Implement

AGENTS.md

@@ -0,0 +1,38 @@
+# 概要
+アプリ名: DummyVPN
+ターゲットAPIレベル: 33 (Android 13)
+言語: Kotlin
+UIフレームワーク: Jetpack Compose
+
+DummyVPNは、実際にリモートのVPNサーバーと接続するのではなく、アプリで受けたパケットをただインターネットに中継するVPNアプリである。
+そのためWi-Fiやモバイルネットワークへのパケット転送のみを行う。
+
+# 画面一覧
+## メイン画面
+シンプルかつモダンなデザインにする。まぶしい色は使わない(白は例外)。
+
+### VPNのオンオフボタン
+ボタンのサイズはなるべく大きく分かりやすいようにする。
+VPNが有効の場合は「ON」、無効の場合は「OFF」と表示する。
+ONの場合はボタンの色を変えて、視覚的に分かりやすくする。
+
+# 機能一覧
+## VPN機能
+メインの機能。概要の通り、すべてのパケットを通常のネットワークに転送する。何も制御しない。
+VPNはすべてのアプリに適用する。
+
+## 通知での操作機能
+VPNが有効の場合は、OSに通知を表示して動作している旨を表示する。
+また、「停止」ボタンを設け、通知から本アプリのVPN機能を停止できるようにする。
+
+# 動作
+バックグラウンドサービスを使用して、アプリを離れても動作できるようにする。
+
+# コーディングの注意事項
+- 権限が必要な機能は、requestPermissionを使用してアプリの起動時にユーザーに権限付与のリクエストをする。
+- テストは作成しない。
+
+# 技術的な事項
+- パケットのパススルーはtun2socksを使用する。
+  そのためローカルにSocksプロキシを作成する。
+  VPN開始時にSocksプロキシも起動し、停止時にプロキシも停止する。

app/build.gradle.kts

@@ -37,6 +37,11 @@ android {
     buildFeatures {
         compose = true
     }
+    packaging {
+        jniLibs {
+            useLegacyPackaging = true
+        }
+    }
 }
 
 dependencies {
@@ -55,4 +60,4 @@ dependencies {
     androidTestImplementation(libs.androidx.junit)
     debugImplementation(libs.androidx.compose.ui.test.manifest)
     debugImplementation(libs.androidx.compose.ui.tooling)
-}
+}

app/src/main/AndroidManifest.xml

@@ -2,6 +2,12 @@
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
     xmlns:tools="http://schemas.android.com/tools">
 
+    <uses-permission android:name="android.permission.FOREGROUND_SERVICE" />
+    <uses-permission android:name="android.permission.FOREGROUND_SERVICE_SPECIAL_USE" />
+    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
+    <uses-permission android:name="android.permission.INTERNET" />
+    <uses-permission android:name="android.permission.POST_NOTIFICATIONS" />
+
     <application
         android:allowBackup="true"
         android:dataExtractionRules="@xml/data_extraction_rules"
@@ -22,6 +28,20 @@
                 <category android:name="android.intent.category.LAUNCHER" />
             </intent-filter>
         </activity>
+
+        <service
+            android:name=".DummyVpnService"
+            android:exported="false"
+            android:foregroundServiceType="specialUse"
+            android:permission="android.permission.BIND_VPN_SERVICE">
+            <intent-filter>
+                <action android:name="android.net.VpnService" />
+            </intent-filter>
+            <property
+                android:name="android.app.PROPERTY_SPECIAL_USE_FGS_SUBTYPE"
+                android:value="Pass-through local VPN service" />
+        </service>
+        <meta-data android:name="android.telephony.PROPERTY_SATELLITE_DATA_OPTIMIZED" android:value="jp.cyberrex.dummyvpn" />
     </application>
 
