Deploying Kubernetes with CoreDNS using kubeadm

DNS（域名系統）解析是網絡瀏覽器訪問在線資源的第一個請求，DNS基於名稱的網絡資源索引機制能夠實現服務和節點按照名稱訪問，從而與具體的IP地址隔離，是實現網絡請求重定向、節點擴容、負載均衡、容錯、故障轉移、在線更新的重要機制，也被某些系統用於網絡阻斷的機制，所以DNS的可靠運行很是重要。Kubernetes內置有DNS機制，但CoreDNS實現了一些重要的特徵，功能更強大，這裏介紹其配置方法。

參考：

• 官方網站，https://coredns.io/
• CoreDNS安裝，http://www.javashuo.com/article/p-zkjxtqex-bk.html
• CoreDNS使用手冊，https://coredns.io/manual/toc/
• CoreDNS源碼，https://github.com/coredns
• CoreDNS配置，http://www.javashuo.com/article/p-nvjbargr-cm.html

Deploying Kubernetes with CoreDNS using kubeadm

A guide to installing CoreDNS in Kubernetes via kubeadm

 8 minute read  Published: 29 Jan, 2018 
Sandeep Rajan  

Kubernetes 1.9 has recently been launched and it ships with CoreDNS being a part of it. We can now install CoreDNS as the default service discovery via Kubeadm, which is the toolkit to install Kubernetes easily in a single step.

Currently, CoreDNS is Alpha in Kubernetes 1.9. We have a roadmap which will make CoreDNS Beta in version 1.10 and eventually be the default DNS, replacing kube-dns.

It is important to note that currently when switching from kube-dns to CoreDNS, the configurations that come with kube-dns (stubzones, federations…) will no longer exist and will switch to a default configuration in CoreDNS. The translation of the configurations from kube-dns shall be supported from the upcoming version of Kubernetes (v1.10) where CoreDNS will be Beta.

Understanding CoreDNS Configuration

This is the default CoreDNS configuration shipped with kubeadm. It is saved in a kubernetes configmap named coredns:

# 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

The Corefile part is the configuration of CoreDNS. This configuration is based on the following plugins of CoreDNS:

• errors: Errors are logged to stdout.
• health: Health of CoreDNS is reported to http://localhost:8080/health.
• kubernetes: CoreDNS will reply to DNS queries based on IP of the services and pods of Kubernetes. You can find more details here.

The Kubernetes plugin has its options Cluster Domain and Service CIDR defined as cluster.local and 10.96.0.0/12 respectively by default through kubeadm. We can modify and choose the desired values through the kubeadm --service-dns-domain and --service-cidr flags.

The pods insecure option is provided for backward compatibility with kube-dns.

Upstream is used for resolving services that point to external hosts (External Services).

• prometheus: Metrics of CoreDNS are available at http://localhost:9153/metrics in Prometheus format.
• proxy: Any queries that are not within the cluster domain of Kubernetes will be forwarded to predefined resolvers (/etc/resolv.conf).
• cache: This enables a frontend cache.

We can modify the default behavior by modifying this configmap. A restart of the CoreDNS pod is required for the changes to take effect.

Installing CoreDNS in fresh Kubernetes cluster

In order to install CoreDNS instead of kube-dns for a fresh Kubernetes cluster, we need to use the feature-gates flag and set it to CoreDNS=true. Use the following command to install CoreDNS as default DNS service while installing a fresh Kubernetes cluster.

# 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!

CoreDNS install is confirmed if we see the following output while deploying Kubernetes.

[addons] Applied essential addon: CoreDNS

Updating your existing cluster to use CoreDNS

In case you have an existing cluster, it is also possible to upgrade your DNS service to CoreDNS, replacing kube-dns, using the kubeadm upgrade command.

It is possible to check the CoreDNS version that will be installed before proceeding to apply the changes by using kubeadm upgrade plan and by setting feature-gates flag as CoreDNS=true.

Checking the CoreDNS version to upgrade:

# 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.

The upgrade of the cluster with CoreDNS as the default DNS can then be performed using kubeadm upgrade apply and feature-gates CoreDNS=true:

# 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.

Verifying CoreDNS Service

To verify CoreDNS is running, we can check the pod status and deployment in the node. Note here that the CoreDNS service will remain as 「kube-dns」 which ensures a smooth transition while upgrading your service discovery from kube-dns to CoreDNS.

Check pod status:

# 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

Check Deployment:

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

We can check if CoreDNS is functioning normally through a few basic dig commands:

# 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   
Published: 29 Jan, 2018 and tagged DNS, Discovery, Documentation, Kube-DNS, Kubeadm, Kubernetesand Service using 1558 words.

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