使用 kubeadm 部署带有 CoreDNS 的 Kubernetes

通过 kubeadm 在 Kubernetes 中安装 CoreDNS 的指南

Kubernetes 1.9 已发布，其中内置 CoreDNS。我们现在可以通过 Kubeadm（一种单步轻松安装 Kubernetes 的工具包）将 CoreDNS 安装为默认服务发现。

当前，CoreDNS 是 Kubernetes 1.9 中的 Alpha 版。我们制定了一份路线图，CoreDNS 将在 1.10 版本中成为 Beta 版，并最终取代 kube-dns 成为默认 DNS。

需要注意的是，当目前从 kube-dns 切换至 CoreDNS 时， kube-dns 随附的配置（stubzone、联合...）将不复存在，并将切换到 CoreDNS 中的默认配置。从 Kubernetes 的即将发布版本（v1.10）开始，将支持从 kube-dns 转换配置，届时 CoreDNS 将成为 Beta 版。

了解 CoreDNS 配置

这是 kubeadm 随附的默认 CoreDNS 配置。它保存在名为 coredns 的 kubernetes configmap 中

# kubectl -n kube-system get configmap coredns -oyaml
apiVersion: v1
data:
  Corefile: |
    .:53 {
        errors
        health
        kubernetes cluster.local 10.96.0.0/12 {
           pods insecure
           upstream /etc/resolv.conf
        }
        prometheus :9153
        proxy . /etc/resolv.conf
        cache 30
    }
kind: ConfigMap
metadata:
  creationTimestamp: 2017-12-21T12:55:15Z
  name: coredns
  namespace: kube-system
  resourceVersion: "161"
  selfLink: /api/v1/namespaces/kube-system/configmaps/coredns
  uid: 30bf0882-e64e-11e7-baf6-0cc47a8055d6

Corefile 部分是 CoreDNS 的配置。此配置基于 CoreDNS 的以下 插件：

  • errors：错误记录在 stdout 中。
  • health：CoreDNS 的运行状况报告到 http://localhost:8080/health。
  • kubernetes：CoreDNS 将根据 Kubernetes 中服务的 IP 和容器的 IP 响应 DNS 查询。你可以在 此处 找到更多详情。

Kubernetes 插件的选项 群集域 和 服务 CIDR 默认通过 kubeadm 分别定义为 cluster.local 和 10.96.0.0/12。我们可通过 kubeadm 的 --service-dns-domain 和 --service-cidr 标志修改并选择所需的值。

提供 pods insecure 选项是为了向后兼容 kube-dns。

Upstream 用于解析指向外部主机（外部服务）的服务。

  • prometheus：CoreDNS 的度量指标采用 Prometheus 格式展示在 http://localhost:9153/metrics。
  • proxy：不在 Kubernetes 群集域内的任何查询都将转发到预定义的解析器 (/etc/resolv.conf)。
  • cache：启用前端缓存。

我们可以通过修改这个 configmap 来修改默认行为。需要重新启动 CoreDNS pod 才能使更改生效。

在全新 Kubernetes 集群中安装 CoreDNS

要为全新 Kubernetes 集群安装 CoreDNS（而不是 kube-dns），我们需要使用 feature-gates 标志并将其设置为 CoreDNS=true。在安装全新 Kubernetes 集群时使用以下命令将 CoreDNS 安装为默认 DNS 服务。

