一文读懂什么是K8s Admission Controller

#作者：曹付江

文章目录

1、什么是 Admission Controllers？
2、如何创建 Admission Controllers？
3、Admission 控制器的最佳实践

K8s 中的操作与安全标准执行机制：
在这里插入图片描述

1、什么是 Admission Controllers？

Admission controllers 是一段拦截请求的代码，它会在请求被持久化之前验证或更改（变更）这些请求。Admission controllers 与外部 WebHook 连接，处理入驻请求。因此，它们也被称为 Admission WebHook。该 WebHook 在对象创建、修改或删除最终确定之前接收 K8s 的入驻请求。

我们可以定义两种类型的 Admission controllers：

MutatingAdmissionWebhook：该控制器既可以验证也可以更改请求。
ValidatingAdmissionWebhook：该控制器只能验证请求。
Admission Controller 是在需要强制执行特定组织政策时最有效的选项之一，例如：
确保所有容器都有资源限制，以避免资源占用过多。
阻止某些常见的镜像标签。
强制执行资源的命名规范。
限制仅允许来自受信任注册表的批准容器镜像。
检查内建的 Admission controllers：

2、如何创建 Admission Controllers？

要创建一个 Mutating Admission Webhook，我们需要以下内容：

用于 Webhook 的 Web 应用程序
运行 Web 应用程序的服务帐户
用于托管 Web 应用程序的 Deployment
用于将流量路由到 Web 应用程序的 Service
一个 ClusterRole，定义 API 级别的访问权限（此博客未详细介绍）
一个 ClusterRoleBinding，将服务帐户与 ClusterRole 关联
一个 Secret，包含 Web 应用程序用于提供 TLS 的证书。（K8s 会向我们的服务发起 HTTPS 调用，因此需要有效的 SSL/TLS 证书）

WebHook 服务器
在本文章中，我将使用基于 Python 的 Webhook，使用 Sanic 框架。

FROM sanicframework/sanic:LTS
RUN pip install jsonpatch
COPY apps.py apps.py
ENTRYPOINT ["sanic", "apps:app", "--host=0.

上面的 Dockerfile 将创建用于运行 Web 服务器的镜像。这里我们挂载了一个卷（来自 secret），该卷将用于 TLS 证书。

apps.py 的内容：

import base64
import json as nativejson
from copy import deepcopy
from pprint import pformatimport jsonpatch
from sanic import Request, Sanic
from sanic.log import logger
from sanic.response import jsonapp = Sanic(name=__name__)@app.post("/validate")
async def validate(request: Request):allowed = Truemessage = ""try:pass # add logic to validate th# update allowed & message accordinglyexcept KeyError:passreturn json({"response": {"allowed": allowed,"uid": request.json["request"]["uid"],"status": {"message": message},}})@app.post("/mutate")
async def mutate(request: Request):logger.info(f"Obtained Request \n {pformat(request.json)}")original_spec = request.json["request"]["object"]modified_spec = deepcopy(spec)try:pass# add logic to update# update the modified_specexcept KeyError:passpatch = jsonpatch.JsonPatch.from_diff(spec, modified_spec)return json({"response": {"allowed": True,"uid": request.json["request"]["uid"],"patch": base64.b64encode(nativejson.dumps(patch).encode()).decode(),"patchtype": "JSONPatch",}})if __name__ == "__main__":app.run(host="0.0.0.0", port=443)

Kubernetes 对验证的期望是返回一个简单的响应，值为 False 或 True。因此，在上述代码中，我们返回 allowed 为 True 或 False。在变更 Webhook 的情况下，我们还返回补丁（新旧对象的差异）。

创建 Webhook 的清单：

apiVersion: v1
kind: ServiceAccount
metadata:name: admission-controllernamespace: admission-webhook-ns
---
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRoleBinding
metadata:name: admission-demo-rbac
subjects:- kind: ServiceAccountname: admission-controllernamespace: admission-webhook-ns
roleRef:kind: ClusterRolename: cluster-adminapiGroup: rbac.authorization.k8s.io---
apiVersion: v1
kind: Service
metadata:name: admission-webhook-svcnamespace: admission-webhook-nslabels:app: admission-webhook-demo
spec:ports:- port: 443targetPort: 3030selector:app: admission-webhook-demo
---
apiVersion: cert-manager.io/v1
kind: Certificate
metadata:name: admission-controller-certnamespace: admission-webhook-ns
spec:dnsNames:- admission-webhook-svc- admission-webhook-svc.admission-webhook-ns- admission-webhook-svc.admission-webhook-ns.svcissuerRef:kind: ClusterIssuername: ca-issuersecretName: admission-controller-secret
---
apiVersion: apps/v1
kind: Deployment
metadata:name: admission-webhook-deploymentnamespace: admission-webhook-nslabels:app: admission-webhook-demo
spec:replicas: 1selector:matchLabels:app: admission-webhook-demotemplate:metadata:labels:app: admission-webhook-demospec:serviceAccountName: admission-controllercontainers:- name: admission-webhook-demoimage: asrathore08/admission-controller:latestimagePullPolicy: Alwaysresources:limits:cpu: 1000mmemory: 512MivolumeMounts:- name: webhook-certsmountPath: /mnt/certsreadOnly: true- name: admission-controller-confmountPath: /mnt/confvolumes:- name: webhook-certssecret:secretName: admission-controller-secret- name: admission-controller-confconfigMap:name: admission-controller-configmap

