Why is dns local cache not enabled out-of-the-box on Kubernetes?

Question

I remember DNS records being cached locally on various linux distros in the past, but this appears to have changed over the years (DNS caching in linux).

Within our environment (non-K8S) we found a noticeable delay (1-2ms) for each request due to DNS lookups because of this.

I also noticed there is no local DNS cache within K8S by default (https://github.com/kubernetes/kubernetes/issues/45363) and the DNS cache within CoreOS is also disabled by default (https://coreos.com/os/docs/latest/configuring-dns.html).

Given we're considering migrating towards K8S I was wondering; why is this not enabled for Kubernetes in particular?

My only theory is within kube-dns records are updated pre-emptively to ensure high-availability; but I'm not sure if K8S actually does that?

As a workaround, if I were to run dnsmasq on every node, would I break things? I noticed there have been attempts to make that setup the default within K8S, but those attempts/PR's appear to have gone stale and I'm not sure why.

Nick_Kh Nick_Kh · Accepted Answer · 2018-07-24T13:07:23

Since Kubernetes 1.9+ was announced, CoreDNS had been included in kubeadm, minikube tools, etc. as a default DNS server and replaced former kube-dns (which was based on dnsmasq).

It was built as a fork of the Caddy web server and middleware chains in a way that each middleware component carries some DNS feature. If you already use kube-dns, it is possible to launch CoreDNS using this Link.

CoreDNS is already equipped with caching and forwarding features, assuming that caching runs as a separate component, and brokes dependency for using dnsmasq.

. {
    proxy . 8.8.8.8:53
    cache example.org
}

There are a lot of plugins which you can use extending DNS functionality, like proxying requests, rewriting requests, doing health checks on endpoints, and publishing metrics into Prometheus.

Why is dns local cache not enabled out-of-the-box on Kubernetes?

1 Answers