Cases3 cases

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1. IL-0147 AKS nodes behind an HTTP proxy failed to provision with CSE exit code 31 on 202508 node images, because a refactoring stopped the proxy from being used when downloading uncached Kubernetes binaries

   Microsoft Azure2025

   Symptoms

   Creating or scaling a node pool on AKS 1.31.8 failed: the VMSS instances never finished provisioning. This started after planned maintenance moved the node image from 202507.* to 202508.*, in a cluster where nodes must send all outbound traffic through an internal HTTP/HTTPS proxy and direct egress is blocked. It happened with both Ubuntu and AzureLinux images.

   Scale-up failureUpgrade stall

   Trigger

   A node image auto-upgrade from 202507.* to 202508.* during planned maintenance, on clusters whose Kubernetes patch version was not pre-cached in the new image.

   Root cause

   Software defect

   Component

   Node operating system imageNode pool

   Contributing factor

   Cache-masked failureAutomation amplification

   Detection

   The customer saw VMSS instance provisioning fail with a vmssCSE error in the VMSS activity logs and reported it as a GitHub issue.

   Remediation

   • The customer pinned the node image to a 202507.* snapshot on clusters running 1.31.8
   • The customer upgraded clusters to AKS 1.31.9 or 1.31.10, whose binaries are cached in the 202508.* images
   • The AKS team hot-patched VHD 20250811 so the proxy configuration is used for uncached binary downloads and made it globally available in public regions on 2025-09-04; it also announced the hotfix for VHD 20250820
   • The AKS team told users to reimage their nodes with az aks nodepool upgrade --node-image-only to pick up the hotfixed image

   Action items

   • Apply the proxy fix in all future VHDs

   Lessons

   A bootstrap path that runs only when an artefact is missing from a cache is easy to break without noticing: here only clusters on Kubernetes versions not pre-cached in the node image ever downloaded binaries, so only they met the proxy regression. Testing node provisioning behind a mandatory egress proxy, on older patch versions as well as the newest, would exercise that path. For clusters with restricted egress, pinning the node image or moving to a Kubernetes version cached in the image were both quick workarounds while the fix was shipped.

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2. IL-0148 A kube-scheduler regression in Kubernetes v1.27.14, v1.28.10 and v1.29.5 made the scheduler panic on a pod whose node affinity named a nonexistent node, so no pods were scheduled on affected AKS clusters

   Microsoft Azure2024

   Symptoms

   On AKS clusters running Kubernetes v1.27.14, v1.28.10 or v1.29.5, kube-scheduler panicked whenever the cluster contained a pod with a bad node affinity, and from then on no pod in the cluster was scheduled.

   Pods Pending

   Trigger

   A pod in the cluster with a required node affinity whose matchFields term names a node that does not exist.

   Root cause

   Software defect

   Component

   Pod scheduling and placement

   Contributing factor

   –

   Detection

   The AKS team found the bug; the issue does not say how.

   Remediation

   • AKS engineers fixed the impacted clusters
   • Rolled out a fixed kube-scheduler to all regions, applied to clusters automatically

   View case
3. IL-0146 New AKS clusters with Azure CNI in Central India lost tunnel connectivity because VMs began getting MAC addresses with a 06 prefix, which exposed a dormant bug in bridge-mode Azure CNI

   Microsoft Azure2020

   Symptoms

   In newly created AKS clusters using Azure CNI in Central India, kubectl exec and kubectl logs could time out, and pod-to-pod or pod-to-node traffic could time out. The cluster diagnostics page showed tunnel exec issues. Clusters using kubenet were not affected.

   Intermittent timeouts

   Trigger

   VMs in Central India started being assigned MAC addresses in a much higher range than before (06 prefix rather than the usual 00).

   Root cause

   Software defect

   Component

   Azure CNINode operating system image

   Contributing factor

   –

   Remediation

   • told owners of affected scale-set clusters to run a node image upgrade, or any Kubernetes upgrade such as a patch version, to get the Azure CNI that does not use a bridge
   • told owners of availability-set clusters to scale out and delete the VMs with a high-range MAC address in bridge mode
   • advised recreating affected clusters that were brand new

   Action items

   • Roll out a hotfix that makes transparent mode the default in all regions

   Lessons

   A change in how the platform assigns MAC addresses can wake a long-dormant bug in the network plugin. Knowing which CNI mode the nodes run (bridge or transparent) and keeping node images current decides whether such a bug reaches you.

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