fix for windows/macos

net/connUDP.go

@@ -24,8 +24,10 @@ type UDPConn struct {
 }
 
 type ControlMessage struct {
-	Dst     net.IP // destination address, receiving only
-	Src     net.IP // source address, specifying only
+	// For connection oriented packetConn the ControlMessage fields are ignored, only linux supports set control message.
+
+	Dst     net.IP // destination address of the packet
+	Src     net.IP // source address of the packet
 	IfIndex int    // interface index, 0 means any interface
 }
 
@@ -538,6 +540,12 @@ func (c *UDPConn) WriteWithContext(ctx context.Context, raddr *net.UDPAddr, cm *
 	if c.closed.Load() {
 		return ErrConnectionIsClosed
 	}
+	if !supportsSetPacketControlMessage(c.connection) {
+		// if remote address is set, we can use it as destination address
+		// because connection is already connected
+		// only linux supports overwriting destination address
+		raddr = nil
+	}
 	n, err := c.packetConn.WriteTo(buffer, cm, raddr)
 	if err != nil {
 		return err

net/connUDP_internal_test.go

@@ -369,20 +369,25 @@ func TestUDPConnWriteToAddr(t *testing.T) {
 	}
 }
 
-func TestPacketConnIPv4ReadFrom(t *testing.T) {
+func TestPacketConnReadFrom(t *testing.T) {
 	readUDP4Conn, err := net.ListenUDP("udp4", &net.UDPAddr{Port: 1234})
 	require.NoError(t, err)
 	defer func() {
 		errC := readUDP4Conn.Close()
 		require.NoError(t, errC)
 	}()
+
+	require.Nil(t, readUDP4Conn.RemoteAddr())
+
 	writeUDP4Conn, err := net.DialUDP("udp4", nil, readUDP4Conn.LocalAddr().(*net.UDPAddr))
 	require.NoError(t, err)
 	defer func() {
 		errC := writeUDP4Conn.Close()
 		require.NoError(t, errC)
 	}()
 
+	require.NotNil(t, writeUDP4Conn.RemoteAddr())
+
 	readUDP6Conn, err := net.ListenUDP("udp6", &net.UDPAddr{Port: 1235})
 	require.NoError(t, err)
 	defer func() {

net/supportsSetPacketControlMessage.go

@@ -0,0 +1,9 @@
+//go:build !linux
+
+package net
+
+import "net"
+
+func supportsSetPacketControlMessage(c *net.UDPConn) bool {
+	return c.RemoteAddr() == nil
+}

net/supportsSetPacketControlMessage_linux.go

@@ -0,0 +1,7 @@
+package net
+
+import "net"
+
+func supportsSetPacketControlMessage(*net.UDPConn) bool {
+	return true
+}