cmd/k8s-operator: support ExternalName Services

Adds support for exposing endpoints, accessible from within
a cluster to the tailnet via DNS names using ExternalName Services.
This can be done by annotating the ExternalName Service with
tailscale.com/expose: "true" annotation.
The operator will deploy a proxy configured to route tailnet
traffic to the backend IPs that service.spec.externalName
resolves to. The backend IPs must be reachable from the operator's
namespace.

Signed-off-by: Irbe Krumina <irbe@tailscale.com>

cmd/k8s-operator/operator_test.go

@@ -1424,6 +1424,73 @@ func Test_clusterDomainFromResolverConf(t *testing.T) {
 	}
 }
 
+func Test_externalNameService(t *testing.T) {
+	fc := fake.NewFakeClient()
+	ft := &fakeTSClient{}
+	zl, err := zap.NewDevelopment()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// 1. A External name Service that should be exposed via Tailscale gets
+	// created.
+	sr := &ServiceReconciler{
+		Client: fc,
+		ssr: &tailscaleSTSReconciler{
+			Client:            fc,
+			tsClient:          ft,
+			defaultTags:       []string{"tag:k8s"},
+			operatorNamespace: "operator-ns",
+			proxyImage:        "tailscale/tailscale",
+		},
+		logger: zl.Sugar(),
+	}
+
+	// 1. Create an ExternalName Service that we should manage, and check that the initial round
+	// of objects looks right.
+	mustCreate(t, fc, &corev1.Service{
+		ObjectMeta: metav1.ObjectMeta{
+			Name:      "test",
+			Namespace: "default",
+			// The apiserver is supposed to set the UID, but the fake client
+			// doesn't. So, set it explicitly because other code later depends
+			// on it being set.
+			UID: types.UID("1234-UID"),
+			Annotations: map[string]string{
+				AnnotationExpose: "true",
+			},
+		},
+		Spec: corev1.ServiceSpec{
+			Type:         corev1.ServiceTypeExternalName,
+			ExternalName: "foo.com",
+		},
+	})
+
+	expectReconciled(t, sr, "default", "test")
+
+	fullName, shortName := findGenName(t, fc, "default", "test", "svc")
+	opts := configOpts{
+		stsName:          shortName,
+		secretName:       fullName,
+		namespace:        "default",
+		parentType:       "svc",
+		hostname:         "default-test",
+		clusterTargetDNS: "foo.com",
+	}
+
+	expectEqual(t, fc, expectedSecret(t, opts), nil)
+	expectEqual(t, fc, expectedHeadlessService(shortName, "svc"), nil)
+	expectEqual(t, fc, expectedSTS(t, fc, opts), removeHashAnnotation)
+
+	// 2. Change the ExternalName and verify that changes get propagated.
+	mustUpdate(t, sr, "default", "test", func(s *corev1.Service) {
+		s.Spec.ExternalName = "bar.com"
+	})
+	expectReconciled(t, sr, "default", "test")
+	opts.clusterTargetDNS = "bar.com"
+	expectEqual(t, fc, expectedSTS(t, fc, opts), removeHashAnnotation)
+}
+
 func toFQDN(t *testing.T, s string) dnsname.FQDN {
 	t.Helper()
 	fqdn, err := dnsname.ToFQDN(s)

cmd/k8s-operator/sts.go

@@ -87,6 +87,7 @@ const (
 	// ensure that it does not get removed when a ProxyClass configuration
 	// is applied.
 	podAnnotationLastSetClusterIP         = "tailscale.com/operator-last-set-cluster-ip"
+	podAnnotationLastSetClusterDNSName    = "tailscale.com/operator-last-set-cluster-dns-name"
 	podAnnotationLastSetTailnetTargetIP   = "tailscale.com/operator-last-set-ts-tailnet-target-ip"
 	podAnnotationLastSetTailnetTargetFQDN = "tailscale.com/operator-last-set-ts-tailnet-target-fqdn"
 	// podAnnotationLastSetConfigFileHash is sha256 hash of the current tailscaled configuration contents.
@@ -109,8 +110,9 @@ type tailscaleSTSConfig struct {
 	ParentResourceUID   string
 	ChildResourceLabels map[string]string
 
