feat: support dual stack for gateway api

docs/sources/gateway.md

@@ -80,3 +80,45 @@ the values from that.
 adding each address's `value`. 
 
 The targets from each parent Gateway matching the *Route are then combined and de-duplicated.
+
+## Dualstack Routes
+
+Gateway resources may be served from an external-loadbalancer which may support both IPv4 and "dualstack" (both IPv4 and IPv6) interfaces.
+External DNS Controller uses the `external-dns.alpha.kubernetes.io/dualstack` annotation to determine this. If this annotation is
+set to `true` then ExternalDNS will create two alias records (one A record
+and one AAAA record) for each hostname associated with the Route resource.
+
+Example:
+
+```yaml
+apiVersion: gateway.networking.k8s.io/v1
+kind: HTTPRoute
+metadata:
+  annotations:
+    konghq.com/strip-path: "true"
+    external-dns.alpha.kubernetes.io/dualstack: "true"
+  name: echo
+spec:
+  hostnames:
+    - echoserver.example.org
+  parentRefs:
+    - group: gateway.networking.k8s.io
+      kind: Gateway
+      name: kong
+      namespace: kong
+  rules:
+    - backendRefs:
+        - group: ""
+          kind: Service
+          name: echo
+          port: 1027
+          weight: 1
+      matches:
+        - path:
+            type: PathPrefix
+            value: /echo
+```
+
+The above HTTPRoute resource is backed by a dualstack Gateway.
+ExternalDNS will create both an A `echoserver.example.org` record and
+an AAAA record of the same name, that each are aliases for the same LB.

source/gateway.go

@@ -44,6 +44,10 @@ import (
 const (
 	gatewayGroup = "gateway.networking.k8s.io"
 	gatewayKind  = "Gateway"
+	// GatewayAPIDualstackAnnotationKey is the annotation used for determining if a Gateway Route is dualstack
+	GatewayAPIDualstackAnnotationKey = "external-dns.alpha.kubernetes.io/dualstack"
+	// GatewayAPIDualstackAnnotationValue is the value of the Gateway Route dualstack annotation that indicates it is dualstack
+	GatewayAPIDualstackAnnotationValue = "true"
 )
 
 type gatewayRoute interface {
@@ -235,6 +239,7 @@ func (src *gatewayRouteSource) Endpoints(ctx context.Context) ([]*endpoint.Endpo
 		for host, targets := range hostTargets {
 			endpoints = append(endpoints, endpointsForHostname(host, targets, ttl, providerSpecific, setIdentifier, resource)...)
 		}
+		setDualstackLabel(rt, endpoints)
 		log.Debugf("Endpoints generated from %s %s/%s: %v", src.rtKind, meta.Namespace, meta.Name, endpoints)
 	}
 	return endpoints, nil
@@ -609,3 +614,13 @@ func selectorsEqual(a, b labels.Selector) bool {
 	}
 	return true
 }
+
+func setDualstackLabel(rt gatewayRoute, endpoints []*endpoint.Endpoint) {
+	val, ok := rt.Metadata().Annotations[GatewayAPIDualstackAnnotationKey]
+	if ok && val == GatewayAPIDualstackAnnotationValue {
+		log.Debugf("Adding dualstack label to GatewayRoute %s/%s.", rt.Metadata().Namespace, rt.Metadata().Name)
+		for _, ep := range endpoints {
+			ep.Labels[endpoint.DualstackLabelKey] = "true"
+		}
+	}
+}

source/gateway_test.go

@@ -20,6 +20,7 @@ import (
 	"strings"
 	"testing"
 
+	"sigs.k8s.io/external-dns/endpoint"
 	v1 "sigs.k8s.io/gateway-api/apis/v1"
 )
 
@@ -245,3 +246,47 @@ func TestIsDNS1123Domain(t *testing.T) {
 		})
 	}
 }
+
+func TestDualStackLabel(t *testing.T) {
+	tests := []struct {
+		desc      string
+		in        map[string](string)
+		setsLabel bool
+	}{
+		{
+			desc:      "empty-annotation",
+			setsLabel: false,
+		},
+		{
+			desc:      "correct-annotation-key-and-value",
+			in:        map[string]string{GatewayAPIDualstackAnnotationKey: GatewayAPIDualstackAnnotationValue},
+			setsLabel: true,
+		},
+		{
+			desc:      "correct-annotation-key-incorrect-value",
+			in:        map[string]string{GatewayAPIDualstackAnnotationKey: "foo"},
+			setsLabel: false,
+		},
+		{
+			desc:      "incorrect-annotation-key-correct-value",
+			in:        map[string]string{"FOO": GatewayAPIDualstackAnnotationValue},
+			setsLabel: false,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.desc, func(t *testing.T) {
+			endpoints := make([]*endpoint.Endpoint, 0)
+			endpoints = append(endpoints, endpoint.NewEndpoint("www.example.com", endpoint.RecordTypeA, "10.0.0.2", "10.0.0.3"))
+
+			rt := &gatewayHTTPRoute{}
+			rt.Metadata().Annotations = tt.in
+
+			setDualstackLabel(rt, endpoints)
+			got := endpoints[0].Labels[endpoint.DualstackLabelKey] == "true"
+
+			if got != tt.setsLabel {
+				t.Errorf("setDualstackLabel(%q); got: %v; want: %v", tt.in, got, tt.setsLabel)
+			}
+		})
+	}
+}