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Updated on 2024-06-26 GMT+08:00
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CoreDNS

Introduction

CoreDNS is a DNS server that provides domain name resolution for Kubernetes clusters through a chain add-on.

CoreDNS is an open-source software and has been a part of CNCF. It provides a means for cloud services to discover each other in cloud-native deployments. Each of the plugins chained by CoreDNS provides a particular DNS function. You can integrate CoreDNS with only the plugins you need to make it fast, efficient, and flexible. When used in a Kubernetes cluster, CoreDNS can automatically discover services in the cluster and provide domain name resolution for these services. By working with DNS server, CoreDNS can resolve external domain names for workloads in a cluster.

This add-on is installed by default during cluster creation.

Kubernetes backs CoreDNS as the official default DNS for all clusters going forward.

CoreDNS official website: https://coredns.io/

Open source community: https://github.com/coredns/coredns

For more information about CoreDNS, see DNS.

Constraints

To run CoreDNS or upgrade CoreDNS in a cluster, ensure the number of available nodes in the cluster is greater than or equal to the number of CoreDNS instances and all CoreDNS instances are running. Otherwise, the add-on will malfunction or the upgrade will fail.

Editing the Add-on

This add-on is installed by default. To modify its settings, perform the following steps:

  1. Log in to the CCE console and click the cluster name to access the cluster console. In the navigation pane on the left, choose Add-ons. Locate CoreDNS on the right and click Edit.
  2. On the Edit Add-on page, modify the specifications.

    Table 1 CoreDNS specifications

    Parameter

    Description

    Pods

    Number of pods for the add-on.

    High availability is not possible if there is only one pod. If an error occurs on the node where the pod runs, the add-on will fail.

    Containers

    CPU and memory quotas of the containers for running the add-on. Queries per second (QPS) of the add-on is positively correlated with the CPU consumption. If the number of nodes or containers in the cluster grows, the CoreDNS pods will bear heavier workloads.

  3. Modify the parameters.

    Table 2 CoreDNS add-on parameters

    Parameter

    Description

    Stub Domain

    A domain name server for a custom domain name. The format is a key-value pair. The key is a domain name suffix, and the value is one or more DNS IP addresses, for example, acme.local -- 1.2.3.4,6.7.8.9.

    For details, see Configuring the Stub Domain for CoreDNS.

    Advance Config
    • parameterSyncStrategy: indicates whether to configure consistency check when the add-on is upgraded.
      • ensureConsistent: indicates that the configuration consistency check is enabled. If the configuration recorded in the cluster is inconsistent with the actual configuration, the add-on cannot be upgraded.
      • force: indicates that the configuration consistency check is ignored during an upgrade. In this case, you must ensure that the current effective configuration is the same as the original configuration. After the add-on is upgraded, restore the value of parameterSyncStrategy to ensureConsistent to enable the configuration consistency check again.
      • inherit: indicates that custom settings are automatically inherited during an upgrade. After the add-on is upgraded, restore the value of parameterSyncStrategy to ensureConsistent to enable the configuration consistency check again.
    • stub_domains: indicates the subdomains, which allow you to configure a domain name server for a custom domain name. A subdomain is in the format of a key-value pair, where the key is the suffix of a DNS domain name and the value is one or more DNS IP addresses.
    • upstream_nameservers: indicates the IP address of the upstream DNS server.
    • servers: indicates the name servers, which are available in CoreDNS v1.23.1 and later versions. You can customize nameservers. For details, see dns-custom-nameservers.
      plugins indicates the configuration of each component in CoreDNS. Retain the default settings typically to prevent CoreDNS from being unavailable due to configuration errors. Each plugin component contains name, parameters (optional), and configBlock (optional). The format of the generated Corefile is as follows: 
      $name  $parameters {
$configBlock
}

      Table 3 describes common plugins. For details, see Plugins.

      Example:

      {
     "servers": [
		   {
			"plugins": [
				{
					"name": "bind",
					"parameters": "{$POD_IP}"
				},
				{
					"name": "cache",
					"parameters": 30
				},
				{
					"name": "errors"
				},
				{
					"name": "health",
					"parameters": "{$POD_IP}:8080"
				},
                                {
					"name": "ready",
					"{$POD_IP}:8081"
				},
				{
					"configBlock": "pods insecure\nfallthrough in-addr.arpa ip6.arpa",
					"name": "kubernetes",
					"parameters": "cluster.local in-addr.arpa ip6.arpa"
				},
				{
					"name": "loadbalance",
					"parameters": "round_robin"
				},
				{
					"name": "prometheus",
					"parameters": "{$POD_IP}:9153"
				},
				{
					"configBlock": "policy random",
					"name": "forward",
					"parameters": ". /etc/resolv.conf"
				},
				{
					"name": "reload"
				}
			],
			"port": 5353,
			"zones": [
				{
					"zone": "."
				}
			]
		}
	],
	"stub_domains": {
		"acme.local": [
			"1.2.3.4",
			"6.7.8.9"
		]
	},
	"upstream_nameservers": ["8.8.8.8", "8.8.4.4"]
}
    Table 3 Default plugin configuration of the active CoreDNS zone

