具有VpnService的Android防火墙

洪建茗

2023-03-14

问题内容：

我正在尝试使用VpnService for
BS项目为Android实现一个简单的防火墙。我选择VpnService，因为它将在非root用户的设备上运行。它将记录连接并让您过滤连接。（基于IP）

有一个应用程序可以这样做。

GooglePlay应用商店

我进行了一些研究，发现VpnService创建了Tun接口。而已。（没有VPN实现只是一个隧道）它使您可以对此接口进行地址分配并添加路由。它返回一个文件描述符。您可以读取传出的程序包并编写传入的程序包。

我创建了一个VpnService派生类并开始服务。我可以tun0使用VpnService.Builder类进行配置。当我查看mobiwol's与adb shell netcfg它的连接时，会创建一个tun0具有10.2.3.4/32地址的接口。它将所有程序包路由到该专用网络并发送到Internet。我正在尝试相同。创建一个具有10.0.0.2/32地址的接口。使用addRoute函数添加了一条路由。0.0.0.0/0，据我了解，我可以从所有网络捕获所有软件包。（我对这门学科还很陌生，现在还在学习。我在网上找到了一些作品，所以我不确定。如果我错了，请纠正我。）

我在服务中创建了2个线程。一个从文件描述符读取并使用受保护的套接字将其写入127.0.0.1。（我不太确定我是否应该读/写127.0.0.1。也许这是问题所在。）

我分析了从文件描述符读取的数据包。例如：

01000101    byte:69     //ipv4 20byte header
00000000    byte:0      //TOS
00000000    byte:0      //Total Length
00111100    byte:60     //Total Length
11111100    byte:-4     //ID
11011011    byte:-37    //ID
01000000    byte:64     //fragment
00000000    byte:0      //"
01000000    byte:64     //TTL
00000110    byte:6      //Protocol 6 -> TCP
01011110    byte:94     //Header checksum
11001111    byte:-49    //Header checksum
00001010    byte:10     //10.0.0.2
00000000    byte:0
00000000    byte:0
00000010    byte:2
10101101    byte:-83    //173.194.39.78 //google
00111110    byte:-62
00100111    byte:39
********    byte:78

10110100    byte:-76    // IP option
01100101    byte:101
00000001    byte:1
10111011    byte:-69
                //20byte IP haeder
01101101    byte:109
.       .       //40byte data (i couldnt parse TCP header, 
                    I think its not needed when I route this in IP layer)
.       .
.       .
00000110    byte:6

我没有在其余数据中找到任何其他IP标头。我认为应该在10.0.0.2网络到本地网络（192.168.2.1）和Internet之间进行封装。我不确定。

我真正的问题是我停留在传入的包线程上。我什么也看不懂。没有反应。如您在屏幕快照中看到的，没有传入数据：

屏幕截图

我正在尝试从与用于使用受保护套接字写入127.0.0.1的同一连接读取数据。

Android <-> Tun接口（tun0）<-> Internet连接

所有软件包<-> 10.0.0.2 <-> 127.0.0.1？<-> 192.168.2.1 <-> Internet？

我找不到有关VpnService的任何帮助。（ToyVPN示例只是无用的）我阅读了有关Linux Tun /
Tap的文档，但有关主机和远程之间的隧道传输的文档。我希望主机和远程设备位于同一设备上。不喜欢隧道。

我怎样才能做到这一点？

编辑：要求的代码。还处于初期。如前所述，它是VpnService派生的类。在服务线程中创建了2个线程（读和写）。

package com.git.firewall;

public class GITVpnService extends VpnService implements Handler.Callback, Runnable {
    private static final String TAG = "GITVpnService";

    private String mServerAddress = "127.0.0.1";
    private int mServerPort = 55555;
    private PendingIntent mConfigureIntent;

    private Handler mHandler;
    private Thread mThread;

    private ParcelFileDescriptor mInterface;

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        // The handler is only used to show messages.
        if (mHandler == null) {
            mHandler = new Handler(this);
        }

