193 lines
6.8 KiB
Markdown
193 lines
6.8 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 provides hints on configuring DNS Pod and guidance on customizing the
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DNS resolution process.
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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 and above.
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* The cluster must be configured to use the `kube-dns` addon.
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{{% /capture %}}
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{{% capture steps %}}
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## Introduction
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Starting from Kubernetes v1.3, DNS is a built-in 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 Kubernetes DNS pod holds 3 containers:
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- "`kubedns`": The `kubedns` process 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`": The `dnsmasq` container adds DNS caching to improve performance.
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- "`sidecar`": The `sidecar` container provides a single health check endpoint
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while performing dual healthchecks (for `dnsmasq` and `kubedns`).
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The DNS pod is exposed as a Kubernetes Service with a static IP. Once assigned
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the kubelet passes DNS configured using the `--cluster-dns=<dns-service-ip>`
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flag to each container.
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DNS names also need domains. The local domain is configurable in the kubelet
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using the flag `--cluster-domain=<default-local-domain>`.
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The Kubernetes cluster DNS server is based off 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 will prepend the cluster DNS server and search
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paths to the node's own DNS settings. If the node is able to resolve DNS names
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specific to the larger environment, pods should be able to, also.
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See [Known issues](#known-issues) below for a caveat.
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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. Setting it to "" means that pods will
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not inherit DNS. Setting it to a valid file path means that kubelet will use
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this file instead of `/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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As specified, 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 would 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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### Impacts on Pods
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Custom upstream nameservers and stub domains won't impact Pods that have their
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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 (as in the [previous example](#configuring-stub-domain-and-upstream-dns-servers)),
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DNS queries will be 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* (e.g.".cluster.local"):
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The request is sent to kube-dns.
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* *Names with the stub domain suffix* (e.g. ".acme.local"):
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The request is sent to the configured custom DNS resolver (e.g. listening at 1.2.3.4).
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* *Names without a matching suffix* (e.g."widget.com"):
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The request is forwarded to the upstream DNS
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(e.g. 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 (e.g. “acme.local”) and a value consisting of a JSON array of DNS IPs. | The target nameserver may 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. | Note: If specified, then the values specified 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 that they wish 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 simply creates a ConfigMap object as shown below.
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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 did not wish 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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Again, this is easy to accomplish; they just need to 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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## What's next
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- [Debugging DNS Resolution](/docs/tasks/administer-cluster/dns-debugging-resolution/).
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{{% /capture %}}
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