# Debugging DNS Resolution

Debugging DNS ResolutionThis page provides hints on diagnosing DNS problems.Before you beginYou need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. It is recommended to run this tutorial on a cluster with at least two nodes that are not acting as control plane hosts. If you do not already have a cluster, you can create one by using minikube or you can use one of these Kubernetes playgrounds:iximiuz LabsKillercodaKodeKloudPlay with KubernetesYour cluster must be configured to use the CoreDNS addon or its precursor, kube-dns.Your Kubernetes server must be at or later than version v1.6.To check the version, enter kubectl version.Create a simple Pod to use as a test environmentadmin/dns/dnsutils.yaml apiVersion: v1 kind: Pod metadata: name: dnsutils namespace: default spec: containers: - name: dnsutils image: registry.k8s.io/e2e-test-images/agnhost:2.39 imagePullPolicy: IfNotPresent restartPolicy: Always Note:This example creates a pod in the default namespace. DNS name resolution for services depends on the namespace of the pod. For more information, review DNS for Services and Pods.Use that manifest to create a Pod:kubectl apply -f https://k8s.io/examples/admin/dns/dnsutils.yaml pod/dnsutils created …and verify its status:kubectl get pods dnsutils NAME READY STATUS RESTARTS AGE dnsutils 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 -i -t dnsutils -- 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 firstTake a look inside the resolv.conf file. (See Customizing DNS Service and Known issues below for more information)kubectl exec -ti dnsutils -- 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 CoreDNS (or kube-dns) add-on or with associated Services:kubectl exec -i -t dnsutils -- nslookup kubernetes.default Server: 10.0.0.10 Address 1: 10.0.0.10 nslookup: can't resolve 'kubernetes.default' orkubectl exec -i -t dnsutils -- 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 runningUse 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 ... coredns-7b96bf9f76-5hsxb 1/1 Running 0 1h coredns-7b96bf9f76-mvmmt 1/1 Running 0 1h ... Note:The value for label k8s-app is kube-dns for both CoreDNS and kube-dns deployments.If you see that no CoreDNS 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 podUse the kubectl logs command to see logs for the DNS containers.For CoreDNS:kubectl logs --namespace=kube-system -l k8s-app=kube-dns Here is an example of a healthy CoreDNS log:.:53 2018/08/15 14:37:17 [INFO] CoreDNS-1.2.2 2018/08/15 14:37:17 [INFO] linux/amd64, go1.10.3, 2e322f6 CoreDNS-1.2.2 linux/amd64, go1.10.3, 2e322f6 2018/08/15 14:37:17 [INFO] plugin/reload: Running configuration MD5 = 24e6c59e83ce706f07bcc82c31b1ea1c See if there are any suspicious or unexpected messages in the logs.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 TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE ... kube-dns ClusterIP 10.0.0.10 <none> 53/UDP,53/TCP 1h ... Note:The service name is kube-dns for both CoreDNS and kube-dns deployments.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 endpointslice command.kubectl get endpointslices -l k8s.io/service-name=kube-dns --namespace=kube-system NAME ADDRESSTYPE PORTS ENDPOINTS AGE kube-dns-zxoja IPv4 53 10.180.3.17,10.180.3.17 1h If you do not see the endpoints, see the endpoints section in the debugging Services documentation.For additional Kubernetes DNS examples, see the cluster-dns examples in the Kubernetes GitHub repository.Are DNS queries being received/processed?You can verify if queries are being received by CoreDNS by adding the log plugin to the CoreDNS configuration (aka Corefile). The CoreDNS Corefile is held in a ConfigMap named coredns. To edit it, use the command:kubectl -n kube-system edit configmap coredns Then add log in the Corefile section per the example below:apiVersion: v1 kind: ConfigMap metadata: name: coredns namespace: kube-system data: Corefile: | .:53 { log errors health