fix: send mDNS queries on multicast sockets

client.go

@@ -397,25 +397,45 @@ func (c *client) query(params *QueryParam) error {
 	}
 }
 
-// sendQuery is used to multicast a query out
+// sendQuery is used to multicast a query out.
+//
+// Queries are written to the multicast destination (RFC 6762). Some devices
+// only answer queries that originate on a multicast socket, so we send via the
+// multicast connections when available. We also still write from the unicast
+// sockets for environments where loopback/self-discovery relies on them.
+//
+// Error handling: each available socket is attempted. If any write succeeds the
+// query is considered sent and nil is returned. Only when every attempted write
+// fails do we surface the last write error (matching the original "fail if we
+// could not send" contract without aborting mid-attempt).
 func (c *client) sendQuery(q *dns.Msg) error {
 	buf, err := q.Pack()
 	if err != nil {
 		return err
 	}
-	if c.ipv4UnicastConn != nil {
-		_, err = c.ipv4UnicastConn.WriteToUDP(buf, ipv4Addr)
-		if err != nil {
-			return err
+
+	var last error
+	var wrote bool
+	tryWrite := func(conn *net.UDPConn, addr *net.UDPAddr) {
+		if conn == nil {
+			return
 		}
-	}
-	if c.ipv6UnicastConn != nil {
-		_, err = c.ipv6UnicastConn.WriteToUDP(buf, ipv6Addr)
-		if err != nil {
-			return err
+		if _, err := conn.WriteToUDP(buf, addr); err != nil {
+			last = err
+			return
 		}
+		wrote = true
 	}
-	return nil
+
+	tryWrite(c.ipv4MulticastConn, ipv4Addr)
+	tryWrite(c.ipv6MulticastConn, ipv6Addr)
+	tryWrite(c.ipv4UnicastConn, ipv4Addr)
+	tryWrite(c.ipv6UnicastConn, ipv6Addr)
+
+	if wrote {
+		return nil
+	}
+	return last
 }
 
 // recv is used to receive until we get a shutdown

client_test.go

@@ -0,0 +1,87 @@
+// Copyright IBM Corp. 2014, 2026
+// SPDX-License-Identifier: MIT
+
+package mdns
+
+import (
+	"log"
+	"net"
+	"testing"
+
+	"github.com/miekg/dns"
+)
+
+func testQuestion() *dns.Msg {
+	m := new(dns.Msg)
+	m.SetQuestion("_foobar._tcp.local.", dns.TypePTR)
+	m.RecursionDesired = false
+	return m
+}
+
+func TestSendQuery_WritesOnMulticastAndUnicast(t *testing.T) {
+	// Bind ephemeral UDP sockets and wire them into a client so sendQuery can
+	// exercise both multicast and unicast paths without requiring the real
+	// 5353 mDNS port.
+	mconn, err := net.ListenUDP("udp4", &net.UDPAddr{IP: net.IPv4zero, Port: 0})
+	if err != nil {
+		t.Fatalf("listen multicast stand-in: %v", err)
+	}
+	defer mconn.Close()
+
+	uconn, err := net.ListenUDP("udp4", &net.UDPAddr{IP: net.IPv4zero, Port: 0})
+	if err != nil {
+		t.Fatalf("listen unicast stand-in: %v", err)
+	}
+	defer uconn.Close()
+
+	c := &client{
+		ipv4MulticastConn: mconn,
+		ipv4UnicastConn:   uconn,
+		log:               log.Default(),
+	}
+
+	if err := c.sendQuery(testQuestion()); err != nil {
+		t.Fatalf("sendQuery: %v", err)
+	}
+}
+
+func TestSendQuery_SucceedsIfAnySocketWrites(t *testing.T) {
+	// Closed connection always fails WriteToUDP; open one should still make
+	// sendQuery return nil (do not surface partial failures).
+	bad, err := net.ListenUDP("udp4", &net.UDPAddr{IP: net.IPv4zero, Port: 0})
+	if err != nil {
+		t.Fatalf("listen: %v", err)
+	}
+	_ = bad.Close()
+
+	good, err := net.ListenUDP("udp4", &net.UDPAddr{IP: net.IPv4zero, Port: 0})
+	if err != nil {
+		t.Fatalf("listen: %v", err)
+	}
+	defer good.Close()
+
+	c := &client{
+		ipv4MulticastConn: bad,
+		ipv4UnicastConn:   good,
+		log:               log.Default(),
+	}
+	if err := c.sendQuery(testQuestion()); err != nil {
+		t.Fatalf("expected nil when at least one write succeeds, got %v", err)
+	}
+}
+
+func TestSendQuery_AllWritesFail(t *testing.T) {
+	bad, err := net.ListenUDP("udp4", &net.UDPAddr{IP: net.IPv4zero, Port: 0})
+	if err != nil {
+		t.Fatalf("listen: %v", err)
+	}
+	_ = bad.Close()
+
+	c := &client{
+		ipv4UnicastConn: bad,
+		log:             log.Default(),
+	}
+	if err := c.sendQuery(testQuestion()); err == nil {
+		t.Fatal("expected error when every write fails")
+	}
+}