Address PR GoogleCloudPlatform#1198 comments: add host normalization for IPv6

- Implement NormalizeHost to strip brackets from IPv6 addresses.
- Use NormalizeHost in controller and kubelet clients.
- Add unit tests for normalization and regression tests for clients.

Author: courageJ

kubelet-to-gcm/monitor/controller/client.go

@@ -25,6 +25,7 @@ import (
 	"net/url"
 	"strings"
 
+	"github.com/GoogleCloudPlatform/k8s-stackdriver/kubelet-to-gcm/monitor/util"
 	"github.com/prometheus/common/expfmt"
 	"github.com/prometheus/common/model"
 )
@@ -82,7 +83,7 @@ type Client struct {
 // NewClient generates a client to hit the given controller.
 func NewClient(host string, port uint, client *http.Client) (*Client, error) {
 	// Parse our URL upfront, so we can fail fast.
-	urlStr := fmt.Sprintf("http://%s/metrics", net.JoinHostPort(host, fmt.Sprint(port)))
+	urlStr := fmt.Sprintf("http://%s/metrics", net.JoinHostPort(util.NormalizeHost(host), fmt.Sprint(port)))
 	metricsURL, err := url.Parse(urlStr)
 	if err != nil {
 		return nil, err

kubelet-to-gcm/monitor/controller/client_test.go

@@ -40,6 +40,12 @@ func TestNewClient(t *testing.T) {
 			port:        10250,
 			expectedURL: "http://[2001:db8::1]:10250/metrics",
 		},
+		{
+			name:        "Bracketed IPv6",
+			host:        "[2001:db8::1]",
+			port:        10250,
+			expectedURL: "http://[2001:db8::1]:10250/metrics",
+		},
 		{
 			name:        "Hostname",
 			host:        "localhost",

kubelet-to-gcm/monitor/kubelet/client.go

@@ -24,6 +24,7 @@ import (
 	"net/http"
 	"net/url"
 
+	"github.com/GoogleCloudPlatform/k8s-stackdriver/kubelet-to-gcm/monitor/util"
 	stats "k8s.io/kubelet/pkg/apis/stats/v1alpha1"
 )
 
@@ -41,7 +42,7 @@ func NewClient(host string, port uint, client *http.Client, useAuthPort bool) (*
 	if useAuthPort {
 		protocol = "https"
 	}
-	urlStr := fmt.Sprintf("%s://%s/stats/summary", protocol, net.JoinHostPort(host, fmt.Sprint(port)))
+	urlStr := fmt.Sprintf("%s://%s/stats/summary", protocol, net.JoinHostPort(util.NormalizeHost(host), fmt.Sprint(port)))
 	summaryURL, err := url.Parse(urlStr)
 	if err != nil {
 		return nil, err

kubelet-to-gcm/monitor/kubelet/client_test.go

@@ -57,6 +57,20 @@ func TestNewClient(t *testing.T) {
 			useAuthPort: true,
 			expectedURL: "https://[2001:db8::1]:10250/stats/summary",
 		},
+		{
+			name:        "Bracketed IPv6 HTTP",
+			host:        "[2001:db8::1]",
+			port:        10255,
+			useAuthPort: false,
+			expectedURL: "http://[2001:db8::1]:10255/stats/summary",
+		},
+		{
+			name:        "Bracketed IPv6 HTTPS",
+			host:        "[2001:db8::1]",
+			port:        10250,
+			useAuthPort: true,
+			expectedURL: "https://[2001:db8::1]:10250/stats/summary",
+		},
 	}
 
 	for _, tc := range testCases {

kubelet-to-gcm/monitor/util/util.go

@@ -0,0 +1,32 @@
+/*
+Copyright 2026 Google Inc.
+
+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 util
+
+import (
+	"strings"
+)
+
+// NormalizeHost handles host strings that may already be bracketed (like IPv6).
+// net.JoinHostPort expects a raw IP or hostname; if it receives a bracketed IPv6,
+// it will double-bracket it.
+func NormalizeHost(host string) string {
+	host = strings.TrimSpace(host)
+	if strings.HasPrefix(host, "[") && strings.HasSuffix(host, "]") {
+		return host[1 : len(host)-1]
+	}
+	return host
+}

kubelet-to-gcm/monitor/util/util_test.go

@@ -0,0 +1,61 @@
+/*
+Copyright 2026 Google Inc.
+
+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 util
+
+import (
+	"testing"
+)
+
+func TestNormalizeHost(t *testing.T) {
+	testCases := []struct {
+		desc     string
+		input    string
+		expected string
+	}{
+		{
+			desc:     "IPv4",
+			input:    "127.0.0.1",
+			expected: "127.0.0.1",
+		},
+		{
+			desc:     "hostname",
+			input:    "localhost",
+			expected: "localhost",
+		},
+		{
+			desc:     "raw IPv6",
+			input:    "2001:db8::1",
+			expected: "2001:db8::1",
+		},
+		{
+			desc:     "bracketed IPv6",
+			input:    "[2001:db8::1]",
+			expected: "2001:db8::1",
+		},
+		{
+			desc:     "bracketed IPv6 with spaces",
+			input:    " [2001:db8::1] ",
+			expected: "2001:db8::1",
+		},
+	}
+
+	for _, tc := range testCases {
+		if got := NormalizeHost(tc.input); got != tc.expected {
+			t.Errorf("%s: NormalizeHost(%q) = %q, want %q", tc.desc, tc.input, got, tc.expected)
+		}
+	}
+}