Switch mDNS probe to multicast, use mDNS for reachability

Unicast mDNS queries (direct to IP:5353) are ignored by some HomeKit
devices. Switch to multicast (224.0.0.251:5353) and filter responses
by sender IP. Also consider mDNS response as reachability signal.

Split probe timeouts: 100ms for ports/DNS/HTTP, 120ms total to give
mDNS extra time. HomeKit responds in ~0.2ms via multicast.

internal/probe/probe.go

@@ -16,7 +16,7 @@ import (
 	_ "modernc.org/sqlite"
 )
 
-const probeTimeout = 100 * time.Millisecond
+const probeTimeout = 120 * time.Millisecond
 
 var log zerolog.Logger
 var db *sql.DB
@@ -78,14 +78,17 @@ func runProbe(parent context.Context, ip string) *probe.Response {
 		}()
 	}
 
+	fastCtx, fastCancel := context.WithTimeout(ctx, 100*time.Millisecond)
+	defer fastCancel()
+
 	run(func() {
-		r, _ := probe.ScanPorts(ctx, ip, ports)
+		r, _ := probe.ScanPorts(fastCtx, ip, ports)
 		mu.Lock()
 		resp.Probes.Ports = r
 		mu.Unlock()
 	})
 	run(func() {
-		r, _ := probe.ReverseDNS(ctx, ip)
+		r, _ := probe.ReverseDNS(fastCtx, ip)
 		mu.Lock()
 		resp.Probes.DNS = r
 		mu.Unlock()
@@ -107,7 +110,7 @@ func runProbe(parent context.Context, ip string) *probe.Response {
 		mu.Unlock()
 	})
 	run(func() {
-		r, _ := probe.ProbeHTTP(ctx, ip, nil)
+		r, _ := probe.ProbeHTTP(fastCtx, ip, nil)
 		mu.Lock()
 		resp.Probes.HTTP = r
 		mu.Unlock()
@@ -116,7 +119,8 @@ func runProbe(parent context.Context, ip string) *probe.Response {
 	wg.Wait()
 
 	// determine reachable
-	resp.Reachable = resp.Probes.Ports != nil && len(resp.Probes.Ports.Open) > 0
+	resp.Reachable = (resp.Probes.Ports != nil && len(resp.Probes.Ports.Open) > 0) ||
+		resp.Probes.MDNS != nil
 
 	// determine type
 	resp.Type = "standard"

pkg/probe/mdns.go

@@ -22,8 +22,11 @@ const (
 	categoryDoorbell = "18"
 )
 
-// QueryHAP sends unicast mDNS query to ip:5353 for HomeKit service.
-// Returns nil if device is not a HomeKit camera/doorbell.
+var multicastAddr = &net.UDPAddr{IP: net.IP{224, 0, 0, 251}, Port: 5353}
+
+// QueryHAP sends multicast mDNS query for HomeKit service and waits
+// for a response from the specified ip. Returns nil if device is not
+// a HomeKit camera/doorbell.
 func QueryHAP(ctx context.Context, ip string) (*MDNSResult, error) {
 	msg := &dns.Msg{
 		Question: []dns.Question{
@@ -36,7 +39,7 @@ func QueryHAP(ctx context.Context, ip string) (*MDNSResult, error) {
 		return nil, err
 	}
 
-	conn, err := net.ListenPacket("udp4", ":0")
+	conn, err := net.ListenMulticastUDP("udp4", nil, multicastAddr)
 	if err != nil {
 		return nil, err
 	}
@@ -44,27 +47,34 @@ func QueryHAP(ctx context.Context, ip string) (*MDNSResult, error) {
 
 	deadline, ok := ctx.Deadline()
 	if !ok {
-		deadline = time.Now().Add(100 * time.Millisecond)
+		deadline = time.Now().Add(time.Second)
 	}
 	_ = conn.SetDeadline(deadline)
 
-	addr := &net.UDPAddr{IP: net.ParseIP(ip), Port: 5353}
-	if _, err = conn.WriteTo(query, addr); err != nil {
+	if _, err = conn.WriteTo(query, multicastAddr); err != nil {
 		return nil, err
 	}
 
+	targetIP := net.ParseIP(ip)
 	buf := make([]byte, 1500)
-	n, _, err := conn.ReadFrom(buf)
-	if err != nil {
-		return nil, nil // timeout = not a HomeKit device
-	}
 
-	var resp dns.Msg
-	if err = resp.Unpack(buf[:n]); err != nil {
-		return nil, nil
-	}
+	for {
+		n, from, err := conn.ReadFrom(buf)
+		if err != nil {
+			return nil, nil // timeout
+		}
 
-	return parseHAPResponse(&resp)
+		if !from.(*net.UDPAddr).IP.Equal(targetIP) {
+			continue
+		}
+
+		var resp dns.Msg
+		if err = resp.Unpack(buf[:n]); err != nil {
+			continue
+		}
+
+		return parseHAPResponse(&resp)
+	}
 }
 
 // internals