Cases7 cases

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1. IL-0104 After a CoreDNS pod was OOMKilled, the talk names three causes: Istio's aggressive DNS cache refresh on one reused client port, kube-proxy not cleaning up conntrack entries of terminated pods, and the 2-minute UDP conntrack timeout

   Cisco ThousandEyes2025

   Symptoms

   On 4 September 2025 a CoreDNS pod in a Cisco ThousandEyes Kubernetes cluster was OOMKilled.

   DNS resolution failuresStale endpoint routingOOMKilled

   Trigger

   A CoreDNS pod was OOMKilled.

   Root cause

   Software defect

   Component

   CoreDNSkube-proxyconntrack tableIstio

   Contributing factor

   Unreviewed defaults

   Lessons

   The closing slide sets three behaviours side by side: Istio's extremely aggressive DNS cache refresh with one client port reused for all DNS requests, kube-proxy's lack of a cleanup mechanism for stale conntrack entries of terminated pods (fixed in Kubernetes v1.32), and conntrack's default bidirectional UDP stream timeout of 2 minutes. The speaker describes Istio as making some very complex things simple but some very simple things complex, and presents Cilium, which looks up Services in maps instead of walking the KUBE-SERVICES chain and can replace kube-proxy entirely with its eBPF data path.

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2. IL-0079 A GKE node auto-upgrade to an ubuntu_containerd image with a kernel regression left kube-proxy unable to apply its ip6tables rules, so load balancer health checks failed and several clusters lost all ingress traffic

   Altinity2025

   Symptoms

   On a Saturday, an OpsGenie alert fired for a ClickHouse cluster in Altinity.Cloud that had gone silent: no queries, no metrics, and no logs from customers trying to connect. The pods were running and the nodes looked healthy, but no traffic reached the database. Other clusters soon showed the same thing, and external access was blocked across several GKE clusters.

   Service outage

   Trigger

   GKE automatically upgraded worker nodes to the ubuntu_containerd image 1.32.3-gke.1785003, which ships kernel 6.8.0-1022-gke.

   Root cause

   Breaking change in an upgrade

   Component

   kube-proxyNode operating system imageLinux kernelCloud load balancer

   Contributing factor

   Unpatched known bugNo staged rollout

   Detection

   An OpsGenie alert fired when one ClickHouse cluster stopped receiving queries, metrics and customer connection logs.

   Remediation

   • added maintenance exceptions to pause all upgrades on the GKE clusters that were not yet affected
   • changed the load balancer health check policy to ignore failed kube-proxy health checks, so traffic flowed to the nodes again
   • began migrating the affected clusters from the ubuntu_containerd to the cos_containerd node image by recreating nodes

   Action items

   • Expand image pre-testing pipelines to catch regressions earlier
   • Enable notifications for upcoming GKE auto-upgrades
   • Make GKE maintenance windows required across the fleet to control when upgrades roll out
   • Track Google's issue and wait for a patched Ubuntu image

   Lessons

   A managed node upgrade can break kube-proxy in a way that leaves pods and nodes looking healthy while the load balancer sends nothing, because the load balancer health check depends on kube-proxy. kube-proxy's own logs showed the cause. Altinity recommends that GKE users on ubuntu_containerd restore traffic by switching to externalTrafficPolicy: Cluster, which disables the kube-proxy health check in the load balancer, move production workloads to cos_containerd, and pause auto-upgrades where possible until a patched image has been confirmed and tested.

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3. IL-0178 On EKS, HPA scale-in returned 502 errors because containers stopped before their pod IPs left the Service endpoints, and after blue/green switches conntrack kept sending established connections to deleted pods behind an instance-mode ALB

   Lotte E-commerce2022

   Symptoms

   LOTTE ON, Lotte E-commerce's online shopping mall of about 40 microservices on Amazon EKS, returned errors for a period of time whenever the Horizontal Pod Autoscaler (HPA) scaled pods in after load dropped. Requests reached containers that had already been deleted and failed with 502 errors.

