virtual-net: initial (#4751)

pkg/vnet/message.go

@@ -0,0 +1,81 @@
+// Copyright 2025 The frp Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package vnet
+
+import (
+	"encoding/binary"
+	"fmt"
+	"io"
+)
+
+// Maximum message size
+const (
+	maxMessageSize = 1024 * 1024 // 1MB
+)
+
+// Format: [length(4 bytes)][data(length bytes)]
+
+// ReadMessage reads a framed message from the reader
+func ReadMessage(r io.Reader) ([]byte, error) {
+	// Read length (4 bytes)
+	var length uint32
+	err := binary.Read(r, binary.LittleEndian, &length)
+	if err != nil {
+		return nil, fmt.Errorf("read message length error: %v", err)
+	}
+
+	// Check length to prevent DoS
+	if length == 0 {
+		return nil, fmt.Errorf("message length is 0")
+	}
+	if length > maxMessageSize {
+		return nil, fmt.Errorf("message too large: %d > %d", length, maxMessageSize)
+	}
+
+	// Read message data
+	data := make([]byte, length)
+	_, err = io.ReadFull(r, data)
+	if err != nil {
+		return nil, fmt.Errorf("read message data error: %v", err)
+	}
+
+	return data, nil
+}
+
+// WriteMessage writes a framed message to the writer
+func WriteMessage(w io.Writer, data []byte) error {
+	// Get data length
+	length := uint32(len(data))
+	if length == 0 {
+		return fmt.Errorf("message data length is 0")
+	}
+	if length > maxMessageSize {
+		return fmt.Errorf("message too large: %d > %d", length, maxMessageSize)
+	}
+
+	// Write length
+	err := binary.Write(w, binary.LittleEndian, length)
+	if err != nil {
+		return fmt.Errorf("write message length error: %v", err)
+	}
+
+	// Write message data
+	_, err = w.Write(data)
+	if err != nil {
+		return fmt.Errorf("write message data error: %v", err)
+	}
+
+	return nil
+}