昨天收到一个朋友的信息，说不小心把集群的业务 namespace 干掉了，导致整个业务都停滞了，问我有没有禁止删除 namespace 的方案。
在我的记忆里，Kubernetes 的准入里并没有这个控制器，所以我就给他说需要自己开发一个准入控制器来实现自己的目标。
作为人，何为正确！我不能只脱裤子，不放屁。所以这里也整理了一下如何自定义 Kubernetes 的准入控制器。
理论介绍
准入控制器（Admission Controller）位于 API Server 中，在对象被持久化之前，准入控制器拦截对 API Server 的请求，一般用来做身份验证和授权。其中包含两个特殊的控制器：MutatingAdmissionWebhook 和 ValidatingAdmissionWebhook。
- MutatingAdmissionWebhook ：用于变更请求对象，比如 istio 为每个 Pod 注入 sidecar，就是通过它实现。
- ValidatingAdmissionWebhook：用于验证请求对象
整个准入控制器的流程如下：
当 API 请求进入时，mutating 和 validating 控制器使用配置中的外部 webhooks 列表并发调用，规则如下：
- 如果所有的 webhooks 批准请求，准入控制链继续流转。
- 如果有任意一个 webhooks 阻止请求，那么准入控制请求终止，并返回第一个 webhook 阻止的原因。其中，多个 webhooks 阻止也只会返回第一个 webhook 阻止的原因。
- 如果在调用 webhook 过程中发生错误，那么请求会被终止或者忽略 webhook。
准入控制器是在 API Server 的启动参数重配置的。一个准入控制器可能属于以上两者中的一种，也可能两者都属于。
我们在部署 Kubernetes 集群的时候都会默认开启一系列准入控制器，如果没有设置这些准入控制器的话可以说你的 Kubernetes 集群就是在裸奔，应该叫管理员为集群添加准入控制器。
代码实现
实现逻辑
在开发之前先大致了解一下准入控制器的 Webhook 的大致实现逻辑：
- Webhook 是一个标准的 HTTP 服务，接收 HTTP 请求
- 接收到的请求是一个 AdmissionReview 对象
- 然后我们自定义的 Hook 会处理这个 AdmissionReview 对象
- 处理完过后再返回一个 AdmissionReview 对象，这里面会包含处理结果
AdmissionReview 的结构体如下：
// AdmissionReview describes an admission review request/response.
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"`
}
从代码的命名中可以很清晰的看出，在请求发送到 WebHook 时我们只需要关注内部的 AdmissionRequest(实际入参)，在我们编写的 WebHook 处理完成后只需要返回包含有 AdmissionResponse(实际返回体) 的 AdmissionReview 对象即可；总的来说 AdmissionReview 对象是个套壳，请求是里面的 AdmissionRequest，响应是里面的 AdmissionResponse。
具体实现
（1）首先创建一个 HTTP Server，监听端口，接收请求
package main
import (
"context"
"flag"
"github.com/joker-bai/validate-namespace/http"
log "k8s.io/klog/v2"
"os"
"os/signal"
"syscall"
)
var (
tlscert, tlskey, port string
)
func main() {
flag.StringVar(&tlscert, "tlscert", "/etc/certs/cert.pem", "Path to the TLS certificate")
flag.StringVar(&tlskey, "tlskey", "/etc/certs/key.pem", "Path to the TLS key")
flag.StringVar(&port, "port", "8443", "The port to listen")
flag.Parse()
server := http.NewServer(port)
go func() {
if err := server.ListenAndServeTLS(tlscert, tlskey); err != nil {
log.Errorf("Failed to listen and serve: %v", err)
}
}()
log.Infof("Server running in port: %s", port)
// listen shutdown signal
signalChan := make(chan os.Signal, 1)
signal.Notify(signalChan, syscall.SIGINT, syscall.SIGTERM)
<-signalChan
log.Info("Shutdown gracefully...")