# kubeadm init --feature-gates CoreDNS=true

# kubeadm init --feature-gates CoreDNS=true
[init] Using Kubernetes version: v1.9.0
[init] Using Authorization modes: [Node RBAC]
[preflight] Running pre-flight checks.
	[WARNING SystemVerification]: docker version is greater than the most recently validated version. Docker version: 17.09.1-ce. Max validated version: 17.03
	[WARNING FileExisting-crictl]: crictl not found in system path
[preflight] Starting the kubelet service
[certificates] Generated ca certificate and key.
[certificates] Generated apiserver certificate and key.
[certificates] apiserver serving cert is signed for DNS names [sandeep2 kubernetes kubernetes.default kubernetes.default.svc kubernetes.default.svc.cluster.local] and IPs [10.96.0.1 147.75.107.43]
[certificates] Generated apiserver-kubelet-client certificate and key.
[certificates] Generated sa key and public key.
[certificates] Generated front-proxy-ca certificate and key.
[certificates] Generated front-proxy-client certificate and key.
[certificates] Valid certificates and keys now exist in "/etc/kubernetes/pki"
[kubeconfig] Wrote KubeConfig file to disk: "admin.conf"
[kubeconfig] Wrote KubeConfig file to disk: "kubelet.conf"
[kubeconfig] Wrote KubeConfig file to disk: "controller-manager.conf"
[kubeconfig] Wrote KubeConfig file to disk: "scheduler.conf"
[controlplane] Wrote Static Pod manifest for component kube-apiserver to "/etc/kubernetes/manifests/kube-apiserver.yaml"
[controlplane] Wrote Static Pod manifest for component kube-controller-manager to "/etc/kubernetes/manifests/kube-controller-manager.yaml"
[controlplane] Wrote Static Pod manifest for component kube-scheduler to "/etc/kubernetes/manifests/kube-scheduler.yaml"
[etcd] Wrote Static Pod manifest for a local etcd instance to "/etc/kubernetes/manifests/etcd.yaml"
[init] Waiting for the kubelet to boot up the control plane as Static Pods from directory "/etc/kubernetes/manifests".
[init] This might take a minute or longer if the control plane images have to be pulled.
[apiclient] All control plane components are healthy after 31.502217 seconds
[uploadconfig] Storing the configuration used in ConfigMap "kubeadm-config" in the "kube-system" Namespace
[markmaster] Will mark node sandeep2 as master by adding a label and a taint
[markmaster] Master sandeep2 tainted and labelled with key/value: node-role.kubernetes.io/master=""
[bootstraptoken] Using token: 4cd282.a826a13b3705a4ec
[bootstraptoken] Configured RBAC rules to allow Node Bootstrap tokens to post CSRs in order for nodes to get long term certificate credentials
[bootstraptoken] Configured RBAC rules to allow the csrapprover controller automatically approve CSRs from a Node Bootstrap Token
[bootstraptoken] Configured RBAC rules to allow certificate rotation for all node client certificates in the cluster
[bootstraptoken] Creating the "cluster-info" ConfigMap in the "kube-public" namespace
[addons] Applied essential addon: CoreDNS
[addons] Applied essential addon: kube-proxy

Your Kubernetes master has initialized successfully!

如果在部署 Kubernetes 时看到以下输出，则确认 CoreDNS 已安装。

[addons] Applied essential addon: CoreDNS

将现有集群更新为使用 CoreDNS

如果您有现有集群，也可以使用 kubeadm upgrade 命令将您的 DNS 服务升级到 CoreDNS，以替换 kube-dns。

在继续应用更改之前，可以通过使用 kubeadm upgrade plan 和将 feature-gates 标志设置为 CoreDNS=true 来检查将要安装的 CoreDNS 版本。

检查要升级的 CoreDNS 版本

# kubeadm upgrade plan  --feature-gates CoreDNS=true

# kubeadm upgrade plan  --feature-gates CoreDNS=true
...

Components that must be upgraded manually after you have upgraded the control plane with 'kubeadm upgrade apply':
COMPONENT   CURRENT      AVAILABLE
Kubelet     1 x v1.9.0   v1.10.0-alpha.1

Upgrade to the latest experimental version:

COMPONENT            CURRENT   AVAILABLE
API Server           v1.9.0    v1.10.0-alpha.1
Controller Manager   v1.9.0    v1.10.0-alpha.1
Scheduler            v1.9.0    v1.10.0-alpha.1
Kube Proxy           v1.9.0    v1.10.0-alpha.1
CoreDNS              1.0.1     1.0.1
Etcd                 3.1.10    3.1.10

You can now apply the upgrade by executing the following command:

	kubeadm upgrade apply v1.10.0-alpha.1

Note: Before you can perform this upgrade, you have to update kubeadm to v1.10.0-alpha.1.

然后可以执行 kubeadm upgrade apply 和 feature-gates CoreDNS=true，使用 CoreDNS 作为默认 DNS 升级集群

# kubeadm upgrade apply <version> --feature-gates CoreDNS=true 

# kubeadm upgrade apply v1.10.0-alpha.1  --feature-gates CoreDNS=true 
[preflight] Running pre-flight checks.
[upgrade] Making sure the cluster is healthy:
[upgrade/config] Making sure the configuration is correct:
[upgrade/config] Reading configuration from the cluster...
[upgrade/config] FYI: You can look at this config file with 'kubectl -n kube-system get cm kubeadm-config -oyaml'
[upgrade/version] You have chosen to change the cluster version to "v1.10.0-alpha.1"
[upgrade/versions] Cluster version: v1.10.0-alpha.1
[upgrade/versions] kubeadm version: v1.9.0
[upgrade/version] Found 1 potential version compatibility errors but skipping since the --force flag is set: 

