k8s APIServer调用webhook需要域名解析吗？

背景

最近看到测试环境，coredns Pod挂掉了，但k8s APIServer调用webhook仍然正常，对此有点儿疑惑，难道APIServer调用webhook中的service不需要经过coredns域名解析？直接获取到了svc ClusterIP或者直接获取到了endpoint中的pod地址？带着这个问题，深入了解下apiServer请求webhook时的一些源码。

k8s版本：1.18.14
部署方式：kube-apiserver为二进制部署，systemd管理。

webhook示例

$ kubectl get mutatingwebhookconfigurations.admissionregistration.k8s.io  rocketmq-operator-mutating-webhook-configuration  -oyaml
apiVersion: admissionregistration.k8s.io/v1
kind: MutatingWebhookConfiguration
metadata:
  annotations:
    cert-manager.io/inject-ca-from: rocketmq/rocketmq-operator-serving-cert
  name: rocketmq-operator-mutating-webhook-configuration
webhooks:
- admissionReviewVersions:
  - v1
  - v1beta1
  clientConfig:
    caBundle: LS0tLSXXX
    service:
      name: rocketmq-operator-webhook-service
      namespace: rocketmq
      path: /mutate-rocketmq-apache-org-v1alpha1-rocketmqcluster
      port: 443
  failurePolicy: Fail
  matchPolicy: Equivalent
  name: mrocketmqcluster.kb.io
  namespaceSelector: {}
  objectSelector: {}
  reinvocationPolicy: Never
  rules:
  - apiGroups:
    - rocketmq.apache.org
    apiVersions:
    - v1alpha1
    operations:
    - CREATE
    - UPDATE
    resources:
    - rocketmqclusters
    scope: '*'
  sideEffects: None
  timeoutSeconds: 10

源码分析

在源码的kubernetes/staging/src/k8s.io/apiserver/pkg/server/plugins.go的RegisterAllAdmissionPlugins函数中：

// RegisterAllAdmissionPlugins registers all admission plugins
func RegisterAllAdmissionPlugins(plugins *admission.Plugins) {
	lifecycle.Register(plugins)
	validatingwebhook.Register(plugins)
	mutatingwebhook.Register(plugins)
}

注册了三种准入控制插件：lifecycle、validatingwebhook、mutatingwebhook

这里只看mutatingwebhook，其他的后面文章会讲到。
kubernetes/staging/src/k8s.io/apiserver/pkg/admission/plugin/webhook/mutating/plugin.go

//NewMutatingWebhook 返回一个通用的准入 webhook 插件。
func NewMutatingWebhook(configFile io.Reader) (*Plugin, error) {
	handler := admission.NewHandler(admission.Connect, admission.Create, admission.Delete, admission.Update)
	p := &Plugin{}
	var err error
	p.Webhook, err = generic.NewWebhook(handler, configFile, configuration.NewMutatingWebhookConfigurationManager, newMutatingDispatcher(p))
	if err != nil {
		return nil, err
	}

	return p, nil
}

这里的NewWebhook调用：kubernetes/staging/src/k8s.io/apiserver/pkg/admission/plugin/webhook/generic/webhook.go

func NewWebhook(handler *admission.Handler, configFile io.Reader, sourceFactory sourceFactory, dispatcherFactory dispatcherFactory) (*Webhook, error) {
	kubeconfigFile, err := config.LoadConfig(configFile)
	//...省略...
	// Set defaults which may be overridden later.
	cm.SetServiceResolver(webhookutil.NewDefaultServiceResolver())

	return &Webhook{
		Handler:          handler,
		sourceFactory:    sourceFactory,
		clientManager:    &cm,
		namespaceMatcher: &namespace.Matcher{},
		objectMatcher:    &object.Matcher{},
		dispatcher:       dispatcherFactory(&cm),
	}, nil
}

设置为kubernetes/staging/src/k8s.io/apiserver/pkg/util/webhook/serviceresolver.go中的defaultServiceResolver：

//ResolveEndpoint 根据给定的命名空间和名称构造服务 URL，
//请注意名称、命名空间和端口是必需的，并且默认情况下所有创建的地址都使用 HTTPS 方案。
//例如：name=ross namespace=andromeda 解析为 https://ross.andromeda.svc:443
func (sr defaultServiceResolver) ResolveEndpoint(namespace, name string, port int32) (*url.URL, error) {
	if len(name) == 0 || len(namespace) == 0 || port == 0 {
		return nil, errors.New("cannot resolve an empty service name or namespace or port")
	}
	return &url.URL{Scheme: "https", Host: fmt.Sprintf("%s.%s.svc:%d", name, namespace, port)}, nil
}

configuration.NewMutatingWebhookConfigurationManager中设置MutatingWebhookConfigurations对象变化时informer回调函数AddFunc、UpdateFunc、DeleteFunc，全部调用mutatingWebhookConfigurationManager的updateConfiguration方法，代码在kubernetes/staging/src/k8s.io/apiserver/pkg/admission/configuration/mutating_webhook_manager.go
中。

