Cases2 cases

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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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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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