-</manifest>
+</manifest>

app/src/main/java/jp/cyberrex/dummyvpn/DummyVpnService.kt

@@ -0,0 +1,186 @@
+package jp.cyberrex.dummyvpn
+
+import android.app.Notification
+import android.app.NotificationChannel
+import android.app.NotificationManager
+import android.app.PendingIntent
+import android.content.Intent
+import android.net.ConnectivityManager
+import android.net.Network
+import android.net.NetworkCapabilities
+import android.net.VpnService
+import android.os.Build
+import android.os.ParcelFileDescriptor
+import androidx.core.app.NotificationCompat
+import androidx.core.content.ContextCompat
+import androidx.core.content.getSystemService
+import java.net.Inet4Address
+
+class DummyVpnService : VpnService() {
+    private var tunInterface: ParcelFileDescriptor? = null
+    private var socksProxy: LocalSocksProxy? = null
+    private var tun2SocksProcess: Tun2SocksProcess? = null
+
+    override fun onStartCommand(intent: Intent?, flags: Int, startId: Int): Int {
+        when (intent?.action) {
+            ACTION_STOP -> stopVpn()
+            ACTION_START, null -> startVpn()
+        }
+        return START_STICKY
+    }
+
+    override fun onDestroy() {
+        stopVpn()
+        super.onDestroy()
+    }
+
+    private fun startVpn() {
+        if (VpnStatusStore.status.value == VpnStatus.On) {
+            return
+        }
+
+        VpnStatusStore.set(VpnStatus.Starting)
+
+        try {
+            createNotificationChannel()
+            startForeground(
+                NOTIFICATION_ID,
+                buildNotification(),
+                if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.UPSIDE_DOWN_CAKE) {
+                    android.content.pm.ServiceInfo.FOREGROUND_SERVICE_TYPE_SPECIAL_USE
+                } else {
+                    0
+                }
+            )
+
+            val underlyingNetwork = findUnderlyingNetwork()
+            val builder = Builder()
+                .setSession(getString(R.string.app_name))
+                .setMtu(1500)
+                .addAddress("198.18.0.2", 24)
+                .addRoute("0.0.0.0", 0)
+
+            val dnsServers = findDnsServers(underlyingNetwork)
+            dnsServers.ifEmpty { listOf("1.1.1.1") }.forEach { dnsServer ->
+                builder.addDnsServer(dnsServer)
+            }
+            underlyingNetwork?.let { network ->
+                builder.setUnderlyingNetworks(arrayOf(network))
+            }
+
+            val tun = builder.establish()
+                ?: throw IllegalStateException("VPN interface could not be established")
+
+            tunInterface = tun
+
+            val proxy = LocalSocksProxy(this)
+            proxy.start()
+            socksProxy = proxy
+
+            val process = Tun2SocksProcess(this)
+            process.start(tun, proxy.port)
+            tun2SocksProcess = process
+            VpnStatusStore.set(VpnStatus.On)
+        } catch (error: Exception) {
+            stopRuntime()
+            val message = error.message ?: getString(R.string.vpn_error_unknown)
+            VpnStatusStore.set(VpnStatus.Error(message))
+            stopForeground(STOP_FOREGROUND_REMOVE)
+            stopSelf()
+        }
+    }
+
+    private fun stopVpn() {
+        if (VpnStatusStore.status.value != VpnStatus.Off) {
+            VpnStatusStore.set(VpnStatus.Stopping)
+        }
+        stopRuntime()
+        VpnStatusStore.set(VpnStatus.Off)
+        stopForeground(STOP_FOREGROUND_REMOVE)
+        stopSelf()
+    }
+
+    private fun stopRuntime() {
+        tun2SocksProcess?.stop()
+        tun2SocksProcess = null
+
+        socksProxy?.stop()
+        socksProxy = null
+
+        try {
+            tunInterface?.close()
+        } catch (_: Exception) {
+        }
+        tunInterface = null
+    }
+
+    private fun createNotificationChannel() {
+        val manager = getSystemService<NotificationManager>() ?: return
+        val channel = NotificationChannel(
+            CHANNEL_ID,
+            getString(R.string.vpn_notification_channel),
+            NotificationManager.IMPORTANCE_LOW
+        )
+        manager.createNotificationChannel(channel)
+    }
+
+    private fun buildNotification(): Notification {
+        val launchIntent = PendingIntent.getActivity(
+            this,
+            0,
+            Intent(this, MainActivity::class.java),
+            PendingIntent.FLAG_IMMUTABLE or PendingIntent.FLAG_UPDATE_CURRENT
+        )
+        val stopIntent = PendingIntent.getService(
+            this,
+            1,
+            Intent(this, DummyVpnService::class.java).setAction(ACTION_STOP),
+            PendingIntent.FLAG_IMMUTABLE or PendingIntent.FLAG_UPDATE_CURRENT
+        )
+
+        return NotificationCompat.Builder(this, CHANNEL_ID)
+            .setSmallIcon(R.drawable.ic_launcher_foreground)
+            .setContentTitle(getString(R.string.vpn_notification_title))
+            .setContentText(getString(R.string.vpn_notification_text))
+            .setContentIntent(launchIntent)
+            .setOngoing(true)
+            .setPriority(NotificationCompat.PRIORITY_LOW)
+            .addAction(0, getString(R.string.vpn_notification_stop), stopIntent)
+            .build()
+    }
+
+    private fun findUnderlyingNetwork(): Network? {
+        val connectivityManager = getSystemService<ConnectivityManager>() ?: return null
+        return connectivityManager.allNetworks.firstOrNull { network ->
+            val capabilities = connectivityManager.getNetworkCapabilities(network) ?: return@firstOrNull false
+            capabilities.hasCapability(NetworkCapabilities.NET_CAPABILITY_INTERNET) &&
+                !capabilities.hasTransport(NetworkCapabilities.TRANSPORT_VPN)
+        }
+    }
+
+    private fun findDnsServers(network: Network?): List<String> {
+        val connectivityManager = getSystemService<ConnectivityManager>() ?: return emptyList()
+        val linkProperties = connectivityManager.getLinkProperties(network ?: return emptyList())
+            ?: return emptyList()
+        return linkProperties.dnsServers
+            .filterIsInstance<Inet4Address>()
+            .mapNotNull { it.hostAddress }
+    }
+
+    companion object {
+        const val ACTION_START = "jp.cyberrex.dummyvpn.action.START"
+        const val ACTION_STOP = "jp.cyberrex.dummyvpn.action.STOP"
+
+        private const val CHANNEL_ID = "dummy_vpn"
+        private const val NOTIFICATION_ID = 1001
+
+        fun start(context: android.content.Context) {
+            val intent = Intent(context, DummyVpnService::class.java).setAction(ACTION_START)
+            ContextCompat.startForegroundService(context, intent)
+        }
+
+        fun stop(context: android.content.Context) {
+            context.startService(Intent(context, DummyVpnService::class.java).setAction(ACTION_STOP))
+        }
+    }
+}