if err := server.Shutdown(context.Background()); err != nil {
log.Error(err)
}
}
由于准入控制器和 Webhook 之间需要使用 TLS 进行通信，所以上面监听的端口是 TLS 端口，通过 server.ListenAndServeTLS 实现，后续在部署服务的时候需要把证书挂到相应的目录中。
（2）定义 Handler，将请求分发到具体的处理方法
package http
import (
"fmt"
"github.com/joker-bai/validate-namespace/namespace"
"net/http"
)
// NewServer creates and return a http.Server
func NewServer(port string) *http.Server {
// Instances hooks
nsValidation := namespace.NewValidationHook()
// Routers
ah := newAdmissionHandler()
mux := http.NewServeMux()
mux.Handle("/healthz", healthz())
mux.Handle("/validate/delete-namespace", ah.Serve(nsValidation))
return &http.Server{
Addr: fmt.Sprintf(":%s", port),
Handler: mux,
}
}
实现 admissionHandler，主要作用是将 http body 的内容解析成 AdmissionReview 对象，然后调用具体的 Hook 处理，再将结果放到 AdmissionReview 中，返回给客户端。
package http
import (
"encoding/json"
"fmt"
"io"
"net/http"
"github.com/douglasmakey/admissioncontroller"
"k8s.io/api/admission/v1beta1"
admission "k8s.io/api/admission/v1beta1"
meta "k8s.io/apimachinery/pkg/apis/meta/v1"
"k8s.io/apimachinery/pkg/runtime"
"k8s.io/apimachinery/pkg/runtime/serializer"
log "k8s.io/klog/v2"
)
// admissionHandler represents the HTTP handler for an admission webhook
type admissionHandler struct {
decoder runtime.Decoder
}
// newAdmissionHandler returns an instance of AdmissionHandler
func newAdmissionHandler() *admissionHandler {
return &admissionHandler{
decoder: serializer.NewCodecFactory(runtime.NewScheme()).UniversalDeserializer(),
}
}
// Serve returns a http.HandlerFunc for an admission webhook
func (h *admissionHandler) Serve(hook admissioncontroller.Hook) http.HandlerFunc {
return func(w http.ResponseWriter, r *http.Request) {
w.Header().Set("Content-Type", "application/json")
if r.Method != http.MethodPost {
http.Error(w, fmt.Sprint("invalid method only POST requests are allowed"), http.StatusMethodNotAllowed)
return
}
if contentType := r.Header.Get("Content-Type"); contentType != "application/json" {
http.Error(w, fmt.Sprint("only content type 'application/json' is supported"), http.StatusBadRequest)
return
}
body, err := io.ReadAll(r.Body)
if err != nil {
http.Error(w, fmt.Sprintf("could not read request body: %v", err), http.StatusBadRequest)
return
}
var review admission.AdmissionReview
if _, _, err := h.decoder.Decode(body, nil, &review); err != nil {
http.Error(w, fmt.Sprintf("could not deserialize request: %v", err), http.StatusBadRequest)
return
}
if review.Request == nil {
http.Error(w, "malformed admission review: request is nil", http.StatusBadRequest)
return
}
result, err := hook.Execute(review.Request)
if err != nil {
log.Error(err)
w.WriteHeader(http.StatusInternalServerError)
return
}
admissionResponse := v1beta1.AdmissionReview{
Response: &v1beta1.AdmissionResponse{
UID: review.Request.UID,
Allowed: result.Allowed,
Result: &meta.Status{Message: result.Msg},
},
}
res, err := json.Marshal(admissionResponse)
if err != nil {
log.Error(err)
http.Error(w, fmt.Sprintf("could not marshal response: %v", err), http.StatusInternalServerError)
return
}
log.Infof("Webhook [%s - %s] - Allowed: %t", r.URL.Path, review.Request.Operation, result.Allowed)
w.WriteHeader(http.StatusOK)
w.Write(res)
}
}
func healthz() http.HandlerFunc {
return func(w http.ResponseWriter, r *http.Request) {
w.WriteHeader(http.StatusOK)
w.Write([]byte("ok"))
}
}
上面处理是通过 hook.Execute 来处理请求，这是 admissionController 内部实现的一个结构体，它为每个操作定义了一个方法，如下：
// AdmitFunc defines how to process an admission request
type AdmitFunc func(request *admission.AdmissionRequest) (*Result, error)
// Hook represents the set of functions for each operation in an admission webhook.
