Fix sending to IPv4 addresses from an IPv6 dual-stack socket (#640) (#641)

* Fix sending to IPv4 addresses from an IPv6 dual-stack socket (#640)

Author: ltlobaro

net/connUDP.go

@@ -516,14 +516,25 @@ func (c *UDPConn) writeTo(raddr *net.UDPAddr, cm *ControlMessage, buffer []byte)
 		// because the connection is already established.
 		return c.connection.Write(buffer)
 	}
-	// On Linux, UDP network binds both IPv6 and IPv4 addresses to the same socket.
-	// When receiving a packet from an IPv4 address, we cannot send a packet from an IPv6 address.
-	// Therefore, we wrap the connection using an IPv4 packet connection (packetConn).
-	if !IsIPv6(raddr.IP) && c.packetConn.IsIPv6() {
-		pc := packetConnIPv4{packetConn: ipv4.NewPacketConn(c.connection)}
-		return pc.WriteTo(buffer, cm, raddr)
-	}
-	return c.packetConn.WriteTo(buffer, cm, raddr)
+
+	var cmb []byte
+
+	if cm != nil && IsIPv6(raddr.IP) {
+		m := &ipv6.ControlMessage{
+			Src:     cm.Src,
+			IfIndex: cm.IfIndex,
+		}
+		cmb = m.Marshal()
+	} else if cm != nil {
+		m := &ipv4.ControlMessage{
+			Src:     cm.Src,
+			IfIndex: cm.IfIndex,
+		}
+		cmb = m.Marshal()
+	}
+
+	i, _, err := c.connection.WriteMsgUDP(buffer, cmb, raddr)
+	return i, err
 }
 
 type UDPWriteCfg struct {

net/connUDP_internal_test.go

@@ -58,6 +58,16 @@ func TestUDPConnWriteWithContext(t *testing.T) {
 			},
 			wantErr: true,
 		},
+		{
+			name: "send to v4 from v6 socket",
+			args: args{
+				ctx:           context.Background(),
+				listenNetwork: udpNetwork,
+				udpAddr:       b,
+				buffer:        []byte("hello world"),
+			},
+			wantErr: false,
+		},
 	}
 	if runtime.GOOS == "linux" {
 		tests = append(tests, struct {