Fix buffer.UDP destination override (#2356)

common/buf/override.go

@@ -0,0 +1,38 @@
+package buf
+
+import (
+	"github.com/xtls/xray-core/common/net"
+)
+
+type EndpointOverrideReader struct {
+	Reader
+	Dest         net.Address
+	OriginalDest net.Address
+}
+
+func (r *EndpointOverrideReader) ReadMultiBuffer() (MultiBuffer, error) {
+	mb, err := r.Reader.ReadMultiBuffer()
+	if err == nil {
+		for _, b := range mb {
+			if b.UDP != nil && b.UDP.Address == r.OriginalDest {
+				b.UDP.Address = r.Dest
+			}
+		}
+	}
+	return mb, err
+}
+
+type EndpointOverrideWriter struct {
+	Writer
+	Dest         net.Address
+	OriginalDest net.Address
+}
+
+func (w *EndpointOverrideWriter) WriteMultiBuffer(mb MultiBuffer) error {
+	for _, b := range mb {
+		if b.UDP != nil && b.UDP.Address == w.Dest {
+			b.UDP.Address = w.OriginalDest
+		}
+	}
+	return w.Writer.WriteMultiBuffer(mb)
+}