type Hook struct {
Create AdmitFunc
Delete AdmitFunc
Update AdmitFunc
Connect AdmitFunc
}
我们就需要实现具体的 AdmitFunc，并注册。
（3）将自己实现的方法注册到 Hook 中。
package namespace
import (
"github.com/douglasmakey/admissioncontroller"
)
// NewValidationHook delete namespace validation hook
func NewValidationHook() admissioncontroller.Hook {
return admissioncontroller.Hook{
Delete: validateDelete(),
}
}
（4）实现具体的 AdmitFunc
package namespace
import (
"github.com/douglasmakey/admissioncontroller"
log "k8s.io/klog/v2"
"k8s.io/api/admission/v1beta1"
)
func validateDelete() admissioncontroller.AdmitFunc {
return func(r *v1beta1.AdmissionRequest) (*admissioncontroller.Result, error) {
if r.Kind.Kind == "Namespace" {
log.Info("You cannot delete namespace: ", r.Name)
return &admissioncontroller.Result{Allowed: false}, nil
} else {
return &admissioncontroller.Result{Allowed: true}, nil
}
}
}
这里实现很简单，如果 Kind 为 Namespace，就拒绝操作。
部署测试
上面完成了业务逻辑开发，下面就把它部署到 Kubernetes 集群测试一番。
部署
（1）编写 Dockerfile，将应用打包成镜像
FROM golang:1.17.5 AS build-env
ENV GOPROXY https://goproxy.cn
ADD . /go/src/app
WORKDIR /go/src/app
RUN go mod tidy
RUN cd cmd && GOOS=linux GOARCH=amd64 go build -v -a -ldflags '-extldflags "-static"' -o /go/src/app/app-server /go/src/app/cmd/main.go
FROM registry.cn-hangzhou.aliyuncs.com/coolops/ubuntu:22.04
ENV TZ=Asia/Shanghai
COPY --from=build-env /go/src/app/app-server /opt/app-server
WORKDIR /opt
EXPOSE 80
CMD [ "./app-server" ]
（2）创建 TLS 证书，使用脚本进行创建
#!/bin/bash
set -e
usage() {
cat <<EOF
Generate certificate suitable for use with an sidecar-injector webhook service.
This script uses k8s' CertificateSigningRequest API to a generate a
certificate signed by k8s CA suitable for use with sidecar-injector webhook
services. This requires permissions to create and approve CSR. See
https://kubernetes.io/docs/tasks/tls/managing-tls-in-a-cluster for
detailed explantion and additional instructions.
The server key/cert k8s CA cert are stored in a k8s secret.
usage: ${0} [OPTIONS]
The following flags are required.
--service Service name of webhook.
--namespace Namespace where webhook service and secret reside.
--secret Secret name for CA certificate and server certificate/key pair.
EOF
exit 1
}
while [[ $# -gt 0 ]]; do
case ${1} in
--service)
service="$2"
shift
;;
--secret)
secret="$2"
shift
;;
--namespace)
namespace="$2"
shift
;;
*)
usage
;;
esac
shift
done
[ -z ${service} ] && service=validate-delete-namespace
[ -z ${secret} ] && secret=validate-delete-namespace-tls
[ -z ${namespace} ] && namespace=default
if [ ! -x "$(command -v openssl)" ]; then
echo "openssl not found"
exit 1
fi
csrName=${service}.${namespace}
tmpdir=$(mktemp -d)
echo "creating certs in tmpdir ${tmpdir} "
cat <<EOF >> ${tmpdir}/csr.conf
[req]
req_extensions = v3_req
distinguished_name = req_distinguished_name
[req_distinguished_name]
[ v3_req ]
basicConstraints = CA:FALSE
keyUsage = nonRepudiation, digitalSignature, keyEncipherment
extendedKeyUsage = serverAuth
subjectAltName = @alt_names
[alt_names]
DNS.1 = ${service}
DNS.2 = ${service}.${namespace}
DNS.3 = ${service}.${namespace}.svc
EOF
openssl genrsa -out ${tmpdir}/server-key.pem 2048
openssl req -new -key ${tmpdir}/server-key.pem -subj "/CN=${service}.${namespace}.svc" -out ${tmpdir}/server.csr -config ${tmpdir}/csr.conf
# clean-up any previously created CSR for our service. Ignore errors if not present.
kubectl delete csr ${csrName} 2>/dev/null || true
# create server cert/key CSR and send to k8s API
cat <<EOF | kubectl create -f -
apiVersion: certificates.k8s.io/v1beta1
kind: CertificateSigningRequest
metadata:
name: ${csrName}
spec:
groups:
- system:authenticated
request: $(cat ${tmpdir}/server.csr | base64 | tr -d '\n')
usages:
- digital signature
- key encipherment
- server auth
EOF
# verify CSR has been created
while true; do
kubectl get csr ${csrName}
if [ "$?" -eq 0 ]; then
break
fi
done
# approve and fetch the signed certificate
kubectl certificate approve ${csrName}
# verify certificate has been signed
for x in $(seq 10); do
serverCert=$(kubectl get csr ${csrName} -o jsonpath='{.status.certificate}')
if [[ ${serverCert} != '' ]]; then
break
fi
sleep 1
done
