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1. 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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