使用minukube部署kubernetes admission webhook实现etcd pod安全删除

本需求来自于一道面试题😂（本环境使用centos 7）

最好使用阿里云ec2服务器安装minikube，若使用本地pc的vmware可能会出现网络方面的问题。

使用如下命令安装minikube，参见install minikube

# curl -Lo minikube https://storage.googleapis.com/minikube/releases/latest/minikube-linux-amd64 && chmod +x minikube
# sudo cp minikube /usr/local/bin && rm minikube

启动minikube可能会遇到docker版本过旧导致启动失败的问题。默认centos下面yum安装的docker版本比较旧，需要安装最新docker，更新docker参见Get Docker CE for CentOS

如下命令移除已经安装的docker

yum remove docker \
                  docker-client \
                  docker-client-latest \
                  docker-common \
                  docker-latest \
                  docker-latest-logrotate \
                  docker-logrotate \
                  docker-engine

如下命令安装存储驱动和设置docker stable版本的库

yum install -y yum-utils \
  device-mapper-persistent-data \
  lvm2

yum-config-manager \
    --add-repo \
    https://download.docker.com/linux/centos/docker-ce.repo

使用如下命令即可更新为最新的docker

yum install docker-ce docker-ce-cli containerd.io

管理kubernetes上的服务最好使用helm(本次未用到，如无需要可忽略本节)，helm安装如下：

使用如下方式获取helm的二进制版本

Download your desired version
Unpack it (tar -zxvf helm-v2.0.0-linux-amd64.tgz)
Find the helm binary in the unpacked directory, and move it to its desired destination (mv linux-amd64/helm /usr/local/bin/helm)

 tiller安装给出了在不同scope下面安装tiller的方法，最简单的是在cluster-admin下面安装即可（生成环境下建议参考helm安装文档将权限最小化）

# cat rbac-config.yaml
apiVersion: v1
kind: ServiceAccount
metadata:
  name: tiller
  namespace: kube-system
---
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRoleBinding
metadata:
  name: tiller
roleRef:
  apiGroup: rbac.authorization.k8s.io
  kind: ClusterRole
  name: cluster-admin
subjects:
  - kind: ServiceAccount
    name: tiller
    namespace: kube-system

使用如下命令创建即可

kubectl create -f rbac-config.yaml
serviceaccount "tiller" created
clusterrolebinding "tiller" created
$ helm init --service-account tiller

admission webhook原理

kubernetes的认证和授权是对客户端进行认证以及对资源进行授权，但在资源的使用处理上不够细化。admission webhook是在一种在改变资源的持久化之前（比如某些资源的创建或删除，修改等之前）的机制。参见官方资料

如下图，在部署了admission webhook之后，apiserver会发送一个AdmissionReview的json数据到webhook，其中主要包含一个AdmissionRequest的请求，AdmissionRequest.RawExtension.Raw包含了需要处理的kubernetes组件(如pod，deployment等)的详细信息。webhook在接收到该请求之后会根据自定义逻辑进行处理，并返回处理结果AdmissionResponse。admission webhook有两种处理方式：

MutatingAdmissionWebhook：可以修改可以自定义的策略MutatingAdmissionWebhook的处理一般优先于MutatingAdmissionWebhook，这样前者修改的内容就可以由后者进行校验

ValidatingAdmissionWebhook: 允许或拒绝客户自定义的策略

type AdmissionReview struct {
    metav1.TypeMeta `json:",inline"`
    // Request describes the attributes for the admission request.
    // +optional
    Request *AdmissionRequest `json:"request,omitempty" protobuf:"bytes,1,opt,name=request"`
    // Response describes the attributes for the admission response.
    // +optional
    Response *AdmissionResponse `json:"response,omitempty" protobuf:"bytes,2,opt,name=response"`
}

 

 处理流程图如下，可以看到MutatingAdmissionWebhook的处理优先于MutatingAdmissionWebhook

使用如下命令启动minikube

minikube start --vm-driver=none --extra-config=apiserver.enable-admission-plugins="NamespaceLifecycle,LimitRanger,ServiceAccount,DefaultStorageClass,DefaultTolerationSeconds,MutatingAdmissionWebhook,ValidatingAdmissionWebhook,Priority,ResourceQuota"

 使用kubectl api-versions查看是否支持admissionregistration.k8s.io/v1alpha1 API

In-depth introduction to Kubernetes admission webhooks是个很好的例子，里面详细记录了创建admission webhook的方式。这里参照In-depth introduction to Kubernetes admission webhooks实现了根据etcd pod角色(leader和非leader)来删除pod。

