Cases5 cases

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1. IL-0159 Docker Hub rate-limited Linode's IPv6 address space with 429 Too Many Requests, breaking image pulls for LKE and LKE-E clusters

   Linode2025

   Symptoms

   From about 21:38 UTC on 5 February 2025, image pulls from Docker Hub over IPv6 failed with 429 Too Many Requests errors across several Linode regions. Both unauthenticated and authenticated pull requests were refused.

   Image pull failures

   Trigger

   Docker Hub's abuse prevention measures started rate-limiting image pulls coming from Linode's IPv6 address space.

   Root cause

   Dependency failure

   Component

   Container registry and its credentialsVPC DNS resolver

   Contributing factor

   Unknown limit

   Detection

   The rate limiting began at about 21:38 UTC. The first status post, at 22:48 UTC, said Linode was aware of an issue with Docker Hub over IPv6 causing rate limiting and was investigating, and at 01:06 UTC on 6 February the status page said the issue had been identified as rate limiting of image pull requests to Docker Hub over IPv6.

   Remediation

   • changed the DNS resolvers to return an A record instead of an AAAA record so image pulls to Docker Hub used IPv4

   Action items

   • Find long-term solutions so that Docker Hub's abuse prevention measures do not affect Linode services

   Lessons

   A registry's abuse limit that groups IPv6 addresses by their first 64 bits can block a provider's whole address range at once, and authentication does not help. Answering DNS lookups with IPv4 addresses only was the quick way to move image pulls off the blocked range.

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2. IL-0018 A Docker Hub password rotation left EKS nodes and Concourse locked out of Docker Hub and unable to pull images

   UK Ministry of Justice2023

   Symptoms

   At 14:22 a Cloud Platform team member noticed failed Docker Hub logins. Fetching an image returned: failed to fetch manifest: Head "https://registry-1.docker.io/v2/ministryofjustice/cloud-platform-tools/manifests/2.1": toomanyrequests: too many failed login attempts for username or IP address

   Image pull failures

   Trigger

   The Docker Hub password was changed as part of LastPass remediation activities while the EKS cluster nodes and Concourse still logged in with the previous password.

   Root cause

   Operator error

   Component

   Container registry and its credentialsNode launch templateNode poolCI system

   Contributing factor

   Credential in configurationShared credential

   Detection

   A Cloud Platform team member saw the failed Docker Hub login error at 14:22. The Kuberhealthy daemonset check showed at 14:52 that the cluster could not pull images, and a user reported ErrImagePull in #ask-cloud-platform at 14:54.

   Remediation

   • Changed the Docker Hub password back to the previous value still used by the EKS cluster nodes and Concourse
   • Confirmed with the reporting user that the pod deployed and that cluster image pulls worked again

   Action items

   • Switch from the Docker Hub username and password to a Docker Hub token created specifically for Cloud Platform
   • Rotate the password cleanly out of hours with a graceful recycle of the nodes

   Lessons

   The write-up states no separate lessons. Its review action replaces the Docker Hub username and password with a Docker Hub token created specifically for Cloud Platform.

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3. 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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4. IL-0199 A non-essential container exceeding its memory limit made GitHub's production pods unavailable, and their restarts failed because a DNS maintenance change had left the clusters unable to reach the image registry

   GitHub2020

   Symptoms

   At 08:18 UTC on 13 July 2020, production Kubernetes pods began to be marked unavailable. The loss of pods spread through the clusters and reduced capacity until GitHub.com services went down.

   OOMKilledImage pull failuresService outage

   Trigger

   A container in the production pods exceeded its memory limit and was terminated; a routine DNS maintenance operation completed earlier had left the clusters unable to reach the image registry.

   Root cause

   DNS or network fault

   Component

   Container memory limitContainer registry and its credentials

   Contributing factor

   Overlapping failuresHarmful manual actionSingle point of failure

   Remediation

   • restarted the process with the cached DNS records so the clusters could reach the registry
   • fetched the container images, redeployed and recovered the services

   Action items

   • Improve monitoring so pod restarts cannot fail again in the same pattern
   • Minimise the dependency on the image registry
   • Expand validation during DNS changes
   • Re-evaluate all existing Kubernetes deployment policies
   • Improve progressive deployments so the impact of a deployment can be evaluated incrementally

   Lessons

   A container that production traffic does not need can still take pods out of service, because a pod is available only when all of its containers are healthy. ImagePullPolicy Always turns every container restart into a registry dependency, so an unrelated DNS change that breaks registry access turns a one-minute self-healing event into a long outage, and a mass redeploy spreads it.

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5. 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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