How To Customize CoreDNS Configuration for Adding Additional External DNS

Problem

How To Customize CoreDNS Configuration for Adding Additional External DNS.

Environment

• Platform9 Managed Kubernetes - v5.2 and K8s v1.20 and Higher

• AddOn Management

Procedure

1. Generate the TOKEN by following steps mentioned in Keystone Identity.

2. API to retrieve existing CoreDNS ClusterAddon object detail.

API GET Call

curl -X GET -H "X-Auth-Token: $TOKEN" https://<DU_FQDN>/qbert/v4/<PROJECT_ID>/sunpike/apis/sunpike.platform9.com/v1alpha2/namespaces/default/clusteraddons/<CLUSTER_UUID>-coredns

Example Showing Default Set Parameters

$ curl -X GET -H "X-Auth-Token: $TOKEN" https://cs-sanchit.platform9.horse/qbert/v4/1c1003c1b84e47e7bd2a4cdf3b725976/sunpike/apis/sunpike.platform9.com/v1alpha2/namespaces/default/clusteraddons/8c8f1d62-159a-4829-b68f-3432d4a9afc6-coredns | jq
...
...
  "spec": {
    "clusterID": "8c8f1d62-159a-4829-b68f-3432d4a9afc6",
    "version": "1.7.0",
    "type": "coredns",
    "override": {
      "params": [
        {
          "name": "dnsMemoryLimit",
          "value": "170Mi"
        },
        {
          "name": "dnsDomain",
          "value": "cluster.local"
        }
      ]
    },
    "watch": true

Example Existing Associated ConfigMap

# kubectl describe configmap coredns -n kube-system
Name:         corednsNamespace:    kube-system
Labels:       addonmanager.kubernetes.io/mode=EnsureExists
Annotations:  <none>


Data
====
Corefile:
----
.:53 {
    errors
    health {
        lameduck 5s
    }
    ready
    kubernetes cluster.local in-addr.arpa ip6.arpa {
        pods insecure
        fallthrough in-addr.arpa ip6.arpa
        ttl 30
    }
    prometheus :9153
    forward . /etc/resolv.conf {
        max_concurrent 1000
    }
    cache 30
    loop
    reload
    loadbalance
}

Events:  <none>

1. Create a new JSON spec file with the required additional DNS configuration parameter.

Note

The extra DNS configuration parameter one wants to add should have no indentations. The addon operator adds the correct indentation before adding it under the DNS block in the configmap.

Sample DNS configuration without indentation

kfplc.com:53 {
   errors
   cache 30
   forward . 10.246.6.1
}

base64 Encoding of the sample configuration

a2ZwbGMuY29tOjUzIHsKICBlcnJvcnMKICBjYWNoZSAzMAogIGZvcndhcmQgLiAxMC4yNDYuNi4xCn0K

This needs to be inputted as a value for the parameter base64EncAdditionalDnsConfig in the JSON spec file.

Sample JSON File

# cat coredns.json
{
    "apiVersion": "sunpike.platform9.com/v1alpha2",
    "kind": "ClusterAddon",
    "metadata": {
        "labels": {
            "sunpike.pf9.io/cluster": "CLUSTER_UUID",
            "type": "coredns"
        },
        "name": "CLUSTER_UUID-coredns",
        "namespace": "default"
    },
    "spec": {
        "clusterID": "CLUSTER_UUID",
        "override": {
            "params": [
                {
                    "name": "dnsMemoryLimit",
                    "value": "170Mi"
                },
                {
                    "name": "dnsDomain",
                    "value": "cluster.local"
                },
                {
                    "name": "base64EncAdditionalDnsConfig",
                    "value": "<base 64 encoded additional dns config>"
                }
            ]
        },
        "type": "coredns",
        "version": "1.7.0",
        "watch": true
    }
}

You can read more about it here CoreDNS Addon.

1. Patch the existing CoreDNS ClusterAddon object with the newly created Spec file coredns.json.

Command

curl -X PATCH -H "X-Auth-Token: $TOKEN" -H "Content-Type: application/merge-patch+json" -H "Accept: application/json" -d "@coredns.json" https://<DU_FQDN>/qbert/v4/<PROJECT_ID>/sunpike/apis/sunpike.platform9.com/v1alpha2/namespaces/default/clusteraddons/<CLUSTER_UUID>-coredns

Info

Replace the placeholders <DU_FQDN>, <PROJECT_ID> and the <CLUSTER_UUID> with their appropriate values required. The name attribute in the spec must be in the format CLUSTER_UUID-coredns

The CoreDNS version in the above JSON file is currently on 1.7.0. It will be updated along with the pf9-kube version and can be referred from the Support Matrix.

Updated ConfigMap after the PATCH API Call

# kubectl describe configmap coredns -n kube-system
Name:         coredns
Namespace:    kube-system
Labels:       addonmanager.kubernetes.io/mode=EnsureExists
Annotations:  <none>

Data
====
Corefile:
----
.:53 {
    errors
    health {
        lameduck 5s
    }
    ready
    kubernetes cluster.local in-addr.arpa ip6.arpa {
        pods insecure
        fallthrough in-addr.arpa ip6.arpa
        ttl 30
    }

    prometheus :9153
    forward . /etc/resolv.conf {
        max_concurrent 1000
    }
    cache 30
    loop
    reload
    loadbalance

}

kfplc.com:53 {
  errors
  cache 30
  forward . 10.246.6.1
}​


Events:  <none>

Note

The changes made to the AddOn object will persist across management plane & cluster upgrades.