使用 kubeadm 部署 Kubernetes 與 CoreDNS

透過 kubeadm 在 Kubernetes 中安裝 CoreDNS 的指南

Kubernetes 1.9 近期已發佈，其中包含 CoreDNS 作為一部分。我們現在可以透過 Kubeadm 安裝 CoreDNS 作為預設的服務探索工具，Kubeadm 是一個可以單一步驟輕鬆安裝 Kubernetes 的工具組。

目前，CoreDNS 在 Kubernetes 1.9 中為 Alpha 階段。我們有一個路線圖，將在 1.10 版本中使 CoreDNS 進入 Beta 階段，並最終成為預設的 DNS，取代 kube-dns。

請務必注意，目前從 kube-dns 切換到 CoreDNS 時，kube-dns 附帶的組態（stubzones、federations...）將不再存在，並會切換到 CoreDNS 中的預設組態。kube-dns 組態的轉換將在即將推出的 Kubernetes 版本 (v1.10) 中支援，屆時 CoreDNS 將為 Beta 版。

瞭解 CoreDNS 組態

這是 kubeadm 提供的預設 CoreDNS 組態。它儲存在名為 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 的組態。此組態基於 CoreDNS 的下列外掛程式

  • errors：錯誤會記錄到 stdout。
  • health：CoreDNS 的健康狀況會回報至 http://localhost:8080/health。
  • kubernetes：CoreDNS 將根據 Kubernetes 服務和 Pod 的 IP 回應 DNS 查詢。您可以在此處找到更多詳細資訊。

Kubernetes 外掛程式的選項 叢集網域 和 服務 CIDR 預設透過 kubeadm 分別定義為 cluster.local 和 10.96.0.0/12。我們可以透過 kubeadm 的 --service-dns-domain 和 --service-cidr 標誌修改並選擇所需的值。

pods insecure 選項是為了與 kube-dns 向後相容而提供。

Upstream 用於解析指向外部主機的服務 (外部服務)。

  • prometheus：CoreDNS 的指標可在 http://localhost:9153/metrics 以 Prometheus 格式取得。
  • proxy：任何不在 Kubernetes 叢集網域內的查詢都將轉發到預定義的解析器 (/etc/resolv.conf)。
  • cache：這會啟用前端快取。

我們可以透過修改此 configmap 來修改預設行為。需要重新啟動 CoreDNS Pod，變更才會生效。

在新 Kubernetes 叢集中安裝 CoreDNS

為了為新的 Kubernetes 叢集安裝 CoreDNS 而非 kube-dns，我們需要使用 feature-gates 標誌並將其設定為 CoreDNS=true。在安裝新的 Kubernetes 叢集時，使用以下命令安裝 CoreDNS 作為預設的 DNS 服務。

# 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 時看到以下輸出，則表示 CoreDNS 安裝已確認。

[addons] Applied essential addon: CoreDNS

更新您現有的叢集以使用 CoreDNS

如果您有現有的叢集，也可以使用 kubeadm upgrade 命令將您的 DNS 服務升級到 CoreDNS，取代 kube-dns。

可以透過使用 kubeadm upgrade plan 並將 feature-gates 標誌設定為 CoreDNS=true，在套用變更之前檢查將安裝的 CoreDNS 版本。

檢查要升級的 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.

然後可以使用 kubeadm upgrade apply 和 feature-gates CoreDNS=true 執行以 CoreDNS 作為預設 DNS 的叢集升級。

# 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.

驗證 CoreDNS 服務

要驗證 CoreDNS 是否正在執行，我們可以檢查節點中的 Pod 狀態和部署。請注意，此處 CoreDNS 服務將保持為「kube-dns」，這可確保從 kube-dns 升級服務探索到 CoreDNS 時的順利過渡。

檢查 pod 狀態

# 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

檢查 Deployment

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

我們可以透過一些基本的 dig 命令來檢查 CoreDNS 是否正常運作

# 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
發佈時間：，並加上 探索、DNS、文件、Kube-DNS、Kubeadm、Kubernetes 和 服務 的標籤，總計 1558 個字。