   High error rateIntermittent timeoutsStale endpoint routing

   Trigger

   HPA scale-in deleting pods, and the deletion of Blue pods after a blue/green switch of the Service label selector.

   Root cause

   DNS or network fault

   Component

   Horizontal pod autoscalerPod terminationkube-proxyconntrack tableCloud load balancerIngress controller

   Contributing factor

   Unreviewed defaults

   Remediation

   • delayed container shutdown on pod deletion (a wait in the SIGTERM handler or a preStop sleep, capped by terminationGracePeriodSeconds) so containers stop only after the pod has left the Service endpoints
   • switched some workloads' Ingress-created ALBs from instance mode to IP mode so traffic goes straight to pod IPs without iptables and conntrack

   Action items

   • Move all workloads to ALB IP mode gradually

   Lessons

   Removing a pod from Service endpoints and stopping its containers happen at the same time in Kubernetes, so a pod can stop while traffic is still routed to it unless shutdown is delayed with a preStop hook or in the application. With an instance-mode ALB, traffic passes through node iptables and conntrack, and established connections can keep going to a pod IP after its iptables rules are gone. IP mode avoids that path, within the limit of 1000 targets per target group.

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4. IL-0049 Partial DNS outage after CoreDNS scaled down and kube-proxy failed to delete stale conntrack entries

   Preply2020

   Symptoms

   Prometheus detected a high rate of 5xx responses on three services, called A, B and C in the write-up, and paged the on-call engineers through VictorOps at 22:18 EET. The 5xx rate was also visible in Kibana.

   DNS resolution failuresHigh error rateStale endpoint routing

   Trigger

   Because load in the cluster was low, the CoreDNS autoscaler reduced the number of CoreDNS pods from 3 to 2.

   Root cause

   DNS or network fault

   Component

   CoreDNSkube-proxyconntrack table

   Contributing factor

   Automation amplificationIgnored early signalNo caching

   Detection

   Prometheus alerted on a high 5xx rate on services A, B and C and paged on-call through VictorOps.

   Remediation

   • reverted the latest release of the affected service
   • a regular deploy created new nodes, the CoreDNS autoscaler added CoreDNS pods, and the conntrack table was rewritten

   Action items

   • Disable the autoscaler for CoreDNS
   • Run a DNS cache server in production
   • Monitor conntrack usage

   Lessons

   Scaling CoreDNS down is not free: if kube-proxy fails to clear conntrack entries for the removed pod, clients keep sending DNS queries to it. Known DNS weaknesses in a cluster should be prioritised before they cause an outage, and fixes that only treat consequences leave the underlying conntrack problem in place.

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5. IL-0013 The conntrack table filled up on downsized GKE nodes and a high-traffic aggregation proxy saw climbing connection resets

   loveholidays2020

   Symptoms

   Shortly after a round of changes, loveholidays saw connectivity problems and raised error rates on its aggregation service. The service is a proxy between two other services of more than 100 instances each, handles over 10,000 requests per second continuously and can spike to four times that.

   Connection resetsHigh error rateIntermittent connection failures

   Trigger

   Three changes together: a change in service architecture, halving the RAM of the nodes that run the aggregation service, and sharding a downstream service, which doubled the number of connections to track.

   Root cause

   Capacity exhaustion

   Component

   conntrack tablekube-proxyNode poolHAProxy

   Contributing factor

   Unknown limitNo capacity planningNo alert on limit

   Detection

   Rising error rates and climbing connection reset errors in HAProxy, then node_exporter conntrack metrics compared with their limit.

   Remediation

   • moved the aggregation service to nodes with more RAM
   • limited the number of aggregation service pods per node

   Action items

   • Alert when conntrack table use goes above 75%, with (node_nf_conntrack_entries / on (pod) node_nf_conntrack_entries_limit / on (pod) group_right kube_pod_info) > 0.75

   Lessons

   The conntrack limit is tied to node memory, so resizing a node pool silently resizes the connection table, and on GKE the ratio did not match the commonly cited formula. A high-traffic proxy on small nodes can exhaust it, and the symptoms are connection resets and timeouts, so conntrack use needs its own metric and alert.

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