进而调用mergeMutatingWebhookConfigurations函数，为每个webhook生产一个accessors
kubernetes/staging/src/k8s.io/apiserver/pkg/admission/plugin/webhook/accessors.go

func mergeMutatingWebhookConfigurations(configurations []*v1.MutatingWebhookConfiguration) []webhook.WebhookAccessor {
	//每个配置的 webhook 的内部顺序由用户提供，但配置本身可以是任何顺序。
    //由于我们将连续运行这些 webhook，因此它们在此处进行排序以具有确定性的顺序。
	sort.SliceStable(configurations, MutatingWebhookConfigurationSorter(configurations).ByName)
	accessors := []webhook.WebhookAccessor{}
	for _, c := range configurations {
		//webhook 名称未验证唯一性，因此我们检查重复项并添加 int 后缀以区分它们
		names := map[string]int{}
		for i := range c.Webhooks {
			n := c.Webhooks[i].Name
			uid := fmt.Sprintf("%s/%s/%d", c.Name, n, names[n])
			names[n]++
			accessors = append(accessors, webhook.NewMutatingWebhookAccessor(uid, c.Name, &c.Webhooks[i]))
		}
	}
	return accessors
}

这里返回的accessor集合会被Store到一个atomic.Value中。后面会被Load。

当发生mutate webhook调用时，会执行kubernetes/staging/src/k8s.io/apiserver/pkg/admission/plugin/webhook/mutating/plugin.go的

// Admit 根据请求属性做出准入决定。
func (a *Plugin) Admit(ctx context.Context, attr admission.Attributes, o admission.ObjectInterfaces) error {
	return a.Webhook.Dispatch(ctx, attr, o)
}

这里面Dispatch方法中会Load到accessors集合，调用mutatingDispatcher的Dispatch方法。位于：
kubernetes/staging/src/k8s.io/apiserver/pkg/admission/plugin/webhook/mutating/dispatcher.go

func (a *mutatingDispatcher) Dispatch(ctx context.Context, attr admission.Attributes, o admission.ObjectInterfaces, hooks []webhook.WebhookAccessor) error {	
	//...省略...
    changed, err := a.callAttrMutatingHook(ctx, hook, invocation, versionedAttr, o, round, i)
	//...省略...
	return nil
}

func (a *mutatingDispatcher) callAttrMutatingHook(ctx context.Context, h *admissionregistrationv1.MutatingWebhook, invocation *generic.WebhookInvocation, attr *generic.VersionedAttributes, o admission.ObjectInterfaces, round, idx int) (bool, error) {
	configurationName := invocation.Webhook.GetConfigurationName()
	annotator := newWebhookAnnotator(attr, round, idx, h.Name, configurationName)
	changed := false
	defer func() { annotator.addMutationAnnotation(changed) }()
	if attr.Attributes.IsDryRun() {
		if h.SideEffects == nil {
			return false, &webhookutil.ErrCallingWebhook{WebhookName: h.Name, Reason: fmt.Errorf("Webhook SideEffects is nil")}
		}
		if !(*h.SideEffects == admissionregistrationv1.SideEffectClassNone || *h.SideEffects == admissionregistrationv1.SideEffectClassNoneOnDryRun) {
			return false, webhookerrors.NewDryRunUnsupportedErr(h.Name)
		}
	}

	uid, request, response, err := webhookrequest.CreateAdmissionObjects(attr, invocation)
	if err != nil {
		return false, &webhookutil.ErrCallingWebhook{WebhookName: h.Name, Reason: err}
	}
	// Make the webhook request
	client, err := invocation.Webhook.GetRESTClient(a.cm)
	if err != nil {
		return false, &webhookutil.ErrCallingWebhook{WebhookName: h.Name, Reason: err}
	}
	
    //...省略...

	if err := r.Do(ctx).Into(response); err != nil {
		return false, &webhookutil.ErrCallingWebhook{WebhookName: h.Name, Reason: err}
	}
	trace.Step("Request completed")

	result, err := webhookrequest.VerifyAdmissionResponse(uid, true, response)
	if err != nil {
		return false, &webhookutil.ErrCallingWebhook{WebhookName: h.Name, Reason: err}
	}

	//...省略...
	return changed, nil
}

重点在上面的client的构建，即调用invocation.Webhook.GetRESTClient(a.cm)，当webhook为mutate时，这个accessor为mutatingWebhookAccessor，实现了WebhookAccessor接口。

func (m *mutatingWebhookAccessor) GetRESTClient(clientManager *webhookutil.ClientManager) (*rest.RESTClient, error) {
	m.initClient.Do(func() {
		m.client, m.clientErr = clientManager.HookClient(hookClientConfigForWebhook(m))
	})
	return m.client, m.clientErr
}

