fix for windows/macos

Author: jkralik

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
 }
 
@@ -524,6 +526,16 @@ func (c *UDPConn) writeMulticast(ctx context.Context, raddr *net.UDPAddr, buffer
 	return nil
 }
 
+func (c *UDPConn) writeTo(raddr *net.UDPAddr, cm *ControlMessage, buffer []byte) (int, error) {
+	if !supportsOverrideRemoteAddr(c.connection) {
+		// If the remote address is set, we can use it as the destination address
+		// because the connection is already established.
+		// Note: Overwriting the destination address is only supported on Linux.
+		return c.connection.Write(buffer)
+	}
+	return c.packetConn.WriteTo(buffer, cm, raddr)
+}
+
 // WriteWithContext writes data with context.
 func (c *UDPConn) WriteWithContext(ctx context.Context, raddr *net.UDPAddr, cm *ControlMessage, buffer []byte) error {
 	if raddr == nil {
@@ -538,7 +550,7 @@ func (c *UDPConn) WriteWithContext(ctx context.Context, raddr *net.UDPAddr, cm *
 	if c.closed.Load() {
 		return ErrConnectionIsClosed
 	}
-	n, err := c.packetConn.WriteTo(buffer, cm, raddr)
+	n, err := c.writeTo(raddr, cm, buffer)
 	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/supportsOverrideRemoteAddr.go

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

net/supportsOverrideRemoteAddr_linux.go

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