Cases15 cases

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1. IL-0433 Shifting API traffic from EC2 to EKS on Fargate overloaded two CoreDNS pods and caused mass name-resolution errors

   MonotaRO2023

   Symptoms

   When MonotaRO raised the share of requests for the remaining endpoints of its backend API sent to EKS from 1% to 25%, Sentry filled with errors saying the API could not reach the database and outside services. The errors were about DNS name resolution, and the traffic had to be shifted back to the EC2 version.

   DNS resolution failuresHigh error rate

   Trigger

   MonotaRO raised the share of requests for the remaining API endpoints routed by the ALB to the EKS version from 1% to 25%.

   Root cause

   Capacity exhaustion

   Component

   CoreDNSClient-side DNS resolver and cacheContainer CPU limitHorizontal pod autoscalerPod termination

   Contributing factor

   Unreviewed defaultsNo resource requests or limitsUnrepresentative stagingUnverified assumption

   Detection

   Sentry showed a large number of errors about connecting to the database and outside services right after the traffic share was raised.

   Remediation

   • Shifted the requests back to the EC2 version to stop the errors
   • Added an HPA to autoscale the CoreDNS pods as a stopgap, which let the migration to EKS finish

   Action items

   • Set ndots to 1 through dnsConfig in the application Deployments
   • Set cache 600 in the Corefile in the coredns ConfigMap to lengthen the CoreDNS cache TTL
   • Add lameduck 30s to the CoreDNS health plugin so CoreDNS keeps serving while it is scaled in
   • Consider reducing the differences between production and staging, weighed against cost

   Lessons

   MonotaRO assumed that a managed service would not cause problems and so skipped researching other companies' migrations, load testing and monitoring. Staging differed from production in its network layout, so ndots differed between them, and staging databases could not take a high-load test. Looking at the CoreDNS logs and resolv.conf instead of guessing found that ndots:5 made lookups of outside names try every search domain first.

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2. IL-0083 Intermittent 5xx errors in a multi-tenant Kubernetes cluster came from internal DNS: the node-local dnsmasq hit its 150 concurrent query limit, did not cache cluster.local answers, and ndots search lookups flooded CoreDNS

   Adevinta2023

   Symptoms

   On a Friday afternoon a tenant of SCHIP, Adevinta's multi-tenant Kubernetes runtime, reported a spike of HTTP 5xx errors in some of its services. The drops in quality of service came back periodically through the following days, each one short, and were reported mostly around midnight.

   High error rateDNS resolution failuresHigh latency

   Root cause

   DNS or network fault

   Component

   CoreDNSClient-side DNS resolver and cacheTelemetry agent

   Contributing factor

   Unreviewed defaultsUnknown limitIgnored early signalUnverified assumption

   Detection

   A tenant reported a spike of HTTP 5xx errors in some of its services; the DNS Latency SLI had already been degraded for some time without being acted on.

   Remediation

   • raised the dnsmasq concurrent query limit from 150 to 1500
   • added a CoreDNS header block that sets the ra and aa flags and clears rd, so dnsmasq caches cluster.local answers
   • configured dnsmasq on each node to answer NXDOMAIN for unresolvable search-domain names instead of forwarding them to CoreDNS
   • changed the fluent-bit log router hostname to a fully qualified name
   • asked the affected tenants to use fully qualified names for internal calls

   Lessons

   The DNS Latency SLI had shown the problem long before anyone looked at it. An SLO only protects you when its SLIs are trusted and a drop is investigated. The investigation took a month partly because several on-call groups worked on it separately, without one place for observations and hypotheses, and sometimes chased hypotheses already discarded. Disprove one hypothesis before starting the next, and collect data the metrics system lacks: DNStop showed the garbage lookups that no dashboard did. With ndots:5, short service names multiply DNS traffic; a node-local cache only helps if it can cache the cluster DNS answers at all.