kubernetes cluster.local in-addr.arpa ip6.arpa { pods insecure upstream fallthrough in-addr.arpa ip6.arpa } prometheus :9153 forward . /etc/resolv.conf cache 30 loop reload loadbalance } After saving the changes, it may take up to minute or two for Kubernetes to propagate these changes to the CoreDNS pods.Next, make some queries and view the logs per the sections above in this document. If CoreDNS pods are receiving the queries, you should see them in the logs.Here is an example of a query in the log:.:53 2018/08/15 14:37:15 [INFO] CoreDNS-1.2.0 2018/08/15 14:37:15 [INFO] linux/amd64, go1.10.3, 2e322f6 CoreDNS-1.2.0 linux/amd64, go1.10.3, 2e322f6 2018/09/07 15:29:04 [INFO] plugin/reload: Running configuration MD5 = 162475cdf272d8aa601e6fe67a6ad42f 2018/09/07 15:29:04 [INFO] Reloading complete 172.17.0.18:41675 - [07/Sep/2018:15:29:11 +0000] 59925 "A IN kubernetes.default.svc.cluster.local. udp 54 false 512" NOERROR qr,aa,rd,ra 106 0.000066649s Does CoreDNS have sufficient permissions?CoreDNS must be able to list service and endpointslice related resources to properly resolve service names.Sample error message:2022-03-18T07:12:15.699431183Z [INFO] 10.96.144.227:52299 - 3686 "A IN serverproxy.contoso.net.cluster.local. udp 52 false 512" SERVFAIL qr,aa,rd 145 0.000091221s First, get the current ClusterRole of system:coredns:kubectl describe clusterrole system:coredns -n kube-system Expected output:PolicyRule: Resources Non-Resource URLs Resource Names Verbs --------- ----------------- -------------- ----- endpoints [] [] [list watch] namespaces [] [] [list watch] pods [] [] [list watch] services [] [] [list watch] endpointslices.discovery.k8s.io [] [] [list watch] If any permissions are missing, edit the ClusterRole to add them:kubectl edit clusterrole system:coredns -n kube-system Example insertion of EndpointSlices permissions:... - apiGroups: - discovery.k8s.io resources: - endpointslices verbs: - list - watch ... Are you in the right namespace for the service?DNS queries that don't specify a namespace are limited to the pod's namespace.If the namespace of the pod and service differ, the DNS query must include the namespace of the service.This query is limited to the pod's namespace:kubectl exec -i -t dnsutils -- nslookup <service-name> This query specifies the namespace:kubectl exec -i -t dnsutils -- nslookup <service-name>.<namespace> To learn more about name resolution, see DNS for Services and Pods.Known issuesSome Linux distributions (e.g. Ubuntu) use a local DNS resolver by default (systemd-resolved). Systemd-resolved moves and replaces /etc/resolv.conf with a stub file that can cause a fatal forwarding loop when resolving names in upstream servers. This can be fixed manually by using kubelet's --resolv-conf flag to point to the correct resolv.conf (With systemd-resolved, this is /run/systemd/resolve/resolv.conf). kubeadm automatically detects systemd-resolved, and adjusts the kubelet flags accordingly.Kubernetes installs do not configure the nodes' resolv.conf files to use the cluster DNS by default, because that process is inherently distribution-specific. This should probably be implemented eventually.Linux's libc (a.k.a. glibc) has a limit for the DNS nameserver records to 3 by default and Kubernetes needs to consume 1 nameserver record. This means that if a local installation already uses 3 nameservers, some of those entries will be lost. To work around this limit, the node can run dnsmasq, which will provide more nameserver entries. You can also use kubelet's --resolv-conf flag.If you are using Alpine version 3.17 or earlier as your base image, DNS may not work properly due to a design issue with Alpine. Until musl version 1.24 didn't include TCP fallback to the DNS stub resolver meaning any DNS call above 512 bytes would fail. Please upgrade your images to Alpine version 3.18 or above.What's nextSee Autoscaling the DNS Service in a Cluster.Read DNS for Services and Pods