Chapter 5. DNS Operator in OpenShift Container Platform The DNS Operator deploys and manages CoreDNS to provide a name resolution service to pods, enabling DNS-based Kubernetes Service discovery in OpenShift Container Platform. 5.1. DNS Operator The DNS Operator implements the dns API from the operator.openshift.io API group. The Operator deploys CoreDNS using a daemon set, creates a service for the daemon set, and configures the kubelet to instruct pods to use the CoreDNS service IP address for name resolution. Procedure The DNS Operator is deployed during installation with a Deployment object. Use the oc get command to view the deployment status: USD oc get -n openshift-dns-operator deployment/dns-operator Example output NAME READY UP-TO-DATE AVAILABLE AGE dns-operator 1/1 1 1 23h Use the oc get command to view the state of the DNS Operator: USD oc get clusteroperator/dns Example output NAME VERSION AVAILABLE PROGRESSING DEGRADED SINCE dns 4.1.0-0.11 True False False 92m AVAILABLE , PROGRESSING and DEGRADED provide information about the status of the operator. AVAILABLE is True when at least 1 pod from the CoreDNS daemon set reports an Available status condition. 5.2. Changing the DNS Operator managementState DNS manages the CoreDNS component to provide a name resolution service for pods and services in the cluster. The managementState of the DNS Operator is set to Managed by default, which means that the DNS Operator is actively managing its resources. You can change it to Unmanaged , which means the DNS Operator is not managing its resources. The following are use cases for changing the DNS Operator managementState : You are a developer and want to test a configuration change to see if it fixes an issue in CoreDNS. You can stop the DNS Operator from overwriting the fix by setting the managementState to Unmanaged . You are a cluster administrator and have reported an issue with CoreDNS, but need to apply a workaround until the issue is fixed. You can set the managementState field of the DNS Operator to Unmanaged to apply the workaround. Procedure Change managementState DNS Operator: oc patch dns.operator.openshift.io default --type merge --patch '{"spec":{"managementState":"Unmanaged"}}' 5.3. Controlling DNS pod placement The DNS Operator has two daemon sets: one for CoreDNS and one for managing the /etc/hosts file. The daemon set for /etc/hosts must run on every node host to add an entry for the cluster image registry to support pulling images. Security policies can prohibit communication between pairs of nodes, which prevents the daemon set for CoreDNS from running on every node. As a cluster administrator, you can use a custom node selector to configure the daemon set for CoreDNS to run or not run on certain nodes. Prerequisites You installed the oc CLI. You are logged in to the cluster with a user with cluster-admin privileges. Procedure To prevent communication between certain nodes, configure the spec.nodePlacement.nodeSelector API field: Modify the DNS Operator object named default : USD oc edit dns.operator/default Specify a node selector that includes only control plane nodes in the spec.nodePlacement.nodeSelector API field: spec: nodePlacement: nodeSelector: node-role.kubernetes.io/worker: "" To allow the daemon set for CoreDNS to run on nodes, configure a taint and toleration: Modify the DNS Operator object named default : USD oc edit dns.operator/default Specify a taint key and a toleration for the taint: spec: nodePlacement: tolerations: - effect: NoExecute key: "dns-only" operators: Equal value: abc tolerationSeconds: 3600 1 1 If the taint is dns-only , it can be tolerated indefinitely. You can omit tolerationSeconds . 5.4. View the default DNS Every new OpenShift Container Platform installation has a dns.operator named default . Procedure Use the oc describe command to view the default dns : USD oc describe dns.operator/default Example output Name: default Namespace: Labels: <none> Annotations: <none> API Version: operator.openshift.io/v1 Kind: DNS ... Status: Cluster Domain: cluster.local 1 Cluster IP: 172.30.0.10 2 ... 1 The Cluster Domain field is the base DNS domain used to construct fully qualified pod and service domain names. 