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Debugging DNS Resolution

This page provides hints on diagnosing DNS problems.

Before you begin

To check the version, enter kubectl version.

Create a simple Pod to use as a test environment

Create a file named busybox.yaml with the following contents:

busybox.yaml docs/tasks/administer-cluster 
apiVersion: v1
kind: Pod
metadata:
  name: busybox
  namespace: default
spec:
  containers:
  - name: busybox
    image: busybox
    command:
      - sleep
      - "3600"
    imagePullPolicy: IfNotPresent
  restartPolicy: Always

Then create a pod using this file and verify its status:

$ kubectl create -f busybox.yaml
pod "busybox" created

$ kubectl get pods busybox
NAME      READY     STATUS    RESTARTS   AGE
busybox   1/1       Running   0          <some-time>

Once that pod is running, you can exec nslookup in that environment. If you see something like the following, DNS is working correctly.

$ kubectl exec -ti busybox -- nslookup kubernetes.default
Server:    10.0.0.10
Address 1: 10.0.0.10

Name:      kubernetes.default
Address 1: 10.0.0.1

If the nslookup command fails, check the following:

Check the local DNS configuration first

Take a look inside the resolv.conf file. (See Inheriting DNS from the node and Known issues below for more information)

$ kubectl exec busybox cat /etc/resolv.conf

Verify that the search path and name server are set up like the following (note that search path may vary for different cloud providers):

search default.svc.cluster.local svc.cluster.local cluster.local google.internal c.gce_project_id.internal
nameserver 10.0.0.10
options ndots:5

Errors such as the following indicate a problem with the kube-dns add-on or associated Services:

$ kubectl exec -ti busybox -- nslookup kubernetes.default
Server:    10.0.0.10
Address 1: 10.0.0.10

nslookup: can't resolve 'kubernetes.default'

or

$ kubectl exec -ti busybox -- nslookup kubernetes.default
Server:    10.0.0.10
Address 1: 10.0.0.10 kube-dns.kube-system.svc.cluster.local

nslookup: can't resolve 'kubernetes.default'

Check if the DNS pod is running

Use the kubectl get pods command to verify that the DNS pod is running.

$ kubectl get pods --namespace=kube-system -l k8s-app=kube-dns
NAME                    READY     STATUS    RESTARTS   AGE
...
kube-dns-v19-ezo1y      3/3       Running   0           1h
...

If you see that no pod is running or that the pod has failed/completed, the DNS add-on may not be deployed by default in your current environment and you will have to deploy it manually.

Check for Errors in the DNS pod

Use kubectl logs command to see logs for the DNS daemons.

$ kubectl logs --namespace=kube-system $(kubectl get pods --namespace=kube-system -l k8s-app=kube-dns -o name) -c kubedns
$ kubectl logs --namespace=kube-system $(kubectl get pods --namespace=kube-system -l k8s-app=kube-dns -o name) -c dnsmasq
$ kubectl logs --namespace=kube-system $(kubectl get pods --namespace=kube-system -l k8s-app=kube-dns -o name) -c sidecar

See if there is any suspicious log. Letter ‘W’, ‘E’, ‘F’ at the beginning represent Warning, Error and Failure. Please search for entries that have these as the logging level and use kubernetes issues to report unexpected errors.

Is DNS service up?

Verify that the DNS service is up by using the kubectl get service command.

$ kubectl get svc --namespace=kube-system
NAME          CLUSTER-IP     EXTERNAL-IP   PORT(S)             AGE
...
kube-dns      10.0.0.10      <none>        53/UDP,53/TCP        1h
...

If you have created the service or in the case it should be created by default but it does not appear, see debugging services for more information.

Are DNS endpoints exposed?

You can verify that DNS endpoints are exposed by using the kubectl get endpoints command.

$ kubectl get ep kube-dns --namespace=kube-system
NAME       ENDPOINTS                       AGE
kube-dns   10.180.3.17:53,10.180.3.17:53    1h

If you do not see the endpoints, see endpoints section in the debugging services documentation.

For additional Kubernetes DNS examples, see the cluster-dns examples in the Kubernetes GitHub repository.

Known issues

Kubernetes installs do not configure the nodes’ resolv.conf files to use the cluster DNS by default, because that process is inherently distro-specific. This should probably be implemented eventually.

Linux’s libc is impossibly stuck (see this bug from 2005) with limits of just 3 DNS nameserver records and 6 DNS search records. Kubernetes needs to consume 1 nameserver record and 3 search records. This means that if a local installation already uses 3 nameservers or uses more than 3 searches, some of those settings will be lost. As a partial workaround, the node can run dnsmasq which will provide more nameserver entries, but not more search entries. You can also use kubelet’s --resolv-conf flag.

If you are using Alpine version 3.3 or earlier as your base image, DNS may not work properly owing to a known issue with Alpine. Check here for more information.

Kubernetes Federation (Multiple Zone support)

Release 1.3 introduced Cluster Federation support for multi-site Kubernetes installations. This required some minor (backward-compatible) changes to the way the Kubernetes cluster DNS server processes DNS queries, to facilitate the lookup of federated services (which span multiple Kubernetes clusters). See the Cluster Federation Administrators’ Guide for more details on Cluster Federation and multi-site support.

References

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