在 Kubernetes 中注册 Webhook：
创建一个 ValidatingWebhookConfiguration，指向我们的 webhook 服务器。

apiVersion: admissionregistration.k8s.io/v1
kind: ValidatingWebhookConfiguration
metadata:name: admission-service-delete-pod-validatenamespace: admission-webhook-nslabels:app: admission-webhook-demoannotations:cert-manager.io/inject-ca-from: admission-webhook-ns/admission-controller-cert
webhooks:
- name: validate.webhook.democlientConfig:service:name: admission-webhook-svcnamespace: admission-webhook-nspath: "/validate"admissionReviewVersions: ["v1"]sideEffects: Nonerules:- operations: ["CREATE", "UPDATE"]apiGroups: [""]apiVersions: ["v1", "v1beta1"]resources: ["configmaps"]scope: "Namespaced"namespaceSelector:matchLabels:demo-admission-validation: enabledfailurePolicy: Ignore

注意：这里我使用了 cert-manager CRD 来生成 TLS/SSL 证书。注解 cert-manager.io/inject-ca-from 有助于获取证书。

创建一个 MutatingWebhookConfiguration，指向我们的 webhook 服务器：

apiVersion: admissionregistration.k8s.io/v1
kind: MutatingWebhookConfiguration
metadata:name: admission-service-configmap-mutatenamespace: admission-webhook-nslabels:app: mutating-admission-webhookannotations:cert-manager.io/inject-ca-from: admission-webhook-ns/admission-controller-cert
webhooks:
- name: mutate.webhook.democlientConfig:service:name: admission-webhook-svcnamespace: admission-webhook-nspath: "/mutate"admissionReviewVersions: ["v1"]sideEffects: Nonerules:- operations: ["CREATE", "UPDATE"]apiGroups: [""]apiVersions: ["v1", "v1beta1"]resources: ["configmaps"]scope: "Namespaced"namespaceSelector:matchLabels:demo-admission-validation: enabledfailurePolicy: Ignore

上面，我们定义了一个 mutating 和一个 validating webhook。我们只启用了这些 webhooks 处理在具有标签 demo-admission-validation 设置为 enabled 的命名空间中创建或更新的对象。此功能允许我们控制 webhook 的作用范围。

MutatingAdmissionWebhook — 转换器
ValidatingAdmissionWebhook — 审核员

注意：在上述 webhook 中，我将资源限制为 configmap。K8s 上的 Apache Spark 使用 configmap 来存储 Spark 配置。这种 webhook 可以控制用户为某些配置提供的值，并且可以为某些配置注入特定的默认值。

一旦部署，Webhook 将接收以下请求体的请求。

#
{"kind": "AdmissionReview","request": {"kind": {"kind": "Pod","version": "v1","group": ""},"resource": {"resource": "pods","version": "v1","group": ""},"uid": "b06b6ec2-681d-11e9-a645-06b44ed6a042","object": {"status": {},"spec": {"dnsPolicy": "ClusterFirst","securityContext": {},"serviceAccountName": "","schedulerName": "default-scheduler","serviceAccount": "","priority": 0,"terminationGracePeriodSeconds": 30,"restartPolicy": "Always","containers": [{"name": "","image": "","imagePullPolicy": "Always","ports": [{"protocol": "TCP","containerPort": 80}],"resources": {}}]},"metadata": {}},"namespace": "","userInfo": {"username": "","groups": ["system:masters","system:authenticated"]},"oldObject": null,"dryRun": false,"operation": "CREATE"},"apiVersion": "admission.k8s.io/v1beta1"
}

控制器将以特定格式返回响应。响应格式如下所述。

# Response
{"body": {"kind": "AdmissionReview","apiVersion": "admission.k8s.io/v1","response": {"uid": "request.uid","allowed": "True","patch": "patch_base64","patchType": "JSONPatch"},"headers": {"Content-Type": "application/json"},"statusCode": 200
}# patch_base64[{"op": "replace","path": "/spec/containers/0/image","value": "xxxx.dkr.ecr.us-west-2.amazonaws.com/nginx:latest"}
]