2 The Cluster IP is the address pods query for name resolution. The IP is defined as the 10th address in the service CIDR range. To find the service CIDR of your cluster, use the oc get command: USD oc get networks.config/cluster -o jsonpath='{USD.status.serviceNetwork}' Example output [172.30.0.0/16] 5.5. Using DNS forwarding You can use DNS forwarding to override the forwarding configuration identified in /etc/resolv.conf on a per-zone basis by specifying which name server should be used for a given zone. If the forwarded zone is the Ingress domain managed by OpenShift Container Platform, then the upstream name server must be authorized for the domain. Procedure Modify the DNS Operator object named default : USD oc edit dns.operator/default This allows the Operator to create and update the ConfigMap named dns-default with additional server configuration blocks based on Server . If none of the servers has a zone that matches the query, then name resolution falls back to the name servers that are specified in /etc/resolv.conf . Sample DNS apiVersion: operator.openshift.io/v1 kind: DNS metadata: name: default spec: servers: - name: foo-server 1 zones: 2 - example.com forwardPlugin: upstreams: 3 - 1.1.1.1 - 2.2.2.2:5353 - name: bar-server zones: - bar.com - example.com forwardPlugin: upstreams: - 3.3.3.3 - 4.4.4.4:5454 1 name must comply with the rfc6335 service name syntax. 2 zones must conform to the definition of a subdomain in rfc1123 . The cluster domain, cluster.local , is an invalid subdomain for zones . 3 A maximum of 15 upstreams is allowed per forwardPlugin . Note If servers is undefined or invalid, the ConfigMap only contains the default server. View the ConfigMap: USD oc get configmap/dns-default -n openshift-dns -o yaml Sample DNS ConfigMap based on sample DNS apiVersion: v1 data: Corefile: | example.com:5353 { forward . 1.1.1.1 2.2.2.2:5353 } bar.com:5353 example.com:5353 { forward . 3.3.3.3 4.4.4.4:5454 1 } .:5353 { 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 { policy sequential } cache 30 reload } kind: ConfigMap metadata: labels: dns.operator.openshift.io/owning-dns: default name: dns-default namespace: openshift-dns 1 Changes to the forwardPlugin triggers a rolling update of the CoreDNS daemon set. Additional resources For more information on DNS forwarding, see the CoreDNS forward documentation . 5.6. DNS Operator status You can inspect the status and view the details of the DNS Operator using the oc describe command. Procedure View the status of the DNS Operator: USD oc describe clusteroperators/dns 5.7. DNS Operator logs You can view DNS Operator logs by using the oc logs command. Procedure View the logs of the DNS Operator: USD oc logs -n openshift-dns-operator deployment/dns-operator -c dns-operator

Commands:
oc get -n openshift-dns-operator deployment/dns-operator
NAME READY UP-TO-DATE AVAILABLE AGE dns-operator 1/1 1 1 23h
oc get clusteroperator/dns
NAME VERSION AVAILABLE PROGRESSING DEGRADED SINCE dns 4.1.0-0.11 True False False 92m
patch dns.operator.openshift.io default --type merge --patch '{"spec":{"managementState":"Unmanaged"}}'
oc edit dns.operator/default
spec: nodePlacement: nodeSelector: node-role.kubernetes.io/worker: ""
oc edit dns.operator/default
spec: nodePlacement: tolerations: - effect: NoExecute key: "dns-only" operators: Equal value: abc tolerationSeconds: 3600 1
oc describe dns.operator/default
Name: default Namespace: Labels: <none> Annotations: <none> API Version: operator.openshift.io/v1 Kind: DNS Status: Cluster Domain: cluster.local 1 Cluster IP: 172.30.0.10 2
oc get networks.config/cluster -o jsonpath='{USD.status.serviceNetwork}'
[172.30.0.0/16]
oc edit dns.operator/default
apiVersion: operator.openshift.io/v1 kind: DNS metadata: name: default spec: servers: - name: foo-server 1 zones: 2 - example.com forwardPlugin: upstreams: 3 - 1.1.1.1 - 2.2.2.2:5353 - name: bar-server zones: - bar.com - example.com forwardPlugin: upstreams: - 3.3.3.3 - 4.4.4.4:5454
oc get configmap/dns-default -n openshift-dns -o yaml
apiVersion: v1 data: Corefile: | example.com:5353 { forward . 1.1.1.1 2.2.2.2:5353 } bar.com:5353 example.com:5353 { forward . 3.3.3.3 4.4.4.4:5454 1 } .:5353 { 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 { policy sequential } cache 30 reload } kind: ConfigMap metadata: labels: dns.operator.openshift.io/owning-dns: default name: dns-default namespace: openshift-dns
oc describe clusteroperators/dns
oc logs -n openshift-dns-operator deployment/dns-operator -c dns-operator