ClientManager的HookClient方法如下：

// HookClient 从缓存中获取一个 RESTClient，或者根据 webhook 配置构造一个。
func (cm *ClientManager) HookClient(cc ClientConfig) (*rest.RESTClient, error) {
	ccWithNoName := cc
	ccWithNoName.Name = ""
	cacheKey, err := json.Marshal(ccWithNoName)
	if err != nil {
		return nil, err
	}
	if client, ok := cm.cache.Get(string(cacheKey)); ok {
		return client.(*rest.RESTClient), nil
	}

	complete := func(cfg *rest.Config) (*rest.RESTClient, error) {
		//避免与 webhook 后端通信的客户端速率限制。在决定服务多少请求时应该进行速率限制。
		cfg.QPS = -1

		//...省略...

		cfg.ContentConfig.NegotiatedSerializer = cm.negotiatedSerializer
		cfg.ContentConfig.ContentType = runtime.ContentTypeJSON
		client, err := rest.UnversionedRESTClientFor(cfg)
		if err == nil {
			cm.cache.Add(string(cacheKey), client)
		}
		return client, err
	}

	if cc.Service != nil {
		port := cc.Service.Port
		if port == 0 {
			// Default to port 443 if no service port is specified
			port = 443
		}

		restConfig, err := cm.authInfoResolver.ClientConfigForService(cc.Service.Name, cc.Service.Namespace, int(port))
		if err != nil {
			return nil, err
		}
		cfg := rest.CopyConfig(restConfig)
		serverName := cc.Service.Name + "." + cc.Service.Namespace + ".svc"

		host := net.JoinHostPort(serverName, strconv.Itoa(int(port)))
		cfg.Host = "https://" + host
		cfg.APIPath = cc.Service.Path
		// Set the server name if not already set
		if len(cfg.TLSClientConfig.ServerName) == 0 {
			cfg.TLSClientConfig.ServerName = serverName
		}

		delegateDialer := cfg.Dial
		if delegateDialer == nil {
			var d net.Dialer
			delegateDialer = d.DialContext
		}
		cfg.Dial = func(ctx context.Context, network, addr string) (net.Conn, error) {
			if addr == host {
				u, err := cm.serviceResolver.ResolveEndpoint(cc.Service.Namespace, cc.Service.Name, port)
				if err != nil {
					return nil, err
				}
				addr = u.Host
			}
			return delegateDialer(ctx, network, addr)
		}

		return complete(cfg)
	}

	if cc.URL == "" {
		return nil, &ErrCallingWebhook{WebhookName: cc.Name, Reason: errors.New("webhook configuration must have either service or URL")}
	}

	u, err := url.Parse(cc.URL)
	if err != nil {
		return nil, &ErrCallingWebhook{WebhookName: cc.Name, Reason: fmt.Errorf("Unparsable URL: %v", err)}
	}

	hostPort := u.Host
	if len(u.Port()) == 0 {
		// Default to port 443 if no port is specified
		hostPort = net.JoinHostPort(hostPort, "443")
	}

	restConfig, err := cm.authInfoResolver.ClientConfigFor(hostPort)
	if err != nil {
		return nil, err
	}

	cfg := rest.CopyConfig(restConfig)
	cfg.Host = u.Scheme + "://" + u.Host
	cfg.APIPath = u.Path

	return complete(cfg)
}

这个方法作用是生成webhook client。首先构建restConfig，其中包含了参数，cfg.Host、cfg.APIPath、cfg.Dial等，这些参数最终会调用complete函数中rest.UnversionedRESTClientFor(cfg)。

其中确定且需关注的点为：
url：https://rocketmq-operator-webhook-service.rocketmq.svc:443
ApiPath: /mutate-rocketmq-apache-org-v1alpha1-rocketmqcluster

Dial需重点关注，这里赋值为以下函数：

func(ctx context.Context, network, addr string) (net.Conn, error) {
			if addr == host {
				u, err := cm.serviceResolver.ResolveEndpoint(cc.Service.Namespace, cc.Service.Name, port)
				if err != nil {
					return nil, err
				}
				addr = u.Host
			}
			return delegateDialer(ctx, network, addr)
}

这里的cfg.Dial最终会赋值给http.Transport的DialContext，代码在kubernetes/staging/src/k8s.io/client-go/transport/cache.go中tlsTransportCache的get方法：

func (c *tlsTransportCache) get(config *Config) (http.RoundTripper, error) {
	key, canCache, err := tlsConfigKey(config)
	if err != nil {
		return nil, err
	}

	//...省略...

