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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.

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.

  1. Use the oc get command to view the deployment status:

    $ 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

  2. Use the oc get command to view the state of the DNS Operator:

    $ 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.

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"}}'

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:

    1. Modify the DNS Operator object named default:

      $ oc edit dns.operator/default

    2. 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:

    1. Modify the DNS Operator object named default:

      $ oc edit dns.operator/default

    2. 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.

View the default DNS

Every new OpenShift Container Platform installation has a dns.operator named default.

Procedure

  1. Use the oc describe command to view the default dns:

    $ 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.

  2. To find the service CIDR of your cluster, use the oc get command:

    $ oc get networks.config/cluster -o jsonpath='{$.status.serviceNetwork}'
Example output
[172.30.0.0/16]

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

  1. Modify the DNS Operator object named default:

    $ 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.

    If servers is undefined or invalid, the ConfigMap only contains the default server.

  2. View the ConfigMap:

    $ oc get configmap/dns-default -n openshift-dns -o yaml
    Sample DNS ConfigMap based on previous 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

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:

$ oc describe clusteroperators/dns

DNS Operator logs

You can view DNS Operator logs by using the oc logs command.

Procedure

View the logs of the DNS Operator:

$ oc logs -n openshift-dns-operator deployment/dns-operator -c dns-operator