app/src/main/java/jp/cyberrex/dummyvpn/LocalSocksProxy.kt

@@ -0,0 +1,470 @@
+package jp.cyberrex.dummyvpn
+
+import android.net.VpnService
+import android.net.ConnectivityManager
+import android.net.Network
+import android.net.NetworkCapabilities
+import android.util.Log
+import java.io.EOFException
+import java.io.IOException
+import java.io.InputStream
+import java.io.OutputStream
+import java.net.DatagramPacket
+import java.net.DatagramSocket
+import java.net.InetAddress
+import java.net.InetSocketAddress
+import java.net.ServerSocket
+import java.net.Socket
+import java.net.SocketException
+import java.net.SocketTimeoutException
+import java.util.Collections
+import java.util.concurrent.ExecutorService
+import java.util.concurrent.Executors
+import java.util.concurrent.TimeUnit
+
+class LocalSocksProxy(private val vpnService: VpnService) {
+    private var serverSocket: ServerSocket? = null
+    private var executor: ExecutorService? = null
+    private val openSockets = Collections.synchronizedSet(mutableSetOf<Socket>())
+    private val openDatagramSockets = Collections.synchronizedSet(mutableSetOf<DatagramSocket>())
+
+    val port: Int
+        get() = serverSocket?.localPort ?: 0
+
+    fun start() {
+        stop()
+
+        val server = ServerSocket()
+        server.reuseAddress = true
+        server.bind(InetSocketAddress(InetAddress.getByName(LOCALHOST), 0))
+        serverSocket = server
+
+        val pool = Executors.newCachedThreadPool()
+        executor = pool
+        pool.execute {
+            acceptLoop(server, pool)
+        }
+    }
+
+    fun stop() {
+        try {
+            serverSocket?.close()
+        } catch (_: IOException) {
+        }
+        serverSocket = null
+
+        synchronized(openSockets) {
+            openSockets.forEach { socket ->
+                try {
+                    socket.close()
+                } catch (_: IOException) {
+                }
+            }
+            openSockets.clear()
+        }
+        synchronized(openDatagramSockets) {
+            openDatagramSockets.forEach { socket ->
+                socket.close()
+            }
+            openDatagramSockets.clear()
+        }
+
+        executor?.shutdownNow()
+        try {
+            executor?.awaitTermination(1, TimeUnit.SECONDS)
+        } catch (_: InterruptedException) {
+            Thread.currentThread().interrupt()
+        }
+        executor = null
+    }
+
+    private fun acceptLoop(server: ServerSocket, pool: ExecutorService) {
+        while (!server.isClosed) {
+            try {
+                val client = server.accept()
+                openSockets.add(client)
+                pool.execute {
+                    handleClient(client)
+                }
+            } catch (_: SocketException) {
+                break
+            } catch (_: IOException) {
+            }
+        }
+    }
+
+    private fun handleClient(client: Socket) {
+        client.use {
+            try {
+                val input = client.getInputStream()
+                val output = client.getOutputStream()
+                negotiate(input, output)
+
+                val requestVersion = input.readByte()
+                if (requestVersion != SOCKS_VERSION) {
+                    throw IOException("Unsupported SOCKS request version")
+                }
+
+                when (val command = input.readByte()) {
+                    COMMAND_CONNECT -> handleConnect(client, input, output)
+                    COMMAND_UDP_ASSOCIATE -> handleUdpAssociate(client, input, output)
+                    else -> {
+                        skipAddress(input)
+                        sendReply(output, REPLY_COMMAND_NOT_SUPPORTED, InetAddress.getByName(LOCALHOST), 0)
+                    }
+                }
+            } catch (_: IOException) {
+            } finally {
+                openSockets.remove(client)
+            }
+        }
+    }
+
