reason

The CoreDNS of the Kubernetes cluster is unable to resolve the longhorn-backend service, causing the DNS resolution to fail.

Solution

1. Check the longhorn-backend Service is available.

   kubectl get service longhorn-backend -n longhorn-system
   

2. Make sure the longhorn-manager Pod(s) is/are up and running.

   kubectl get pod -l app=longhorn-manager -n longhorn-system
   

3. Make sure the CoreDNS Pod(s) is/are up and running.

   kubectl get pod -n kube-system
   

4. SSH into one of the longhorn-manager Pod, and check the nslookup test result, to make sure the DNS resolution result.

   kubectl exec -it <longhorn-manager-pod-name> -- nslookup longhorn-backend
   

5. If you’re using k3s or rke2, add

   K3S_RESOLV_CONF=/etc/resolv.conf
   RKE2_RESOLV_CONF=/etc/resolv.conf
   

   to k3s or rke2 env files on all nodes, then restart the k3s or rke2 server/agent service.
6. If you’re not using k3s or rke2, check the Kubernetes DNS Resolution Known Issues to see how to set kubelet’s --resolv-conf flag to point to the correct resolv.conf.

Related information

• Longhorn issue comment: https://github.com/longhorn/longhorn/issues/3109#issuecomment-938471504