284 lines
9.4 KiB
Markdown
284 lines
9.4 KiB
Markdown
---
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reviewers:
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- bowei
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- zihongz
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title: Customizing DNS Service
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content_template: templates/task
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---
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{{% capture overview %}}
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This page explains how to configure your DNS Pod and customize the
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DNS resolution process. In Kubernetes version 1.11 and later, CoreDNS is at GA
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and is installed by default with kubeadm. See [Configuring CoreDNS](#config-coredns)
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and [Using CoreDNS for Service Discovery](/docs/tasks/administer-cluster/coredns/).
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{{% /capture %}}
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{{% capture prerequisites %}}
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* {{< include "task-tutorial-prereqs.md" >}} {{< version-check >}}
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* Kubernetes version 1.6 or later. To work with CoreDNS, version 1.9 or later.
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* The appropriate add-on: kube-dns or CoreDNS. To install with kubeadm,
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see [the kubeadm reference documentation](/docs/reference/setup-tools/kubeadm/kubeadm-alpha/#cmd-phase-addon).
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{{% /capture %}}
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{{% capture steps %}}
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## Introduction
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DNS is a built-in Kubernetes service launched automatically
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using the addon manager
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[cluster add-on](http://releases.k8s.io/{{< param "githubbranch" >}}/cluster/addons/README.md).
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The running DNS Pod holds 3 containers:
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- "`kubedns`": watches the Kubernetes master for changes
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in Services and Endpoints, and maintains in-memory lookup structures to serve
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DNS requests.
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- "`dnsmasq`": adds DNS caching to improve performance.
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- "`sidecar`": provides a single health check endpoint
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to perform healthchecks for `dnsmasq` and `kubedns`.
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The DNS Pod is exposed as a Kubernetes Service with a static IP.
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The kubelet passes DNS to each container with the `--cluster-dns=<dns-service-ip>`
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flag.
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DNS names also need domains. You configure the local domain in the kubelet
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with the flag `--cluster-domain=<default-local-domain>`.
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The Kubernetes cluster DNS server is based on the
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[SkyDNS](https://github.com/skynetservices/skydns) library. It supports forward
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lookups (A records), service lookups (SRV records), and reverse IP address
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lookups (PTR records).
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## Inheriting DNS from the node
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When running a Pod, kubelet prepends the cluster DNS server and searches
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paths to the node's DNS settings. If the node is able to resolve DNS names
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specific to the larger environment, Pods should also be able to resolve.
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But see [Known issues](/docs/tasks/administer-cluster/dns-debugging-resolution/#known-issues).
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If you don't want this, or if you want a different DNS config for pods, you can
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use the kubelet's `--resolv-conf` flag. Set this flag to "" to prevent Pods from
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inheriting DNS. Set it to a valid file path to specify a file other than
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`/etc/resolv.conf` for DNS inheritance.
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## Configure stub-domain and upstream DNS servers
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Cluster administrators can specify custom stub domains and upstream nameservers
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by providing a ConfigMap for kube-dns (`kube-system:kube-dns`).
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For example, the following ConfigMap sets up a DNS configuration with a single stub domain and two
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upstream nameservers:
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```yaml
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apiVersion: v1
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kind: ConfigMap
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metadata:
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name: kube-dns
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namespace: kube-system
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data:
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stubDomains: |
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{"acme.local": ["1.2.3.4"]}
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upstreamNameservers: |
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["8.8.8.8", "8.8.4.4"]
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```
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DNS requests with the “.acme.local” suffix
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are forwarded to a DNS listening at 1.2.3.4. Google Public DNS
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serves the upstream queries.
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The table below describes how queries with certain domain names map to
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their destination DNS servers:
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| Domain name | Server answering the query |
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| ----------- | -------------------------- |
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| kubernetes.default.svc.cluster.local| kube-dns |
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| foo.acme.local| custom DNS (1.2.3.4) |
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| widget.com | upstream DNS (one of 8.8.8.8, 8.8.4.4) |
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See [ConfigMap options](#configmap-options) for
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details about the configuration option format.
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{{% /capture %}}
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{{% capture discussion %}}
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### Effects on Pods
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Custom upstream nameservers and stub domains do not affect Pods with a
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`dnsPolicy` set to "`Default`" or "`None`".
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If a Pod's `dnsPolicy` is set to "`ClusterFirst`", its name resolution is
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handled differently, depending on whether stub-domain and upstream DNS servers
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are configured.
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**Without custom configurations**: Any query that does not match the configured
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cluster domain suffix, such as "www.kubernetes.io", is forwarded to the upstream
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nameserver inherited from the node.
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**With custom configurations**: If stub domains and upstream DNS servers are
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configured,
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DNS queries are routed according to the following flow:
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1. The query is first sent to the DNS caching layer in kube-dns.
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1. From the caching layer, the suffix of the request is examined and then
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forwarded to the appropriate DNS, based on the following cases:
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* *Names with the cluster suffix*, for example ".cluster.local":
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The request is sent to kube-dns.
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* *Names with the stub domain suffix*, for example ".acme.local":
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The request is sent to the configured custom DNS resolver, listening for example at 1.2.3.4.
