Re-add missing changes from #556 back after they got lost in #651 (#868)

The generic handling of the overlay handling unintentionally removed the
source NAT to upstream DNS server setting. This change introduces it
again.

pkg/admission/mutator/shoot.go

@@ -31,8 +31,9 @@ type shoot struct {
 }
 
 const (
-	overlayKey = "overlay"
-	enabledKey = "enabled"
+	overlayKey           = "overlay"
+	snatToUpstreamDNSKey = "snatToUpstreamDNS"
+	enabledKey           = "enabled"
 )
 
 // Mutate mutates the given shoot object.
@@ -95,6 +96,10 @@ func (s *shoot) Mutate(_ context.Context, new, old client.Object) error {
 			}
 		}
 
+		if networkConfig[overlayKey] != nil && !networkConfig[overlayKey].(map[string]interface{})[enabledKey].(bool) {
+			networkConfig[snatToUpstreamDNSKey] = map[string]interface{}{enabledKey: false}
+		}
+
 		modifiedJSON, err := json.Marshal(networkConfig)
 		if err != nil {
 			return err

pkg/admission/mutator/shoot_test.go

@@ -161,7 +161,7 @@ var _ = Describe("Shoot mutator", func() {
 				err := shootMutator.Mutate(ctx, shoot, nil)
 				Expect(err).NotTo(HaveOccurred())
 				Expect(shoot.Spec.Networking.ProviderConfig).To(Equal(&runtime.RawExtension{
-					Raw: []byte(`{"overlay":{"enabled":false}}`),
+					Raw: []byte(`{"overlay":{"enabled":false},"snatToUpstreamDNS":{"enabled":false}}`),
 				}))
 			})
 
@@ -172,7 +172,7 @@ var _ = Describe("Shoot mutator", func() {
 				err := shootMutator.Mutate(ctx, shoot, nil)
 				Expect(err).NotTo(HaveOccurred())
 				Expect(shoot.Spec.Networking.ProviderConfig).To(Equal(&runtime.RawExtension{
-					Raw: []byte(`{"foo":{"enabled":true},"overlay":{"enabled":false}}`),
+					Raw: []byte(`{"foo":{"enabled":true},"overlay":{"enabled":false},"snatToUpstreamDNS":{"enabled":false}}`),
 				}))
 			})