        // Stop the previous session by interrupting the thread.
        if (mThread != null) {
            mThread.interrupt();
        }
        // Start a new session by creating a new thread.
        mThread = new Thread(this, "VpnThread");
        mThread.start();
        return START_STICKY;
    }

    @Override
    public void onDestroy() {
        if (mThread != null) {
            mThread.interrupt();
        }
    }

    @Override
    public boolean handleMessage(Message message) {
        if (message != null) {
            Toast.makeText(this, (String)message.obj, Toast.LENGTH_SHORT).show();
        }
        return true;
    }

    @Override
    public synchronized void run() {
        try {
            Log.i(TAG, "Starting");
            InetSocketAddress server = new InetSocketAddress(
                    mServerAddress, mServerPort);

            run(server);

              } catch (Exception e) {
            Log.e(TAG, "Got " + e.toString());
            try {
                mInterface.close();
            } catch (Exception e2) {
                // ignore
            }
            Message msgObj = mHandler.obtainMessage();
            msgObj.obj = "Disconnected";
            mHandler.sendMessage(msgObj);

        } finally {

        }
    }

    DatagramChannel mTunnel = null;


    private boolean run(InetSocketAddress server) throws Exception {
        boolean connected = false;

        android.os.Debug.waitForDebugger();

        // Create a DatagramChannel as the VPN tunnel.
        mTunnel = DatagramChannel.open();

        // Protect the tunnel before connecting to avoid loopback.
        if (!protect(mTunnel.socket())) {
            throw new IllegalStateException("Cannot protect the tunnel");
        }

        // Connect to the server.
        mTunnel.connect(server);

        // For simplicity, we use the same thread for both reading and
        // writing. Here we put the tunnel into non-blocking mode.
        mTunnel.configureBlocking(false);

        // Authenticate and configure the virtual network interface.
        handshake();

        // Now we are connected. Set the flag and show the message.
        connected = true;
        Message msgObj = mHandler.obtainMessage();
        msgObj.obj = "Connected";
        mHandler.sendMessage(msgObj);

        new Thread ()
        {
            public void run ()
                {
                    // Packets to be sent are queued in this input stream.
                    FileInputStream in = new FileInputStream(mInterface.getFileDescriptor());
                    // Allocate the buffer for a single packet.
                    ByteBuffer packet = ByteBuffer.allocate(32767);
                    int length;
                    try
                    {
                        while (true)
                        {
                            while ((length = in.read(packet.array())) > 0) {
                                    // Write the outgoing packet to the tunnel.
                                    packet.limit(length);
                                    debugPacket(packet);    // Packet size, Protocol, source, destination
                                    mTunnel.write(packet);
                                    packet.clear();

                                }
                            }
                    }
                    catch (IOException e)
                    {
                            e.printStackTrace();
                    }

            }
        }.start();

        new Thread ()
        {

            public void run ()
            {
                    DatagramChannel tunnel = mTunnel;
                    // Allocate the buffer for a single packet.
                    ByteBuffer packet = ByteBuffer.allocate(8096);
                    // Packets received need to be written to this output stream.
                    FileOutputStream out = new FileOutputStream(mInterface.getFileDescriptor());

                    while (true)
                    {
                        try
                        {
                            // Read the incoming packet from the tunnel.
                            int length;
                            while ((length = tunnel.read(packet)) > 0)
                            {
                                    // Write the incoming packet to the output stream.
                                out.write(packet.array(), 0, length);

                                packet.clear();

                            }
                        }
                        catch (IOException ioe)
                        {
                                ioe.printStackTrace();
                        }
                    }
            }
        }.start();

        return connected;
    }

    private void handshake() throws Exception {

        if (mInterface == null)
        {
            Builder builder = new Builder();

            builder.setMtu(1500);
            builder.addAddress("10.0.0.2",32);
            builder.addRoute("0.0.0.0", 0);
            //builder.addRoute("192.168.2.0",24);
            //builder.addDnsServer("8.8.8.8");

