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1. IL-0056 Calls from EKS pods to services behind an NLB in another VPC failed intermittently because the AWS VPC CNI did source NAT on the node

   MindTickle2020

   Symptoms

   On the Saturday the production cluster was moved from kops to EKS, the platform became very slow and too unstable to keep running, and the team reverted.

   Intermittent connection failuresHigh latency

   Trigger

   Production was moved from a kops cluster using Weave networking to a new EKS cluster using the AWS VPC CNI, without external SNAT enabled.

   Root cause

   DNS or network fault

   Component

   AWS VPC CNI

   Contributing factor

   Unreviewed defaultsUnverified assumption

   Detection

   The team lead saw the platform behaving very slowly after the switch to EKS and phoned the author.

   Remediation

   • reverted production to the kops cluster
   • set AWS_VPC_K8S_CNI_EXTERNALSNAT=true in the AWS VPC CNI

   Action items

   • Monitor and alert on the AWS VPC CNI

   Lessons

   In a networking problem every component in the stack is a suspect; ruling out EKS early cost a lot of time. A DaemonSet that pinged every component from every other one exposed the anomaly that led to the fix. The team had only skimmed the AWS CNI documentation before the migration; knowing how the plugin works internally would have led to better judgements and hypotheses while debugging.

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