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1. IL-0144 A systemd security update installed by unattended-upgrades lost the DNS configuration of Ubuntu 18.04 VMs, so AKS kubelets could not resolve registry names and pods failed with ImagePullBackOff

   Microsoft Azure2022

   Symptoms

   From 06:00 UTC on 30 August 2022, Azure VMs running Ubuntu 18.04 (bionic) with unattended-upgrades enabled began failing DNS resolution and losing network connectivity. Several Azure teams, among them the AKS and Azure Container Apps teams, were alerted by production alerts shortly after the update packages were published.

   DNS resolution failuresImage pull failuresMonitoring outage

   Trigger

   Unattended-upgrades on Ubuntu 18.04 VMs installed a Canonical security update published at 06:00 UTC on 30 August 2022, including systemd version 237-3ubuntu10.54.

   Root cause

   Breaking change in an upgrade

   Component

   Node operating system imagekubeletContainer registry and its credentialsClient-side DNS resolver and cache

   Contributing factor

   No staged rolloutExcessive blast radiusAutomation amplification

   Detection

   Several Azure teams, including AKS and Azure Container Apps, were alerted by production alerts shortly after the packages were published; incoming support cases then confirmed which Ubuntu versions were affected.

   Remediation

   • pointed customers to Canonical's description of the systemd bug and its fix
   • recommended rebooting affected VMs or running a template script that repairs DNS resolution
   • built automatic detection and remediation for AKS nodes and rolled it out across all regions
   • mitigated the remaining AKS customers manually through support

   Action items

   • Review the process that lets critical security updates bypass Safe Deployment Practices
   • Improve monitoring of the AKS data plane to alert on an uptick of errors
   • Review AKS monitoring algorithms so that similar VM errors are detected and alerted on
   • Establish an improved escalation path to Canonical during outages and add it to internal Technical Service Guides
   • AKS takes full control of the node security patch mechanism, with additional testing and controlled release of patches (estimated December 2022)
   • Work with Canonical to run dedicated tests on proposed packages before they are published for Azure IaaS VMs
   • Establish a process with Canonical to close the testing gap for the upgrade scenario (estimated March 2023)
   • Give AKS customers maintenance window control and environment staging for these patches (estimated December 2023)

   Lessons

   Security patches applied automatically by unattended-upgrades, once a day by default, and outside Azure's Safe Deployment Practices reached Ubuntu 18.04 VMs in every Azure region, so one systemd regression became a DNS outage across Azure. On AKS, lost node DNS showed up as image pull failures such as ImagePullBackoff, because kubelet could not resolve the registry. Azure plans to put node security patching under AKS's control with testing and controlled release; customers can use automatic node image upgrade for AKS, or automatic VM guest patching for VMs, instead of unattended-upgrades.

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2. IL-0037 Image registry rate-limited a whole region after a crash-looping DaemonSet with imagePullPolicy Always pulled through three NAT instances

   Datadog2019

   Symptoms

   Users reported that their jobs were not starting, with some sign that image pulls were involved. Many pods across the cluster were in error states related to image pulls, while applications that were already running were not affected.

   Image pull failuresService outage

   Trigger

   A change to the permissions on a storage bucket that one application needed to read. The application could no longer start, and its DaemonSet, running on roughly a thousand nodes of the cluster, went into a crash loop with imagePullPolicy: Always, so every restart pulled the image again although it was already on the node.

   Root cause

   Capacity exhaustion

   Component

   Container registry and its credentialsNAT gatewayApplication databaseAdmission webhook

   Contributing factor

   Automation amplificationSingle point of failureUnknown limitHarmful manual actionExcessive blast radius

   Detection

   Users reported that their jobs were not starting; monitoring had not caught it first.

   Remediation

   • replaced the affected NAT instances so registry traffic came from new source IPs
   • reconfigured the static IPs and firewall rules so applications could reach the Cloud SQL database again

   Action items

   • Add an admission webhook that rejects floating image tags, starting with latest
   • Apply the image tag webhook to workload objects only, not to pods, after it blocked pods created by in-cluster controllers

   Lessons

   The speakers' takeaway is to be very careful with DaemonSets in large clusters: they say they had broken everything DaemonSets depend on, including DNS, Vault and the image repository, and that it is very easy to break things when managing them by the thousand. imagePullPolicy: Always made every crash-loop restart pull the image again. Sending all registry traffic through three NAT instances put the whole region behind three source IPs, which the registry rate-limited, and changing those IPs cut the connections to a critical database reached over the internet through them. An admission webhook on pods rejects the pods that controllers in the cluster create, where no user sees the error.

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