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3. IL-0230 kube-proxy sync failures during a Kubernetes upgrade corrupted node iptables, and CircleCI jobs stopped starting

   CircleCI2023

   Symptoms

   On 14 March 2023 at about 18:00 UTC, communication errors rose on one CircleCI service, and pipelines for all customers began to start late until jobs did not run at all. Within an hour the errors had spread to many services and to every function of the application, including the CircleCI UI and API.

   Service outageIntermittent connection failuresDNS resolution failures

   Trigger

   At about 17:40 UTC on 14 March the infrastructure team resumed the upgrade of the main production Kubernetes cluster with a changed approach for some components, after a first attempt on 13 March had been rolled back. The ongoing upgrade left kubelet and kube-proxy at mismatched versions.

   Root cause

   Breaking change in an upgrade

   Component

   kube-proxykubeletCoreDNSClient-side DNS resolver and cache

   Contributing factor

   Partial rolloutIneffective rollbackMonitoring blind spotHarmful manual action

   Detection

   At 18:00 UTC engineers saw communication errors rise on one service, then job processing delays climbing and job queues backing up across several services. The kube-proxy "Sync failed" errors were found only in the analysis after the incident.

   Remediation

   • Restarted every node in the cluster so each node built a fresh iptables ruleset
   • Restarted a separate pool of dependencies so jobs using contexts or the machine executor ran again
   • Added capacity to work through the backlog of jobs
   • Restarted the RabbitMQ nodes to fix a few corrupted queues

   Action items

   • Improve the Kubernetes upgrade process to limit the risk of future upgrades
   • Add metrics and alerts for kube-proxy sync errors
   • Evaluate ways to make the RabbitMQ infrastructure more resilient

   Lessons

   No single trigger caused the incident. Because networking underlies everything in the cluster, Kubernetes upgrades carry inherent risk, and CircleCI's risk mitigation did not hold up when several events interacted.

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4. 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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5. IL-0461 Every rollout of Datadog's metrics query service on Kubernetes raised errors that the logs blamed on DNS

   Datadog2022

   Symptoms

   From September 2021, each rollout of an update to Datadog's metrics query service, which serves real-time and historical metric data to the web frontend and to monitor evaluation, came with a rise in errors. Clients retried failed queries, so few errors reached users, but the retries slowed dashboards and monitors. The service's logs showed DNS errors stopping it from connecting to the data stores it depends on.

   High error rateDNS resolution failuresHigh latencyOOMKilledPacket lossStale endpoint routing

   Trigger

   Rollouts of the metrics query service, which delete the old pods. The client behaviour behind the failures came from an August 2021 rollout of the metrics query clients that applied round_robin as the new default gRPC load balancing policy.

   Root cause

   Configuration change

   Component

   conntrack tableCiliumLinux kernelCoreDNSClient-side DNS resolver and cacheexternal-dnsHTTP and RPC client library

   Contributing factor

   Change with a delayed effectRetry stormUnknown limitMonitoring blind spot

   Detection

   Errors rose during rollouts of the metrics query service, and the service's logs showed DNS errors when connecting to its data stores.

   Remediation

   • Raised the memory of the node-local-dns pods from 64 MB to 256 MB, which stopped the OOM kills but not the DNS errors
   • Tested larger instance types and confirmed that scaling up the service's infrastructure resolved the errors
   • Removed the custom gRPC reconnect parameters from the clients to restore the defaults

   Action items

   • Contribute a change to Cilium that adds an unreachable route for a deleted pod's IP so clients get an ICMP error

   Lessons

   The errors looked like DNS but were not. ENA metrics and VPC Flow Logs gave the visibility into VPC connection tracking that host metrics did not, and Cilium, Kubernetes and AWS networking interact in ways that can make reverse path filtering drop packets it should allow. DNS propagation time depends on many settings. The causes were changes Datadog had made itself, to work around a dependency bug, to resolve earlier incidents and to override default reconnect parameters, which looked safe at the time and showed their effects much later, when the link was hard to see.

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