if [[ ${serverCert} == '' ]]; then
echo "ERROR: After approving csr ${csrName}, the signed certificate did not appear on the resource. Giving up after 10 attempts." >&2
exit 1
fi
echo ${serverCert} | openssl base64 -d -A -out ${tmpdir}/server-cert.pem
# create the secret with CA cert and server cert/key
kubectl create secret generic ${secret} \
--from-file=key.pem=${tmpdir}/server-key.pem \
--from-file=cert.pem=${tmpdir}/server-cert.pem \
--dry-run -o yaml |
kubectl -n ${namespace} apply -f -
（3）编写 Deployment 部署服务
apiVersion: apps/v1
kind: Deployment
metadata:
name: validate-delete-namespace
labels:
app: validate-delete-namespace
spec:
replicas: 1
selector:
matchLabels:
app: validate-delete-namespace
template:
metadata:
labels:
app: validate-delete-namespace
spec:
containers:
- name: server
image: registry.cn-hangzhou.aliyuncs.com/coolops/validate-delete-namespace:latest
imagePullPolicy: Always
livenessProbe:
httpGet:
path: /healthz
port: 8443
scheme: HTTPS
ports:
- containerPort: 8443
volumeMounts:
- name: tls-certs
mountPath: /etc/certs
readOnly: true
volumes:
- name: tls-certs
secret:
secretName: validate-delete-namespace-tls
---
apiVersion: v1
kind: Service
metadata:
name: validate-delete-namespace
spec:
selector:
app: validate-delete-namespace
ports:
- port: 443
targetPort: 8443
（4）部署 Webhook
apiVersion: admissionregistration.k8s.io/v1beta1
kind: ValidatingWebhookConfiguration
metadata:
name: validate-delete-namespace
webhooks:
- name: validate-delete-namespace.default.svc.cluster.local
clientConfig:
service:
namespace: default
name: validate-delete-namespace
path: "/validate/delete-namespace"
caBundle: "${CA_BUNDLE}"
rules:
- operations:
- DELETE
apiGroups:
- ""
apiVersions:
- "v1"
resources:
- namespaces
failurePolicy: Ignore
这里有一个 ${CA_BUNDLE} 占位符，在创建 Webhook 的时候要将起替换掉，使用如下命令：
cat ./validate-delete-namespace.yaml | sh ./patch-webhook-ca.sh > ./webhook.yaml
然后创建 webhook.yaml 即可。
kubectl apply -f webhook.yaml
上面的所有文件都在代码库里，可以直接使用脚本进行部署。
# sh deploy.sh
creating certs in tmpdir /tmp/tmp.SvMHWcPI6x
Generating RSA private key, 2048 bit long modulus
..........................................+++
.............................................................+++
e is 65537 (0x10001)
certificatesigningrequest.certificates.k8s.io/validate-delete-namespace.default created
NAME AGE REQUESTOR CONDITION
validate-delete-namespace.default 0s kubernetes-admin Pending
certificatesigningrequest.certificates.k8s.io/validate-delete-namespace.default approved
secret/validate-delete-namespace-tls created
Creating k8s admission deployment
deployment.apps/validate-delete-namespace created
service/validate-delete-namespace created
validatingwebhookconfiguration.admissionregistration.k8s.io/validate-delete-namespace created
执行完成过后，可以查看具体的信息。
# kubectl get po
NAME READY STATUS RESTARTS AGE
validate-delete-namespace-74c9b8b7bd-5g9zv 1/1 Running 0 3s
# kubectl get secret
NAME TYPE DATA AGE
default-token-kx5wf kubernetes.io/service-account-token 3 72d
validate-delete-namespace-tls Opaque 2 53s
# kubectl get ValidatingWebhookConfiguration
NAME CREATED AT
validate-delete-namespace 2022-06-24T09:39:26Z
测试
（1）首先打开 webhook 的 pod 日志
# kubectl logs validate-delete-namespace-74c9b8b7bd-5g9zv -f
I0624 17:39:27.858753 1 main.go:30] Server running in port: 8443
（2）创建一个 namespace 并删除
# kubectl create ns joker
# kubectl get ns | grep joker
joker Active 4h5m
# kubectl delete ns joker
Error from server: admission webhook "validate-delete-namespace.default.svc.cluster.local" denied the request without explanation
# kubectl get ns | grep joker
joker Active 4h5m
可以发现我们的删除操作被拒绝了，并且查看 namespace 还存在。
我们也可以到日志中查看，如下：
# kubectl logs validate-delete-namespace-74c9b8b7bd-5g9zv -f
I0624 17:39:27.858753 1 main.go:30] Server running in port: 8443
2022/06/24 17:43:34 You cannot delete namespace: joker
I0624 17:43:34.664945 1 handler.go:94] Webhook [/validate/delete-namespace - DELETE] - Allowed: false
2022/06/24 17:43:34 You cannot delete namespace: joker
I0624 17:43:34.667043 1 handler.go:94] Webhook [/validate/delete-namespace - DELETE] - Allowed: false
上面就是简单的实现了一个准入控制器.
只要思想不滑坡，办法总比困难多。
感谢万能的百度，感谢牛逼的网友。