Cases7 cases

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1. IL-0001 Cluster-wide DNS outage after CoreDNS pods were OOM-killed again and again under a burst of DNS queries

   Zalando2019

   Symptoms

   Customers of the Zalando Fashion Store got a high rate of errors on all web product pages and outfit pages for more than an hour. The errors came from the main data aggregation layer service, which runs in one of Zalando's Kubernetes clusters.

   High error rateDNS resolution failuresOOMKilledMonitoring outage

   Trigger

   Calls from the aggregation layer to a single, not very important downstream service started timing out. The aggregation layer returned 404s for them, clients retried, and the resulting spike in requests became a spike in DNS queries to CoreDNS.

   Root cause

   Capacity exhaustion

   Component

   CoreDNSContainer memory limitClient-side DNS resolver and cache

   Contributing factor

   Unknown limitNo cachingRetry stormUnreviewed defaultsExcessive blast radiusMonitoring blind spot

   Detection

   The on-call engineer for the aggregation layer phoned the on-call Kubernetes engineer. Automatic paging did not happen because the cluster's monitoring and alerting ran inside the cluster and failed along with DNS. The Kubernetes engineer then saw that the CoreDNS pods were being OOM-killed.

   Remediation

   • Raised the CoreDNS memory requests and limits by hand from 100Mi to 2000Mi

   Action items

   • Introduce a more resilient DNS setup: CoreDNS fronted by dnsmasq on every node
   • Improve high-level alerting from outside the clusters, so that a cluster problem is known before or at the same time as applications are affected
   • Improve incident response processes and tooling so that any on-call engineer can escalate directly to other teams when automatic paging fails, for example through a chat bot

   Lessons

   The Kubernetes DNS setup was far from resilient enough: a single application could take down DNS for everything in the cluster. The blast radius of one application sending more DNS queries should be limited to one node at most, ideally to single pods. Known Kubernetes DNS problems, such as Linux kernel races in conntrack handling and the ndots 5 problem, were familiar to the team and partly mitigated, but their combined effect on the DNS infrastructure was worse than the team had understood. Monitoring that runs inside the cluster was not robust enough: when the cluster's DNS broke, it could not page anyone, and recovery took longer than it should have.

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2. IL-0050 Pods on Kubernetes failed to resolve an external Redis hostname because of ndots:5, the musl resolver in Alpine images, and Netty's DNS resolver

   Toyota Connected2018

   Symptoms

   Services that used the team's Redis-dependent library crashed on start-up, and their teams reported it to the ops members. The Spring Boot logs showed the applications failing to resolve the Redis hostname during bean initialisation.

   DNS resolution failuresIntermittent connection failures

   Root cause

   Software defect

   Component

   Client-side DNS resolver and cache

   Contributing factor

   Unreviewed defaultsUnverified assumption

   Detection

   Service teams reported start-up crashes to the ops members of the team; later the resolution failures were seen in Kibana.

   Remediation

   • set ndots to 4 with a dnsConfig block in the Kubernetes deployment manifests
   • corrected the maximum connection pool size in the Redis library, cutting Redis connections by about 70%

   Action items

   • Test Debian glibc-based base images added to the image library to see whether they resolve the DNS problems
   • Update dependencies to newer Netty versions

   Lessons

   With ndots:5, a name outside the cluster that has fewer than five dots is first tried with every cluster search domain. Adding a trailing dot avoids that, but only if every library in the path accepts it, and the Netty version in use did not. The ndots change is a workaround that will not hold forever. The author would like better alerting for this kind of failure but notes that alerts on one-off DNS failures are not wanted.

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