Cases2 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-0047 Pod-to-pod requests delayed by 5, 10 or 15 seconds because of DNS lookups lost to a conntrack race

   MindTickle2019

   Symptoms

   Pod-to-pod calls were intermittently slow and the web application felt slow overall. A request that pod A sent at time x reached pod B at x + 5s, x + 10s or x + 15s, always a multiple of five seconds.

   Intermittent timeoutsHigh latencyDNS resolution failures

   Trigger

   The C library resolver sent the A and AAAA queries for each lookup on the same port; where DNAT or SNAT translation takes place, a race condition in conntrack sometimes left one of the queries unanswered, and the resolver waited 5 seconds before retrying.

   Root cause

   DNS or network fault

   Component

   conntrack tableClient-side DNS resolver and cacheLinux kernel

   Contributing factor

   Unreviewed defaults

   Detection

   The team noticed intermittent delays in pod-to-pod calls and slowness in the web application, and that the delays were always multiples of five seconds.

   Remediation

   • added the single-request-reopen option to dnsConfig in the deployment specs
   • set GRPC_DNS_RESOLVER=native in the Dockerfiles of gRPC applications so they used the libc resolver
   • checked with tcpdump port 53 that A and AAAA queries left from different ports

   Lessons

   Delays that are exact multiples of five seconds point to DNS lookups being retried after the resolver's 5-second timeout. A resolver option set in the pod spec only helps processes that use the libc resolver; runtimes with their own resolver, such as gRPC with c-ares, ignore it.

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