Updates Kubeadm Master Endpoint for IPv6

Previously, kubeadm would use <ip>:<port> to construct a master
endpoint. This works fine for IPv4 addresses, but not for IPv6.
IPv6 requires the ip to be encased in brackets when being joined
to a port with a colon.

This patch updates kubeadm to support wrapping a v6 address with
[] to form the master endpoint url. Since this functionality is
needed in multiple areas, a dedicated util function was created.

Fixes: https://github.com/kubernetes/kubernetes/issues/48227

cmd/kubeadm/app/util/endpoint.go

@@ -0,0 +1,51 @@
+/*
+Copyright 2017 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 util
+
+import (
+	"fmt"
+	"net"
+	"strconv"
+
+	kubeadmapi "k8s.io/kubernetes/cmd/kubeadm/app/apis/kubeadm"
+)
+
+// GetMasterEndpoint returns a properly formatted Master Endpoint
+// or passes the error from GetMasterHostPort.
+func GetMasterEndpoint(cfg *kubeadmapi.MasterConfiguration) (string, error) {
+	hostPort, err := GetMasterHostPort(cfg)
+	if err != nil {
+		return "", err
+	}
+	return fmt.Sprintf("https://%s", hostPort), nil
+}
+
+// GetMasterHostPort returns a properly formatted Master IP/port pair or error
+// if the IP address can not be parsed or port is outside the valid TCP range.
+func GetMasterHostPort(cfg *kubeadmapi.MasterConfiguration) (string, error) {
+	masterIP := net.ParseIP(cfg.API.AdvertiseAddress)
+	if masterIP == nil {
+		return "", fmt.Errorf("error parsing address %s", cfg.API.AdvertiseAddress)
+	}
+
+	if cfg.API.BindPort < 0 || cfg.API.BindPort > 65535 {
+		return "", fmt.Errorf("api server port must be between 0 and 65535")
+	}
+
+	hostPort := net.JoinHostPort(masterIP.String(), strconv.Itoa(int(cfg.API.BindPort)))
+	return hostPort, nil
+}