Kubernetes - how to check current domain set by --cluster-domain from pod?

Question

Kubernetes admin can use --cluster-domain to customize cluster domain instead of using default one: cluster.local Kubelet Configs.

So the question is, how does an application pod check this domain in runtime?

Answer 1

It needs to be configured on the DNS server.

Either kube-dns or coredns (Favored on newer K8s versions)

kube-dns: it's a cli option --domain

core-dns: you can configure the K8s ConfigMap

And you see here:

The kubelet passes DNS to each container with the --cluster-dns= flag.

If you'd like to know how a pod resolves cluster.local it does it through the /etc/resolv.conf that the kubelet mounts on every pod. The content is something like this:

$ cat /etc/resolv.conf
nameserver 10.96.0.10
search <namespace>.svc.cluster.local svc.cluster.local cluster.local <nod-domain>
options ndots:5

10.96.0.10 is your coredns or kube-dns cluster IP address.

Answer 2

Running a DNS query against service kubernetes.default is a possible solution. Here is a one-liner example in shell:

kubectl run -it --image=ubuntu --restart=Never shell -- \
sh -c 'apt-get update > /dev/null && apt-get install -y dnsutils > /dev/null && \
nslookup kubernetes.default | grep Name | sed "s/Name:\skubernetes.default//"'

This will returns as last line:

.svc.cluster.local

However, I think it would be more robust to implement this algorithm in a programming language like go which have a good DNS client implemented in net library, here is an example you can run in a pod:

package main

import (
    "fmt"
    "net"
    "strings"
)

// GetClusterDomain returns Kubernetes cluster domain, default to "cluster.local"
func getClusterDomain() string {
    apiSvc := "kubernetes.default.svc"

    cname, err := net.LookupCNAME(apiSvc)
    if err != nil {
        defaultClusterDomain := "cluster.local"
        return defaultClusterDomain
    }

    clusterDomain = strings.TrimPrefix(cname, apiSvc)
    clusterDomain = strings.TrimSuffix(clusterDomain, ".")

    return clusterDomain
}

func main() {
    fmt.Println(getClusterDomain())
}