	// Get the TLS options for this client config
	tlsConfig, err := TLSConfigFor(config)
	if err != nil {
		return nil, err
	}
	//...省略...
	dial := config.Dial
	if dial == nil {
		dial = (&net.Dialer{
			Timeout:   30 * time.Second,
			KeepAlive: 30 * time.Second,
		}).DialContext
	}
    
	//...省略...
	transport := utilnet.SetTransportDefaults(&http.Transport{
		Proxy:               http.ProxyFromEnvironment,
		TLSHandshakeTimeout: 10 * time.Second,
		TLSClientConfig:     tlsConfig,
		MaxIdleConnsPerHost: idleConnsPerHost,
		DialContext:         dial,
		DisableCompression:  config.DisableCompression,
	})

	if canCache {
		// Cache a single transport for these options
		c.transports[key] = transport
	}

	return transport, nil
}

上面重点关注的cfg.Dial函数中的：

cm.serviceResolver.ResolveEndpoint(cc.Service.Namespace, cc.Service.Name, port)

serviceResolver实现为loopbackResolver，代码在kubernetes/staging/src/k8s.io/kube-aggregator/pkg/apiserver/resolvers.go中：

type loopbackResolver struct {
	delegate ServiceResolver
	host     *url.URL
}

func (r *loopbackResolver) ResolveEndpoint(namespace, name string, port int32) (*url.URL, error) {
	if namespace == "default" && name == "kubernetes" && port == 443 {
		return r.host, nil
	}
	return r.delegate.ResolveEndpoint(namespace, name, port)
}

在设置了–enable-aggregator-routing=true时，delegate又去调用实现为aggregatorEndpointRouting：位于kubernetes/staging/src/k8s.io/kube-aggregator/pkg/apiserver/resolvers.go中

type aggregatorEndpointRouting struct {
	services  listersv1.ServiceLister
	endpoints listersv1.EndpointsLister
}

func (r *aggregatorEndpointRouting) ResolveEndpoint(namespace, name string, port int32) (*url.URL, error) {
	return proxy.ResolveEndpoint(r.services, r.endpoints, namespace, name, port)
}

proxy.ResolveEndpoint如下：

// ResourceLocation returns a URL to which one can send traffic for the specified service.
func ResolveEndpoint(services listersv1.ServiceLister, endpoints listersv1.EndpointsLister, namespace, id string, port int32) (*url.URL, error) {
	svc, err := services.Services(namespace).Get(id)
	if err != nil {
		return nil, err
	}

	svcPort, err := findServicePort(svc, port)
	if err != nil {
		return nil, err
	}

	switch {
	case svc.Spec.Type == v1.ServiceTypeClusterIP, svc.Spec.Type == v1.ServiceTypeLoadBalancer, svc.Spec.Type == v1.ServiceTypeNodePort:
		// these are fine
	default:
		return nil, fmt.Errorf("unsupported service type %q", svc.Spec.Type)
	}

	eps, err := endpoints.Endpoints(namespace).Get(svc.Name)
	if err != nil {
		return nil, err
	}
	if len(eps.Subsets) == 0 {
		return nil, errors.NewServiceUnavailable(fmt.Sprintf("no endpoints available for service %q", svc.Name))
	}

	// Pick a random Subset to start searching from.
	ssSeed := rand.Intn(len(eps.Subsets))

	// Find a Subset that has the port.
	for ssi := 0; ssi < len(eps.Subsets); ssi++ {
		ss := &eps.Subsets[(ssSeed+ssi)%len(eps.Subsets)]
		if len(ss.Addresses) == 0 {
			continue
		}
		for i := range ss.Ports {
			if ss.Ports[i].Name == svcPort.Name {
				// Pick a random address.
				ip := ss.Addresses[rand.Intn(len(ss.Addresses))].IP
				port := int(ss.Ports[i].Port)
				return &url.URL{
					Scheme: "https",
					Host:   net.JoinHostPort(ip, strconv.Itoa(port)),
				}, nil
			}
		}
	}
	return nil, errors.NewServiceUnavailable(fmt.Sprintf("no endpoints available for service %q", id))
}

可以看到这里在lister缓存中获取到svc及对应的endpoint对象，返回https://podip:443。

总结

具体golang的http.Transport中的DialContext作用，这里不展开。

具体为什么实现为loopbackResolver->aggregatorEndpointRouting，会在后面webhook源码分析中专门介绍。

这里只定位APIServer webhook调用时，可以直接获取webhook podip地址，而不需要去coredns做解析，这在一定程度上做到解耦合（不依赖coredns）。

其实不止webhook，APIService（聚合API）也遵循上面的过程。

即使APIServer为静态Pod方式，由kubelet管理，创建出来的mirror Pod的spec.dnsPolicy依然为ClusterFirst，而Pod为hostNetwork网络，即Pod中的/etc/resolv.conf继承自主机，不经过coredns解析。

以上讨论的前提是webhook的clientConfig是service指定。如果是用url指定的，还需另当别论。