dns: Add preferred_by rule item

dns/transport/hosts/hosts.go

@@ -19,7 +19,10 @@ func RegisterTransport(registry *dns.TransportRegistry) {
 	dns.RegisterTransport[option.HostsDNSServerOptions](registry, C.DNSTypeHosts, NewTransport)
 }
 
-var _ adapter.DNSTransport = (*Transport)(nil)
+var (
+	_ adapter.DNSTransport                    = (*Transport)(nil)
+	_ adapter.DNSTransportWithPreferredDomain = (*Transport)(nil)
+)
 
 type Transport struct {
 	dns.TransportAdapter
@@ -66,6 +69,18 @@ func (t *Transport) Close() error {
 func (t *Transport) Reset() {
 }
 
+func (t *Transport) PreferredDomain(domain string) bool {
+	if _, loaded := t.predefined[domain]; loaded {
+		return true
+	}
+	for _, file := range t.files {
+		if len(file.Lookup(domain)) > 0 {
+			return true
+		}
+	}
+	return false
+}
+
 func (t *Transport) Exchange(ctx context.Context, message *mDNS.Msg) (*mDNS.Msg, error) {
 	question := message.Question[0]
 	domain := mDNS.CanonicalName(question.Name)

dns/transport/local/local.go

@@ -23,7 +23,10 @@ func RegisterTransport(registry *dns.TransportRegistry) {
 	dns.RegisterTransport[option.LocalDNSServerOptions](registry, C.DNSTypeLocal, NewTransport)
 }
 
-var _ adapter.DNSTransport = (*Transport)(nil)
+var (
+	_ adapter.DNSTransport                    = (*Transport)(nil)
+	_ adapter.DNSTransportWithPreferredDomain = (*Transport)(nil)
+)
 
 type Transport struct {
 	dns.TransportAdapter
@@ -97,6 +100,15 @@ func (t *Transport) Close() error {
 func (t *Transport) Reset() {
 }
 
+func (t *Transport) PreferredDomain(domain string) bool {
+	if t.hosts != nil && t.resolved == nil {
+		if len(t.hosts.Lookup(dns.FqdnToDomain(domain))) > 0 {
+			return true
+		}
+	}
+	return t.hasNeighborHost(domain)
+}
+
 func (t *Transport) Exchange(ctx context.Context, message *mDNS.Msg) (*mDNS.Msg, error) {
 	if t.resolved != nil {
 		response := t.lookupNeighbor(message)

dns/transport/local/local_darwin.go

@@ -24,7 +24,10 @@ func RegisterTransport(registry *dns.TransportRegistry) {
 	dns.RegisterTransport[option.LocalDNSServerOptions](registry, C.DNSTypeLocal, NewTransport)
 }
 
-var _ adapter.DNSTransport = (*Transport)(nil)
+var (
+	_ adapter.DNSTransport                    = (*Transport)(nil)
+	_ adapter.DNSTransportWithPreferredDomain = (*Transport)(nil)
+)
 
 type Transport struct {
 	dns.TransportAdapter
@@ -106,3 +109,12 @@ func (t *Transport) Reset() {
 		t.dhcpTransport.Reset()
 	}
 }
+
+func (t *Transport) PreferredDomain(domain string) bool {
+	if t.hosts != nil {
+		if len(t.hosts.Lookup(dns.FqdnToDomain(domain))) > 0 {
+			return true
+		}
+	}
+	return t.hasNeighborHost(domain)
+}

dns/transport/local/local_neighbor.go

@@ -43,6 +43,17 @@ func (t *Transport) lookupNeighbor(message *mDNS.Msg) *mDNS.Msg {
 	return dns.FixedResponse(message.Id, question, addresses, C.DefaultDNSTTL)
 }
 
+func (t *Transport) hasNeighborHost(domain string) bool {
+	if t.neighborResolver == nil {
+		return false
+	}
+	host := extractNeighborHost(domain, t.neighborSuffixes)
+	if host == "" {
+		return false
+	}
+	return len(t.neighborResolver.LookupAddresses(host)) > 0
+}
+
 func extractNeighborHost(canonical string, suffixes []string) string {
 	for _, suffix := range suffixes {
 		if !strings.HasSuffix(canonical, suffix) || len(canonical) <= len(suffix) {