-	ServeConfig     *ipn.ServeConfig // if serve config is set, this is a proxy for Ingress
-	ClusterTargetIP string           // ingress target
+	ServeConfig          *ipn.ServeConfig // if serve config is set, this is a proxy for Ingress
+	ClusterTargetIP      string           // ingress target IP
+	ClusterTargetDNSName string           // ingress target DNS name
 	// If set to true, operator should configure containerboot to forward
 	// cluster traffic via the proxy set up for Kubernetes Ingress.
 	ForwardClusterTrafficViaL7IngressProxy bool
@@ -536,6 +538,12 @@ func (a *tailscaleSTSReconciler) reconcileSTS(ctx context.Context, logger *zap.S
 			Value: sts.ClusterTargetIP,
 		})
 		mak.Set(&ss.Spec.Template.Annotations, podAnnotationLastSetClusterIP, sts.ClusterTargetIP)
+	} else if sts.ClusterTargetDNSName != "" {
+		container.Env = append(container.Env, corev1.EnvVar{
+			Name:  "TS_DEST_DNS_NAME",
+			Value: sts.ClusterTargetDNSName,
+		})
+		mak.Set(&ss.Spec.Template.Annotations, podAnnotationLastSetClusterDNSName, sts.ClusterTargetDNSName)
 	} else if sts.TailnetTargetIP != "" {
 		container.Env = append(container.Env, corev1.EnvVar{
 			Name:  "TS_TAILNET_TARGET_IP",

cmd/k8s-operator/svc.go

@@ -208,10 +208,14 @@ func (a *ServiceReconciler) maybeProvision(ctx context.Context, logger *zap.Suga
 	}
 
 	a.mu.Lock()
-	if a.shouldExpose(svc) {
+	if a.shouldExposeClusterIP(svc) {
 		sts.ClusterTargetIP = svc.Spec.ClusterIP
 		a.managedIngressProxies.Add(svc.UID)
 		gaugeIngressProxies.Set(int64(a.managedIngressProxies.Len()))
+	} else if a.shouldExposeDNSName(svc) {
+		sts.ClusterTargetDNSName = svc.Spec.ExternalName
+		a.managedIngressProxies.Add(svc.UID)
+		gaugeIngressProxies.Set(int64(a.managedIngressProxies.Len()))
 	} else if ip := a.tailnetTargetAnnotation(svc); ip != "" {
 		sts.TailnetTargetIP = ip
 		a.managedEgressProxies.Add(svc.UID)
@@ -303,15 +307,22 @@ func validateService(svc *corev1.Service) []string {
 }
 
 func (a *ServiceReconciler) shouldExpose(svc *corev1.Service) bool {
+	return a.shouldExposeClusterIP(svc) || a.shouldExposeDNSName(svc)
+}
+
+func (a *ServiceReconciler) shouldExposeClusterIP(svc *corev1.Service) bool {
 	// Headless services can't be exposed, since there is no ClusterIP to
 	// forward to.
 	if svc.Spec.ClusterIP == "" || svc.Spec.ClusterIP == "None" {
 		return false
 	}
-
 	return a.hasLoadBalancerClass(svc) || a.hasExposeAnnotation(svc)
 }
 
+func (a *ServiceReconciler) shouldExposeDNSName(svc *corev1.Service) bool {
+	return a.hasExposeAnnotation(svc) && svc.Spec.Type == corev1.ServiceTypeExternalName && svc.Spec.ExternalName != ""
+}
+
 func (a *ServiceReconciler) hasLoadBalancerClass(svc *corev1.Service) bool {
 	return svc != nil &&
 		svc.Spec.Type == corev1.ServiceTypeLoadBalancer &&

cmd/k8s-operator/testutils_test.go

@@ -43,6 +43,7 @@ type configOpts struct {
 	tailnetTargetIP                                string
 	tailnetTargetFQDN                              string
 	clusterTargetIP                                string
+	clusterTargetDNS                               string
 	subnetRoutes                                   string
 	isExitNode                                     bool
 	confFileHash                                   string
@@ -126,6 +127,12 @@ func expectedSTS(t *testing.T, cl client.Client, opts configOpts) *appsv1.Statef
 			Value: opts.clusterTargetIP,
 		})
 		annots["tailscale.com/operator-last-set-cluster-ip"] = opts.clusterTargetIP
+	} else if opts.clusterTargetDNS != "" {
+		tsContainer.Env = append(tsContainer.Env, corev1.EnvVar{
+			Name:  "TS_DEST_DNS_NAME",
+			Value: opts.clusterTargetDNS,
+		})
+		annots["tailscale.com/operator-last-set-cluster-dns-name"] = opts.clusterTargetDNS
 	}
 	if opts.serveConfig != nil {
 		tsContainer.Env = append(tsContainer.Env, corev1.EnvVar{

wgengine/router/router_linux_test.go

@@ -470,6 +470,10 @@ func (n *fakeIPTablesRunner) AddDNATRule(origDst, dst netip.Addr) error {
 	return errors.New("not implemented")
 }
 
+func (n *fakeIPTablesRunner) DNATWithLoadBalancer(netip.Addr, []netip.Addr) error {
+	return errors.New("not implemented")
+}
+
 func (n *fakeIPTablesRunner) AddSNATRuleForDst(src, dst netip.Addr) error {
 	return errors.New("not implemented")
 }