Use http/2 for localhost webhook

Signed-off-by: Eric Lin <exlin@google.com>

Kubernetes-commit: 246e69fb99007412c4903fe8e7ad1d8c5f25cd8e

pkg/util/webhook/client.go

@@ -24,6 +24,7 @@ import (
 	"net"
 	"net/url"
 	"strconv"
+	"strings"
 
 	"k8s.io/apimachinery/pkg/runtime"
 	"k8s.io/apimachinery/pkg/runtime/schema"
@@ -32,6 +33,7 @@ import (
 	"k8s.io/apiserver/pkg/util/x509metrics"
 	"k8s.io/client-go/rest"
 	"k8s.io/utils/lru"
+	netutils "k8s.io/utils/net"
 )
 
 const (
@@ -128,7 +130,20 @@ func (cm *ClientManager) HookClient(cc ClientConfig) (*rest.RESTClient, error) {
 		return client.(*rest.RESTClient), nil
 	}
 
-	complete := func(cfg *rest.Config) (*rest.RESTClient, error) {
+	cfg, err := cm.hookClientConfig(cc)
+	if err != nil {
+		return nil, err
+	}
+
+	client, err := rest.UnversionedRESTClientFor(cfg)
+	if err == nil {
+		cm.cache.Add(string(cacheKey), client)
+	}
+	return client, err
+}
+
+func (cm *ClientManager) hookClientConfig(cc ClientConfig) (*rest.Config, error) {
+	complete := func(cfg *rest.Config) (*rest.Config, error) {
 		// Avoid client-side rate limiting talking to the webhook backend.
 		// Rate limiting should happen when deciding how many requests to serve.
 		cfg.QPS = -1
@@ -139,11 +154,6 @@ func (cm *ClientManager) HookClient(cc ClientConfig) (*rest.RESTClient, error) {
 		}
 		cfg.TLSClientConfig.CAData = append(cfg.TLSClientConfig.CAData, cc.CABundle...)
 
-		// Use http/1.1 instead of http/2.
-		// This is a workaround for http/2-enabled clients not load-balancing concurrent requests to multiple backends.
-		// See https://issue.k8s.io/75791 for details.
-		cfg.NextProtos = []string{"http/1.1"}
-
 		cfg.ContentConfig.NegotiatedSerializer = cm.negotiatedSerializer
 		cfg.ContentConfig.ContentType = runtime.ContentTypeJSON
 
@@ -153,12 +163,7 @@ func (cm *ClientManager) HookClient(cc ClientConfig) (*rest.RESTClient, error) {
 			x509MissingSANCounter,
 			x509InsecureSHA1Counter,
 		))
-
-		client, err := rest.UnversionedRESTClientFor(cfg)
-		if err == nil {
-			cm.cache.Add(string(cacheKey), client)
-		}
-		return client, err
+		return cfg, nil
 	}
 
 	if cc.Service != nil {
@@ -173,6 +178,12 @@ func (cm *ClientManager) HookClient(cc ClientConfig) (*rest.RESTClient, error) {
 			return nil, err
 		}
 		cfg := rest.CopyConfig(restConfig)
+
+		// Use http/1.1 instead of http/2.
+		// This is a workaround for http/2-enabled clients not load-balancing concurrent requests to multiple backends.
+		// See https://issue.k8s.io/75791 for details.
+		cfg.NextProtos = []string{"http/1.1"}
+
 		serverName := cc.Service.Name + "." + cc.Service.Namespace + ".svc"
 
 		host := net.JoinHostPort(serverName, strconv.Itoa(int(port)))
@@ -225,6 +236,22 @@ func (cm *ClientManager) HookClient(cc ClientConfig) (*rest.RESTClient, error) {
 	cfg := rest.CopyConfig(restConfig)
 	cfg.Host = u.Scheme + "://" + u.Host
 	cfg.APIPath = u.Path
+	if !isLocalHost(u) {
+		cfg.NextProtos = []string{"http/1.1"}
+	}
 
 	return complete(cfg)
 }
+
+func isLocalHost(u *url.URL) bool {
+	host := u.Hostname()
+	if strings.EqualFold(host, "localhost") {
+		return true
+	}
+
+	netIP := netutils.ParseIPSloppy(host)
+	if netIP != nil {
+		return netIP.IsLoopback()
+	}
+	return false
+}

pkg/util/webhook/client_test.go

@@ -0,0 +1,91 @@
+/*
+Copyright 2024 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package webhook
+
+import (
+	"testing"
+
+	"golang.org/x/net/http2"
+	"k8s.io/apimachinery/pkg/runtime/schema"
+)
+
+func TestWebhookClientConfig(t *testing.T) {
+	cm, _ := NewClientManager([]schema.GroupVersion{})
+	authInfoResolver, err := NewDefaultAuthenticationInfoResolver("")
+	if err != nil {
+		t.Fatal(err)
+	}
+	cm.SetAuthenticationInfoResolver(authInfoResolver)
+	cm.SetServiceResolver(NewDefaultServiceResolver())
+
+	tests := []struct {
+		name             string
+		url              string
+		expectAllowHTTP2 bool
+	}{
+		{
+			name:             "force http1",
+			url:              "https://webhook.example.com",
+			expectAllowHTTP2: false,
+		},
+		{
+			name:             "allow http2 for localhost",
+			url:              "https://localhost",
+			expectAllowHTTP2: true,
+		},
+		{
+			name:             "allow http2 for 127.0.0.1",
+			url:              "https://127.0.0.1",
+			expectAllowHTTP2: true,
+		},
+		{
+			name:             "allow http2 for [::1]:0",
+			url:              "https://[::1]",
+			expectAllowHTTP2: true,
+		},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+
+			cc := ClientConfig{
+				URL: tc.url,
+			}
+			cfg, err := cm.hookClientConfig(cc)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if tc.expectAllowHTTP2 && !allowHTTP2(cfg.NextProtos) {
+				t.Errorf("expected allow http/2, got: %v", cfg.NextProtos)
+			}
+		})
+	}
+}
+
+func allowHTTP2(nextProtos []string) bool {
+	if len(nextProtos) == 0 {
+		// the transport expressed no NextProto preference, allow
+		return true
+	}
+	for _, p := range nextProtos {
+		if p == http2.NextProtoTLS {
+			// the transport explicitly allowed http/2
+			return true
+		}
+	}
+	// the transport explicitly set NextProtos and excluded http/2
+	return false
+}