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Troubleshoot node not ready failures caused by CSE errors

This article helps you troubleshoot scenarios in which a Microsoft Azure Kubernetes Service (AKS) cluster isn't in the Succeeded state and an AKS node isn't ready within a node pool because of custom script extension (CSE) errors.

Prerequisites

Symptoms

Because of CSE errors, an AKS cluster node isn't ready within a node pool, and the AKS cluster isn't in the Succeeded state.

Cause

The node extension deployment fails and returns more than one error code when you provision the kubelet and other components. This is the most common cause of errors. To verify that the node extension deployment is failing when you provision the kubelet, follow these steps:

  1. To better understand the current failure on the cluster, run the az aks show and az resource update commands to set up debugging:

    Set your environment variables and run the commands to view the cluster's status and debug information.

    export RG_NAME="my-aks-rg"
export CLUSTER_NAME="myakscluster"
clusterResourceId=$(az aks show \
    --resource-group $RG_NAME --name $CLUSTER_NAME --output tsv --query id)
az resource update --debug --verbose --ids $clusterResourceId

    Results:

    {
  "id": "/subscriptions/xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx/resourceGroups/my-aks-rg-xxx/providers/Microsoft.ContainerService/managedClusters/myaksclusterxxx",
  "name": "myaksclusterxxx",
  "type": "Microsoft.ContainerService/managedClusters",
  "location": "eastus2",
  "tags": null,
  "properties": {
    ...
  }
}

  2. Check the debugging output and the error messages that you received from the az resource update command against the error list in the CSE helper executable file on GitHub.

If any of the errors involve the CSE deployment of the kubelet, then you've verified that the scenario that's described here's the cause of the Node Not Ready failure.

In general, exit codes identify the specific issue that's causing the failure. For example, you see messages such as "Unable to communicate with API server" or "Unable to connect to internet." Or the exit codes might alert you to API network time-outs, or a node fault that needs a replacement.

Solution 1: Make sure your custom DNS server is configured correctly

Set up your custom Domain Name System (DNS) server so that it can do name resolution correctly. Configure the server to meet the following requirements:

  • If you're using custom DNS servers, make sure that the servers are healthy and reachable over the network.

  • Make sure that custom DNS servers have the required conditional forwarders to the Azure DNS IP address (or the forwarder to that address).

  • Make sure that your private AKS DNS zone is linked to your custom DNS virtual networks if they're hosted on Azure.

  • Don't use the Azure DNS IP address with the IP addresses of your custom DNS server. Doing this isn't recommended.

  • Avoid using IP addresses instead of the DNS server in DNS settings. You can use Azure CLI commands to check for this situation on a Virtual Machine Scale Set or availability set.

    • For Virtual Machine Scale Set nodes, use the az vmss run-command invoke command:

      Important: You must specify the --instance-id of the VM scale set. Here, we demonstrate querying for a valid instance ID (e.g., 0) and a likely VMSS in an AKS node resource group. Update values appropriately to match your environment.

      export NODE_RESOURCE_GROUP=$(az aks show --resource-group $RG_NAME --name $CLUSTER_NAME --query nodeResourceGroup -o tsv)
export VMSS_NAME=$(az vmss list --resource-group $NODE_RESOURCE_GROUP --query "[0].name" -o tsv)
export DNS_IP_ADDRESS="10.0.0.10"
export INSTANCE_ID=$(az vmss list-instances --resource-group $NODE_RESOURCE_GROUP --name $VMSS_NAME --query "[0].instanceId" -o tsv)
export API_FQDN=$(az aks show --resource-group $RG_NAME --name $CLUSTER_NAME --query fqdn -o tsv)

az vmss run-command invoke \
    --resource-group $NODE_RESOURCE_GROUP \
    --name $VMSS_NAME \
    --instance-id $INSTANCE_ID \
    --command-id RunShellScript \
    --output tsv \
    --query "value[0].message" \
    --scripts "telnet $DNS_IP_ADDRESS 53"
az vmss run-command invoke \
    --resource-group $NODE_RESOURCE_GROUP \
    --name $VMSS_NAME \
    --instance-id $INSTANCE_ID \
    --command-id RunShellScript \
    --output tsv \
    --query "value[0].message" \
    --scripts "nslookup $API_FQDN $DNS_IP_ADDRESS"

    • For VM availability set nodes, use the az vm run-command invoke command:

      Important: You must specify the --name of a valid VM in an availability set in your resource group. Here is a template for running network checks.

      az vm run-command invoke \
    --resource-group $RG_NAME \
    --name $AVAILABILITY_SET_VM \
    --command-id RunShellScript \
    --output tsv \
    --query "value[0].message" \
    --scripts "telnet $DNS_IP_ADDRESS 53"
az vm run-command invoke \
    --resource-group $RG_NAME \
    --name $AVAILABILITY_SET_VM \
    --command-id RunShellScript \
    --output tsv \
    --query "value[0].message" \
    --scripts "nslookup $API_FQDN $DNS_IP_ADDRESS"

For more information, see Name resolution for resources in Azure virtual networks and Hub and spoke with custom DNS.

Solution 2: Fix API network time-outs

Make sure that the API server can be reached and isn't subject to delays. To do this, follow these steps:

  • Check the AKS subnet to see whether the assigned network security group (NSG) is blocking the egress traffic port 443 to the API server.

  • Check the node itself to see whether the node has another NSG that's blocking the traffic.

  • Check the AKS subnet for any assigned route table. If a route table has a network virtual appliance (NVA) or firewall, make sure that port 443 is available for egress traffic. For more information, see Control egress traffic for cluster nodes in AKS.

  • If the DNS resolves names successfully and the API is reachable, but the node CSE failed because of an API time-out, take the appropriate action as shown in the following table.

    Set type Action
    VM availability set Delete the node from the Azure portal and the AKS API by using the kubectl delete node command, and then scale up the cluster again.
    Virtual Machine Scale Set Either reimage the node from the Azure portal, or delete the node, and then scale up the cluster again. To delete the specific node, use az aks nodepool delete-machines command. It will cordon & drain first and then delete the node.

  • If the requests are being throttled by the AKS API server, upgrade to a higher service tier. For more information, see Pricing tiers for AKS.

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