    Plugin Name

    Description

    bind

    Host IP address listened by CoreDNS. Retain the default value {$POD_IP}. For details, see bind.

    cache

    Enables DNS cache. For details, see cache.

    errors

    Errors are logged to stdout. For details, see errors.

    health

    Health check for CoreDNS. {$POD_IP}:8080 is listened to. Retain the default setting. Otherwise, the CoreDNS health check will fail and the add-on will restart repeatedly. For details, see health.

    ready

    Whether the backend server is ready to receive traffic. {$POD_IP}:8081 is listened to. If the backend server is not ready, CoreDNS will suspend DNS resolution until the backend server is ready. For details, see ready.

    kubernetes

    CoreDNS Kubernetes plugin, which provides the service parsing capability in a cluster. For details, see kubernetes.

    loadbalance

    Round-robin DNS load balancer that randomizes the order of A, AAAA, and MX records in an answer. For details, see loadbalance.

    prometheus

    API for obtaining CoreDNS metrics. {$POD_IP}:9153 is listened to in the default zone. Retain the default setting. Otherwise, Prometheus cannot collect CoreDNS metrics. For details, see Prometheus.

    forward

    Forwards any queries that are not within the cluster domain of Kubernetes to predefined resolvers (/etc/resolv.conf). For details, see forward.

    reload

    Automatically reloads modified Corefiles. After you modify a ConfigMap, wait for two minutes for the modification to take effect. For details, see reload.

    log

    Enables CoreDNS logging. For details, see log.

    Example:

    {
   "name": "log"
}

    template

    A quick response template, where AAAA indicates an IPv6 request. If NXDOMAIN is returned in an rcode response, no IPv6 resolution result is returned. For details, see Template.

    Example:

    {
   "configBlock": "rcode NXDOMAIN",
   "name": "template",
   "parameters": "ANY AAAA" 
}

  4. Click Install.

Components

Table 4 CoreDNS components

Component

Description

Resource Type

CoreDNS

DNS server for clusters

Deployment

How Does Domain Name Resolution Work in Kubernetes?

DNS policies can be configured for each pod. Kubernetes supports DNS policies Default, ClusterFirst, ClusterFirstWithHostNet, and None. For details, see DNS for Services and Pods. These policies are specified in the dnsPolicy field in the pod-specific.

  • Default: Pods inherit the name resolution configuration from the node that the pods run on. The custom upstream DNS server and the stub domain cannot be used together with this policy.
  • ClusterFirst: Any DNS query that does not match the configured cluster domain suffix, such as www.kubernetes.io, is forwarded to the upstream name server inherited from the node. Cluster administrators may have extra stub domains and upstream DNS servers configured.
  • ClusterFirstWithHostNet: For pods running with hostNetwork, set its DNS policy ClusterFirstWithHostNet.
  • None: It allows a pod to ignore DNS settings from the Kubernetes environment. All DNS settings should be provided using the dnsPolicy field in dnsConfigPod.
  • Clusters of Kubernetes v1.10 and later support Default, ClusterFirst, ClusterFirstWithHostNet, and None. Clusters earlier than Kubernetes v1.10 support only Default, ClusterFirst, and ClusterFirstWithHostNet.
  • Default is not the default DNS policy. If dnsPolicy is not explicitly specified, ClusterFirst is used.

Routing

Without stub domain configurations: Any query that does not match the configured cluster domain suffix, such as www.kubernetes.io, is forwarded to the upstream DNS server inherited from the node.

With stub domain configurations: If stub domains and upstream DNS servers are configured, DNS queries are routed according to the following flow:

  1. The query is first sent to the DNS caching layer in CoreDNS.
  2. From the caching layer, the suffix of the request is examined and then the request is forwarded to the corresponding DNS:
    • Names with the cluster suffix, for example, .cluster.local: The request is sent to CoreDNS.
    • Names with the stub domain suffix, for example, .acme.local: The request is sent to the configured custom DNS resolver that listens, for example, on 1.2.3.4.
    • Names that do not match the suffix (for example, widget.com): The request is forwarded to the upstream DNS.
Figure 1 Routing
Parent topic: Add-ons