feat: add support for ingress backed istio gateways

docs/tutorials/istio.md

@@ -270,6 +270,34 @@ transfer-encoding: chunked
 
 **Note:** The `-H` flag in the original Istio tutorial is no longer necessary in the `curl` commands.
 
+### Optional Gateway Annotation
+
+To support setups where an Ingress resource is used provision an external LB you can add the following annotation to your Gateway
+
+**Note:** The Ingress namespace can be omitted if its in the same namespace as the gateway
+
+```bash
+$ cat <<EOF | kubectl apply -f -
+apiVersion: networking.istio.io/v1alpha3
+kind: Gateway
+metadata:
+  name: httpbin-gateway
+  namespace: istio-system
+  annotations:
+    "external-dns.alpha.kubernetes.io/ingress": "$ingressNamespace/$ingressName"
+spec:
+  selector:
+    istio: ingressgateway # use Istio default gateway implementation
+  servers:
+  - port:
+      number: 80
+      name: http
+      protocol: HTTP
+    hosts:
+    - "*"
+EOF
+```
+
 ### Debug ExternalDNS
 
 * Look for the deployment pod to see the status

source/istio_gateway.go

@@ -38,6 +38,10 @@ import (
 	"sigs.k8s.io/external-dns/endpoint"
 )
 
+// IstioGatewayIngressSource is the annotation used to determine if the gateway is implemented by an Ingress object
+// instead of a standard LoadBalancer service type
+const IstioGatewayIngressSource = "external-dns.alpha.kubernetes.io/ingress"
+
 // gatewaySource is an implementation of Source for Istio Gateway objects.
 // The gateway implementation uses the spec.servers.hosts values for the hostnames.
 // Use targetAnnotationKey to explicitly set Endpoint.
@@ -166,7 +170,7 @@ func (sc *gatewaySource) Endpoints(ctx context.Context) ([]*endpoint.Endpoint, e
 			continue
 		}
 
-		gwEndpoints, err := sc.endpointsFromGateway(gwHostnames, gateway)
+		gwEndpoints, err := sc.endpointsFromGateway(ctx, gwHostnames, gateway)
 		if err != nil {
 			return nil, err
 		}
@@ -232,12 +236,55 @@ func (sc *gatewaySource) setResourceLabel(gateway *networkingv1alpha3.Gateway, e
 	}
 }
 
-func (sc *gatewaySource) targetsFromGateway(gateway *networkingv1alpha3.Gateway) (targets endpoint.Targets, err error) {
+func parseIngress(ingress string) (namespace, name string, err error) {
+	parts := strings.Split(ingress, "/")
+	if len(parts) == 2 {
+		namespace, name = parts[0], parts[1]
+	} else if len(parts) == 1 {
+		name = parts[0]
+	} else {
+		err = fmt.Errorf("invalid ingress name (name or namespace/name) found %q", ingress)
+	}
+
+	return
+}
+
+func (sc *gatewaySource) targetsFromIngress(ctx context.Context, ingressStr string, gateway *networkingv1alpha3.Gateway) (targets endpoint.Targets, err error) {
+	namespace, name, err := parseIngress(ingressStr)
+	if err != nil {
+		return nil, fmt.Errorf("failed to parse Ingress annotation on Gateway (%s/%s): %w", gateway.Namespace, gateway.Name, err)
+	}
+	if namespace == "" {
+		namespace = gateway.Namespace
+	}
+
+	ingress, err := sc.kubeClient.NetworkingV1().Ingresses(namespace).Get(ctx, name, metav1.GetOptions{})
+	if err != nil {
+		log.Error(err)
+		return
+	}
+	for _, lb := range ingress.Status.LoadBalancer.Ingress {
+		if lb.IP != "" {
+			targets = append(targets, lb.IP)
+		} else if lb.Hostname != "" {
+			targets = append(targets, lb.Hostname)
+		}
+	}
+	return
+}
+
+func (sc *gatewaySource) targetsFromGateway(ctx context.Context, gateway *networkingv1alpha3.Gateway) (targets endpoint.Targets, err error) {
 	targets = getTargetsFromTargetAnnotation(gateway.Annotations)
 	if len(targets) > 0 {
 		return
 	}
 
+	ingressStr, ok := gateway.Annotations[IstioGatewayIngressSource]
+	if ok && ingressStr != "" {
+		targets, err = sc.targetsFromIngress(ctx, ingressStr, gateway)
+		return
+	}
+
 	services, err := sc.serviceInformer.Lister().Services(sc.namespace).List(labels.Everything())
 	if err != nil {
 		log.Error(err)
@@ -262,7 +309,7 @@ func (sc *gatewaySource) targetsFromGateway(gateway *networkingv1alpha3.Gateway)
 }
 
 // endpointsFromGatewayConfig extracts the endpoints from an Istio Gateway Config object
-func (sc *gatewaySource) endpointsFromGateway(hostnames []string, gateway *networkingv1alpha3.Gateway) ([]*endpoint.Endpoint, error) {
+func (sc *gatewaySource) endpointsFromGateway(ctx context.Context, hostnames []string, gateway *networkingv1alpha3.Gateway) ([]*endpoint.Endpoint, error) {
 	var endpoints []*endpoint.Endpoint
 
 	annotations := gateway.Annotations
@@ -274,7 +321,7 @@ func (sc *gatewaySource) endpointsFromGateway(hostnames []string, gateway *netwo
 	targets := getTargetsFromTargetAnnotation(annotations)
 
 	if len(targets) == 0 {
-		targets, err = sc.targetsFromGateway(gateway)
+		targets, err = sc.targetsFromGateway(ctx, gateway)
 		if err != nil {
 			return nil, err
 		}