首先使用webhook-create-signed-cert.sh文件来生成自签证书，后续ValidatingWebhookConfiguration中会用到

# ./deployment/webhook-create-signed-cert.sh

部署admission webhook

# kubectl create -f deployment/deployment.yaml
# kubectl create -f deployment/service.yaml

生成待CA bundle的config文件以及validatingwebhook.yaml原始文件如下

# cat ./deployment/validatingwebhook.yaml | ./deployment/webhook-patch-ca-bundle.sh > ./deployment/validatingwebhook-ca-bundle.yaml

apiVersion: admissionregistration.k8s.io/v1beta1
kind: ValidatingWebhookConfiguration
metadata:
  name: validation-webhook-example-cfg
  labels:
    app: admission-webhook-example
webhooks:
  - name: required-labels.banzaicloud.com
    clientConfig:
      service:
        name: admission-webhook-example-svc #service名称
        namespace: default
        path: "/validate" #访问的后缀路径
      caBundle: ${CA_BUNDLE} #上述命令生成的认证字段
    rules:
      - operations: [ "DELETE" ] #操作的动作
        apiGroups: ["apps", ""]  #api groups
        apiVersions: ["v1"]      #api version
        resources: ["pods"]      #操作的资源
    namespaceSelector:
      matchLabels:
        admission-webhook-example: enabled # 限制default的命名空间

给default namespace打上标签

$ kubectl label namespace default admission-webhook-example=enabled

创建ValidatingWebhookConfiguration

$ kubectl create -f deployment/validatingwebhook-ca-bundle.yaml

admission-webhook-example/build用于编译和生成容器镜像

etcd的部署直接使用admission-webhook-example\deployment\etcd中的配置文件即可，这样在删除etcd的leader时会显示如下内容

admission webhook的实现只有2个文件，main.go和webhook.go。main.go中通过mux.HandleFunc("/validate", whsvr.serve)来启动一个http服务处理路径/validate(对应validatingwebhook.yaml的webhooks.clientConfig.service.path)的请求。主要逻辑是现在函数func (whsvr *WebhookServer) validate(ar *v1beta1.AdmissionReview) *v1beta1.AdmissionRespons中

FAQ：

• 执行yum install -y socat可解决如下问题：

an error occurred forwarding 40546 -> 44134: error forwarding port 44134 to pod 3ea221f842e1446a5fd9da9fc29e7e415a1ecb6dd6d07c56f64fb4788f2c3915, uid : unable to do port forwarding: socat not found.

TIPS:

•  使用如下方式可以安装etcdctl命令行工具

# choose either URL
GOOGLE_URL=https://storage.googleapis.com/etcd
GITHUB_URL=https://github.com/coreos/etcd/releases/download
DOWNLOAD_URL=${GOOGLE_URL}

rm -f /tmp/etcd-${ETCD_VER}-linux-amd64.tar.gz
rm -rf /tmp/etcd-download-test && mkdir -p /tmp/etcd-download-test

curl -L ${DOWNLOAD_URL}/${ETCD_VER}/etcd-${ETCD_VER}-linux-amd64.tar.gz -o /tmp/etcd-${ETCD_VER}-linux-amd64.tar.gz
tar xzvf /tmp/etcd-${ETCD_VER}-linux-amd64.tar.gz -C /tmp/etcd-download-test --strip-components=1
rm -f /tmp/etcd-${ETCD_VER}-linux-amd64.tar.gz

/tmp/etcd-download-test/etcd --version
ETCDCTL_API=3 /tmp/etcd-download-test/etcdctl version
# start a local etcd server
/tmp/etcd-download-test/etcd

# write,read to etcd
ETCDCTL_API=3 /tmp/etcd-download-test/etcdctl --endpoints=localhost:2379 put foo bar
ETCDCTL_API=3 /tmp/etcd-download-test/etcdctl --endpoints=localhost:2379 get foo

• go get -u github.com/golang/dep/cmd/dep 安装dep
• etcdv3的API使用可以参考package clientv3

参考：

https://github.com/helm/helm/blob/master/docs/install.md

https://container-solutions.com/some-admission-webhook-basics/

https://kubernetes.io/docs/reference/access-authn-authz/extensible-admission-controllers/

https://github.com/kubernetes/kubernetes/tree/master/test/e2e/testing-manifests/statefulset/etcd

https://github.com/morvencao/kube-mutating-webhook-tutorial/blob/master/medium-article.md