            // Close the old interface since the parameters have been changed.
            try {
                mInterface.close();
            } catch (Exception e) {
                // ignore
            }


            // Create a new interface using the builder and save the parameters.
            mInterface = builder.setSession("GIT VPN")
                    .setConfigureIntent(mConfigureIntent)
                    .establish();
        }
    }

    private void debugPacket(ByteBuffer packet)
    {
        /*
        for(int i = 0; i < length; ++i)
        {
            byte buffer = packet.get();

            Log.d(TAG, "byte:"+buffer);
        }*/



        int buffer = packet.get();
        int version;
        int headerlength;
        version = buffer >> 4;
        headerlength = buffer & 0x0F;
        headerlength *= 4;
        Log.d(TAG, "IP Version:"+version);
        Log.d(TAG, "Header Length:"+headerlength);

        String status = "";
        status += "Header Length:"+headerlength;

        buffer = packet.get();      //DSCP + EN
        buffer = packet.getChar();  //Total Length

        Log.d(TAG, "Total Length:"+buffer);

        buffer = packet.getChar();  //Identification
        buffer = packet.getChar();  //Flags + Fragment Offset
        buffer = packet.get();      //Time to Live
        buffer = packet.get();      //Protocol

        Log.d(TAG, "Protocol:"+buffer);

        status += "  Protocol:"+buffer;

        buffer = packet.getChar();  //Header checksum

        String sourceIP  = "";
        buffer = packet.get();  //Source IP 1st Octet
        sourceIP += buffer;
        sourceIP += ".";

        buffer = packet.get();  //Source IP 2nd Octet
        sourceIP += buffer;
        sourceIP += ".";

        buffer = packet.get();  //Source IP 3rd Octet
        sourceIP += buffer;
        sourceIP += ".";

        buffer = packet.get();  //Source IP 4th Octet
        sourceIP += buffer;

        Log.d(TAG, "Source IP:"+sourceIP);

        status += "   Source IP:"+sourceIP;

        String destIP  = "";
        buffer = packet.get();  //Destination IP 1st Octet
        destIP += buffer;
        destIP += ".";

        buffer = packet.get();  //Destination IP 2nd Octet
        destIP += buffer;
        destIP += ".";

        buffer = packet.get();  //Destination IP 3rd Octet
        destIP += buffer;
        destIP += ".";

        buffer = packet.get();  //Destination IP 4th Octet
        destIP += buffer;

        Log.d(TAG, "Destination IP:"+destIP);

        status += "   Destination IP:"+destIP;
        /*
        msgObj = mHandler.obtainMessage();
        msgObj.obj = status;
        mHandler.sendMessage(msgObj);
        */

        //Log.d(TAG, "version:"+packet.getInt());
        //Log.d(TAG, "version:"+packet.getInt());
        //Log.d(TAG, "version:"+packet.getInt());

    }

}

问题答案：

几个月前，有人提出了类似的问题，虽然答案并不十分有见地，但接受的答案中的评论使您可以洞悉可能出了什么问题。

您应该牢记逻辑驻留在OSI模型的哪一层：

VpnService的传入和传出流在网络层中。正如您在问题中所描述的，您正在接收（并且应该依次传输）原始IP数据包。

在示例字节流中，您可以看到传入的字节流是IPv4数据报，因为前四个位是0100（4）。有关IPv4的详细信息，请查阅此数据包结构规范。

转发请求时，您在应用程序层中。您应该分别使用DatagramSocket或Socket 传输UDP或TCP有效负载的内容（即，仅传输其数据，而不传输标头本身）。

请记住，这会跳过传输层，因为那些实现会负责构造UDP标头（对于DatagramSocket）和TCP标头和选项（对于Socket）。

您的应用程序本质上将需要能够解释和构造IPv4和IPv6标头和选项，以及作为IP有效负载的UDP标头和TCP标头和选项。

具有VpnService的Android防火墙

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