Bracket IPv6 addresses in control-plane join URLs

internal/controller/bootstrap/controlplane_bootstrap_controller.go

@@ -21,6 +21,7 @@ import (
 	"crypto/tls"
 	"errors"
 	"fmt"
+	"net"
 	"net/http"
 	"net/netip"
 	"path/filepath"
@@ -690,7 +691,7 @@ func (c *ControlPlaneController) detectJoinHost(ctx context.Context, scope *Cont
 	if found && k0sAPIPort > 0 {
 		port = strconv.Itoa(int(k0sAPIPort))
 	}
-	host := fmt.Sprintf("https://%s:%s", scope.Cluster.Spec.ControlPlaneEndpoint.Host, port)
+	host := joinHostPortURL(scope.Cluster.Spec.ControlPlaneEndpoint.Host, port)
 
 	_, err = httpClient.Get(fmt.Sprintf("%s/v1beta1/ca", host))
 	if err == nil {
@@ -702,7 +703,13 @@ func (c *ControlPlaneController) detectJoinHost(ctx context.Context, scope *Cont
 		return "", fmt.Errorf("failed to get first controller IP: %w", err)
 	}
 
-	return fmt.Sprintf("https://%s:%s", firstControllerIP, port), nil
+	return joinHostPortURL(firstControllerIP, port), nil
+}
+
+// joinHostPortURL builds an https URL for the given host and port. It uses
+// net.JoinHostPort so that IPv6 literals are bracketed correctly.
+func joinHostPortURL(host, port string) string {
+	return "https://" + net.JoinHostPort(host, port)
 }
 
 func (c *ControlPlaneController) findFirstControllerIP(ctx context.Context, firstControllerMachine *clusterv1.Machine) (string, error) {

internal/controller/bootstrap/controlplane_bootstrap_controller_test.go

@@ -164,3 +164,22 @@ func TestController_genK0sCommands(t *testing.T) {
 		})
 	}
 }
+
+func Test_joinHostPortURL(t *testing.T) {
+	tests := []struct {
+		name string
+		host string
+		port string
+		want string
+	}{
+		{"hostname", "cp.example.com", "9443", "https://cp.example.com:9443"},
+		{"ipv4", "10.0.0.1", "9443", "https://10.0.0.1:9443"},
+		{"ipv6", "2001:db8::1", "9443", "https://[2001:db8::1]:9443"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			require.Equal(t, tt.want, joinHostPortURL(tt.host, tt.port))
+		})
+	}
+}

internal/controller/bootstrap/worker_bootstrap_controller.go

@@ -21,7 +21,9 @@ import (
 	"errors"
 	"fmt"
 	"maps"
+	"net"
 	"path/filepath"
+	"strconv"
 	"strings"
 	"time"
 
@@ -529,9 +531,9 @@ func (r *Controller) getK0sToken(ctx context.Context, scope *Scope) (string, err
 	}
 
 	var joinToken string
-	joinURL := fmt.Sprintf("https://%s:%d", scope.Cluster.Spec.ControlPlaneEndpoint.Host, scope.Cluster.Spec.ControlPlaneEndpoint.Port)
+	joinURL := controlPlaneJoinURL(scope.Cluster.Spec.ControlPlaneEndpoint.Host, int64(scope.Cluster.Spec.ControlPlaneEndpoint.Port))
 	if scope.ingressSpec != nil {
-		joinURL = fmt.Sprintf("https://%s:%d", scope.ingressSpec.APIHost, scope.ingressSpec.Port)
+		joinURL = controlPlaneJoinURL(scope.ingressSpec.APIHost, scope.ingressSpec.Port)
 	}
 
 	joinToken, err := kutil.CreateK0sJoinToken(ca.KeyPair.Cert, token, joinURL, "kubelet-bootstrap")
@@ -541,6 +543,12 @@ func (r *Controller) getK0sToken(ctx context.Context, scope *Scope) (string, err
 	return joinToken, nil
 }
 
+// controlPlaneJoinURL builds the k0s join URL for the given host and port.
+// It uses net.JoinHostPort so that IPv6 literals are bracketed correctly.
+func controlPlaneJoinURL(host string, port int64) string {
+	return "https://" + net.JoinHostPort(host, strconv.FormatInt(port, 10))
+}
+
 func createIngressCommands(scope *Scope) []string {
 	if scope.ingressSpec == nil {
 		return []string{}

internal/controller/bootstrap/worker_bootstrap_controller_test.go

@@ -145,3 +145,22 @@ func Test_getWindowsCommands(t *testing.T) {
 	}
 
 }
+
+func Test_controlPlaneJoinURL(t *testing.T) {
+	tests := []struct {
+		name string
+		host string
+		port int64
+		want string
+	}{
+		{"hostname", "cp.example.com", 443, "https://cp.example.com:443"},
+		{"ipv4", "10.0.0.1", 6443, "https://10.0.0.1:6443"},
+		{"ipv6", "2001:db8::1", 443, "https://[2001:db8::1]:443"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			require.Equal(t, tt.want, controlPlaneJoinURL(tt.host, tt.port))
+		})
+	}
+}