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1. IL-0080 kube-dns on Mercari's GKE clusters could not keep up with peak-hour DNS query rates, and a Sev1 production outage followed when several services failed DNS resolution

   Mercari2025sev1

   Symptoms

   During peak hours, when Mercari's GKE clusters handle very high request rates, the platform team saw kube-dns reaching its limits. DNS query rates were high enough to overwhelm the kube-dns service.

   DNS resolution failuresIntermittent timeoutsService outage

   Trigger

   Peak-hour traffic on the GKE clusters produced more DNS queries than the few kube-dns pods could answer.

   Root cause

   Capacity exhaustion

   Component

   kube-dns

   Contributing factor

   No cachingSingle point of failure

   Detection

   During peak hours the team saw frequent NXDOMAIN and truncated-response errors and recurring "failed to refresh DNS cache" errors on the clusters, which showed that kube-dns had reached its limits.

   Remediation

   • deployed NodeLocal DNSCache on the Laboratory GKE cluster with a specific nodeAffinity to measure its effect
   • rolled NodeLocal DNSCache out gradually to the Dev and Prod environments by labelling node pools so its pods could be deployed there

   Lessons

   When every pod in a busy cluster resolves names through a few central kube-dns pods, DNS becomes a shared bottleneck, and a DNS failure quickly turns into higher latency, timeouts and cascading outages across microservices. A cache on each node takes most of that load off kube-dns and keeps resolution local to the node. Rolling the cache out first to an internal test cluster with nodeAffinity, and then node pool by node pool through labels, kept the blast radius of the change small.

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2. IL-0027 All CoreDNS pods landed on one node, the node ran out of memory, and DNS for the whole service went down

   Backpackr2025

   Symptoms

   Services on the production Amazon EKS cluster behind the idus marketplace threw intermittent errors whose cause was hard to pin down. Calls to external services such as RDS and Redis failed now and then, and whenever DNS traffic spiked, domain lookups answered slowly or failed outright.

   DNS resolution failuresIntermittent connection failuresHigh latencyOOMKilledService outage

   Trigger

   All CoreDNS pods were scheduled onto the same node, and that node's memory usage spiked until it ran out of memory.

   Root cause

   Capacity exhaustion

   Component

   CoreDNSClient-side DNS resolver and cachekubeletContainer memory limitNode pool

   Contributing factor

   Unreviewed defaultsSingle point of failureNo caching

   Detection

   The write-up does not say how the full outage was noticed. The earlier DNS trouble showed up as intermittent service errors, and investigating failed calls to RDS and Redis led to DNS resolution failures.

   Remediation

   • deployed NodeLocal DNSCache as a DaemonSet in front of CoreDNS
   • added a PodDisruptionBudget (maxUnavailable 1) and topologySpreadConstraints across zones for CoreDNS
   • lowered ndots from 5 to 2 with dnsConfig in the Deployments

   Lessons

   The CoreDNS setup that EKS creates by default (two pods) can be enough for a small or development cluster but becomes a bottleneck in a large production cluster, and DNS is a single point of failure that needs high availability and room to scale. Tuning the CoreDNS cache and buffer size only relieved the symptoms; the real load came from the extra queries that ndots:5 generates for external names. A CoreDNS pod without a memory limit can share a node's fate with everything else on it.

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