148 lines
5.4 KiB
Markdown
148 lines
5.4 KiB
Markdown
---
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approvers:
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- bowei
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- zihongz
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title: Configure private DNS zones and upstream nameservers in Kubernetes
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---
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{% capture overview %}
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This page shows how to add custom private DNS zones (stub domains) and upstream
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nameservers.
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{% endcapture %}
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{% capture prerequisites %}
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* {% include task-tutorial-prereqs.md %}
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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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{% endcapture %}
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{% capture steps %}
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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 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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{% endcapture %}
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{% capture discussion %}
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## Understanding name resolution in Kubernetes
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DNS policies can be set on a per-pod basis. Currently Kubernetes supports two pod-specific DNS policies: “Default” and “ClusterFirst”. These policies are specified with the `dnsPolicy` flag.
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*NOTE: "Default" is not the default DNS policy. If `dnsPolicy` is not
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explicitly specified, then “ClusterFirst” is used.*
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### "Default" DNS Policy
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If `dnsPolicy` is set to “Default”, then the name resolution configuration is
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inherited from the node that the pods run on. Custom upstream nameservers and stub domains cannot be used in conjunction with this policy.
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### "ClusterFirst" DNS Policy
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If the `dnsPolicy` is set to "ClusterFirst", name resolution is handled differently, *depending on whether stub-domain and upstream DNS servers are configured*.
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**Without custom configurations**: Any query that does not match the configured cluster domain suffix, such as "www.kubernetes.io", is forwarded to the upstream nameserver inherited from the node.
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**With custom configurations**: If stub domains and upstream DNS servers are configured (as in the [previous example](#configuring-stub-domain-and-upstream-dns-servers)), DNS queries will be
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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 forwarded to the appropriate DNS, based on the following cases:
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* *Names with the cluster suffix* (e.g.".cluster.local"): The request is sent to kube-dns.
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* *Names with the stub domain suffix* (e.g. ".acme.local"): 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"): The request is forwarded to the upstream DNS (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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## Additional 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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{% endcapture %}
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{% include templates/task.md %}
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