	- Specified version to upgrade to "v1.10.0-alpha.1" is at least one minor release higher than the kubeadm minor release (10 > 9). Such an upgrade is not supported
[upgrade/prepull] Will prepull images for components [kube-apiserver kube-controller-manager kube-scheduler]
[upgrade/apply] Upgrading your Static Pod-hosted control plane to version "v1.10.0-alpha.1"...
[upgrade/staticpods] Writing new Static Pod manifests to "/etc/kubernetes/tmp/kubeadm-upgraded-manifests781134294"
[controlplane] Wrote Static Pod manifest for component kube-apiserver to "/etc/kubernetes/tmp/kubeadm-upgraded-manifests781134294/kube-apiserver.yaml"
[controlplane] Wrote Static Pod manifest for component kube-controller-manager to "/etc/kubernetes/tmp/kubeadm-upgraded-manifests781134294/kube-controller-manager.yaml"
[controlplane] Wrote Static Pod manifest for component kube-scheduler to "/etc/kubernetes/tmp/kubeadm-upgraded-manifests781134294/kube-scheduler.yaml"
[upgrade/staticpods] Moved new manifest to "/etc/kubernetes/manifests/kube-apiserver.yaml" and backed up old manifest to "/etc/kubernetes/tmp/kubeadm-backup-manifests038673725/kube-apiserver.yaml"
[upgrade/staticpods] Waiting for the kubelet to restart the component
[apiclient] Found 1 Pods for label selector component=kube-apiserver
[upgrade/staticpods] Component "kube-apiserver" upgraded successfully!
[upgrade/staticpods] Moved new manifest to "/etc/kubernetes/manifests/kube-controller-manager.yaml" and backed up old manifest to "/etc/kubernetes/tmp/kubeadm-backup-manifests038673725/kube-controller-manager.yaml"
[upgrade/staticpods] Waiting for the kubelet to restart the component
[apiclient] Found 1 Pods for label selector component=kube-controller-manager
[upgrade/staticpods] Component "kube-controller-manager" upgraded successfully!
[upgrade/staticpods] Moved new manifest to "/etc/kubernetes/manifests/kube-scheduler.yaml" and backed up old manifest to "/etc/kubernetes/tmp/kubeadm-backup-manifests038673725/kube-scheduler.yaml"
[upgrade/staticpods] Waiting for the kubelet to restart the component
[apiclient] Found 1 Pods for label selector component=kube-scheduler
[upgrade/staticpods] Component "kube-scheduler" upgraded successfully!
[uploadconfig] Storing the configuration used in ConfigMap "kubeadm-config" in the "kube-system" Namespace
[bootstraptoken] Configured RBAC rules to allow Node Bootstrap tokens to post CSRs in order for nodes to get long term certificate credentials
[bootstraptoken] Configured RBAC rules to allow the csrapprover controller automatically approve CSRs from a Node Bootstrap Token
[bootstraptoken] Configured RBAC rules to allow certificate rotation for all node client certificates in the cluster
[addons] Applied essential addon: CoreDNS
[addons] Applied essential addon: kube-proxy

[upgrade/successful] SUCCESS! Your cluster was upgraded to "v1.10.0-alpha.1". Enjoy!

[upgrade/kubelet] Now that your control plane is upgraded, please proceed with upgrading your kubelets in turn.

验证 CoreDNS 服务

要验证 CoreDNS 是否正在运行，我们可以检查节点中的 pod 状态和部署。请注意，CoreDNS 服务将保持为 “kube-dns”，这确保了从 kube-dns 向 CoreDNS 升级服务发现期间的平稳过渡。

检查 pod 状态

# kubectl -n kube-system get pods -o wide
NAME                               READY     STATUS    RESTARTS   AGE       IP              NODE
coredns-546545bc84-ttsh4           1/1       Running   0          5h        10.32.0.61      sandeep2
etcd-sandeep2                      1/1       Running   0          5h        147.75.107.43   sandeep2
kube-apiserver-sandeep2            1/1       Running   0          4h        147.75.107.43   sandeep2
kube-controller-manager-sandeep2   1/1       Running   0          4h        147.75.107.43   sandeep2
kube-proxy-fkxmg                   1/1       Running   0          4h        147.75.107.43   sandeep2
kube-scheduler-sandeep2            1/1       Running   4          5h        147.75.107.43   sandeep2
weave-net-jhjtc                    2/2       Running   0          5h        147.75.107.43   sandeep2

检查 部署

# kubectl -n kube-system get deployments
NAME      DESIRED   CURRENT   UP-TO-DATE   AVAILABLE   AGE
coredns   1            1          1           1        4h

我们可以通过一些基本的 dig 命令来检查 CoreDNS 是否正常运行

# dig @10.32.0.61 kubernetes.default.svc.cluster.local +noall +answer

; <<>> DiG 9.10.3-P4-Ubuntu <<>> @10.32.0.61 kubernetes.default.svc.cluster.local +noall +answer
; (1 server found)
;; global options: +cmd
kubernetes.default.svc.cluster.local. 23 IN A	10.96.0.1

# dig @10.32.0.61 ptr 1.0.96.10.in-addr.arpa. +noall +answer

; <<>> DiG 9.10.3-P4-Ubuntu <<>> @10.32.0.61 ptr 1.0.96.10.in-addr.arpa. +noall +answer
; (1 server found)
;; global options: +cmd
1.0.96.10.in-addr.arpa.	30	IN	PTR	kubernetes.default.svc.cluster.local.

Sandeep Rajan
发布：并标记 服务发现、DNS、文档、Kube-DNS、kubeadm、Kubernetes 以及 服务，使用 1558 个单词。