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* *Names without a matching suffix*, for example "widget.com":
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The request is forwarded to the upstream DNS,
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for example Google public DNS servers at 8.8.8.8 and 8.8.4.4.
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## ConfigMap options
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Options for the kube-dns `kube-system:kube-dns` ConfigMap:
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| Field | Format | Description |
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| ----- | ------ | ----------- |
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| `stubDomains` (optional) | A JSON map using a DNS suffix key such as “acme.local”, and a value consisting of a JSON array of DNS IPs. | The target nameserver can itself be a Kubernetes Service. For instance, you can run your own copy of dnsmasq to export custom DNS names into the ClusterDNS namespace. |
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| `upstreamNameservers` (optional) | A JSON array of DNS IPs. | If specified, the values replace the nameservers taken by default from the node’s `/etc/resolv.conf`. Limits: a maximum of three upstream nameservers can be specified. |
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### Examples
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#### Example: Stub domain
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In this example, the user has a Consul DNS service discovery system they want to
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integrate with kube-dns. The consul domain server is located at 10.150.0.1, and
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all consul names have the suffix `.consul.local`. To configure Kubernetes, the
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cluster administrator creates the following ConfigMap:
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```yaml
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apiVersion: v1
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kind: ConfigMap
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metadata:
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name: kube-dns
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namespace: kube-system
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data:
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stubDomains: |
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{"consul.local": ["10.150.0.1"]}
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```
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Note that the cluster administrator does not want to override the node’s
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upstream nameservers, so they did not specify the optional
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`upstreamNameservers` field.
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#### Example: Upstream nameserver
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In this example the cluster administrator wants to explicitly force all
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non-cluster DNS lookups to go through their own nameserver at 172.16.0.1.
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In this case, they create a ConfigMap with the
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`upstreamNameservers` field specifying the desired nameserver:
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```yaml
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apiVersion: v1
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kind: ConfigMap
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metadata:
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name: kube-dns
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namespace: kube-system
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data:
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upstreamNameservers: |
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["172.16.0.1"]
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```
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{{% /capture %}}
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## Configuring CoreDNS {#config-coredns}
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You can configure [CoreDNS](https://coredns.io/) as a service discovery.
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CoreDNS is available as an option in Kubernetes starting with version 1.9.
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It is currently a [GA feature](https://github.com/kubernetes/community/blob/master/keps/sig-network/0010-20180314-coredns-GA-proposal.md) and is on course to be [the default](https://github.com/kubernetes/community/blob/master/keps/sig-network/0012-20180518-coredns-default-proposal.md), replacing kube-dns.
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## CoreDNS ConfigMap options
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CoreDNS chains plugins and can be configured by maintaining a Corefile with the ConfigMap. CoreDNS supports all the functionalities and more that is provided by kube-dns.
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A ConfigMap created for kube-dns to support `StubDomains`and `upstreamNameservers` translates to the `proxy` plugin in CoreDNS.
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Similarly, the `Federation` plugin translates to the `federation` plugin in CoreDNS.
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### Example
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This example ConfigMap for kubedns specifies federations, stubdomains and upstreamnameservers:
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```yaml
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apiVersion: v1
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data:
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federations: |
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{"foo" : "foo.feddomain.com"}
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stubDomains: |
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{"abc.com" : ["1.2.3.4"], "my.cluster.local" : ["2.3.4.5"]}
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upstreamNameservers: |
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["8.8.8.8", "8.8.4.4"]
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kind: ConfigMap
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```
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The equivalent configuration in CoreDNS creates a Corefile:
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* For federations:
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```yaml
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federation cluster.local {
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foo foo.feddomain.com
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}
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```
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* For stubDomains:
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```yaml
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abc.com:53 {
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errors
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cache 30
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proxy . 1.2.3.4
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}
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my.cluster.local:53 {
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errors
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cache 30
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proxy . 2.3.4.5
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}
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```
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The complete Corefile with the default plugins:
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```yaml
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.:53 {
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errors
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health
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kubernetes cluster.local in-addr.arpa ip6.arpa {
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upstream 8.8.8.8 8.8.4.4
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pods insecure
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fallthrough in-addr.arpa ip6.arpa
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}
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federation cluster.local {
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foo foo.feddomain.com
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}
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prometheus :9153
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proxy . 8.8.8.8 8.8.4.4
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cache 30
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}
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abc.com:53 {
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errors
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cache 30
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proxy . 1.2.3.4
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}
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my.cluster.local:53 {
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errors
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cache 30
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proxy . 2.3.4.5
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}
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```
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In Kubernetes version 1.10 and later, kubeadm supports automatic translation of the CoreDNS ConfigMap from the kube-dns ConfigMap.
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## Migration to CoreDNS
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A number of tools support the installation of CoreDNS instead of kube-dns.
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To migrate from kube-dns to CoreDNS, [a detailed blog](https://coredns.io/2018/05/21/migration-from-kube-dns-to-coredns/) is available to help users adapt CoreDNS in place of kube-dns.
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## What's next
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- [Debugging DNS Resolution](/docs/tasks/administer-cluster/dns-debugging-resolution/).
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