+    private fun negotiate(input: InputStream, output: OutputStream) {
+        val version = input.readByte()
+        if (version != SOCKS_VERSION) throw IOException("Unsupported SOCKS version")
+
+        val methodCount = input.readByte()
+        var supportsNoAuth = false
+        repeat(methodCount) {
+            if (input.readByte() == METHOD_NO_AUTH) supportsNoAuth = true
+        }
+
+        if (supportsNoAuth) {
+            output.write(byteArrayOf(SOCKS_VERSION.toByte(), METHOD_NO_AUTH.toByte()))
+        } else {
+            output.write(byteArrayOf(SOCKS_VERSION.toByte(), METHOD_NOT_ACCEPTABLE.toByte()))
+            throw IOException("No acceptable SOCKS authentication method")
+        }
+    }
+
+    private fun handleConnect(client: Socket, input: InputStream, output: OutputStream) {
+        input.readByte()
+        val destination = readAddress(input)
+        val remote = createOutboundTcpSocket()
+        openSockets.add(remote)
+
+        try {
+            remote.connect(InetSocketAddress(destination.host, destination.port), CONNECT_TIMEOUT_MS)
+            sendReply(output, REPLY_SUCCEEDED, remote.localAddress, remote.localPort)
+            pipeBothWays(client, remote)
+        } catch (error: IOException) {
+            Log.w(TAG, "CONNECT ${destination.host}:${destination.port} failed: ${error.message}")
+            sendReply(output, REPLY_HOST_UNREACHABLE, InetAddress.getByName(LOCALHOST), 0)
+        } finally {
+            openSockets.remove(remote)
+            try {
+                remote.close()
+            } catch (_: IOException) {
+            }
+        }
+    }
+
+    private fun handleUdpAssociate(client: Socket, input: InputStream, output: OutputStream) {
+        input.readByte()
+        skipAddress(input)
+        val relay = DatagramSocket(InetSocketAddress(InetAddress.getByName(LOCALHOST), 0))
+        openDatagramSockets.add(relay)
+
+        try {
+            protectAndBind(relay)
+            sendReply(output, REPLY_SUCCEEDED, InetAddress.getByName(LOCALHOST), relay.localPort)
+
+            val udpTask = executor?.submit {
+                relayUdpPackets(relay)
+            }
+
+            try {
+                while (client.getInputStream().read() != -1) {
+                }
+            } catch (_: IOException) {
+            }
+            udpTask?.cancel(true)
+        } finally {
+            openDatagramSockets.remove(relay)
+            relay.close()
+        }
+    }
+
+    private fun relayUdpPackets(relay: DatagramSocket) {
+        var clientAddress: InetSocketAddress? = null
+        val buffer = ByteArray(UDP_BUFFER_SIZE)
+
+        while (!relay.isClosed) {
+            try {
+                val packet = DatagramPacket(buffer, buffer.size)
+                relay.receive(packet)
+                val source = InetSocketAddress(packet.address, packet.port)
+                val data = packet.data.copyOfRange(packet.offset, packet.offset + packet.length)
+
+                if (clientAddress == null || source == clientAddress) {
+                    clientAddress = source
+                    val request = parseUdpRequest(data) ?: continue
+                    executor?.execute {
+                        relaySingleUdpRequest(relay, source, request)
+                    }
+                }
+            } catch (_: SocketException) {
+                break
+            } catch (_: IOException) {
+            }
+        }
+    }
+
+    private fun relaySingleUdpRequest(relay: DatagramSocket, client: InetSocketAddress, request: UdpRequest) {
+        val remote = DatagramSocket()
+        openDatagramSockets.add(remote)
+
+        try {
+            protectAndBind(remote)
+            remote.soTimeout = UDP_REPLY_TIMEOUT_MS
+            val targetAddress = InetAddress.getByName(request.host)
+            remote.send(DatagramPacket(request.payload, request.payload.size, targetAddress, request.port))
+
+            val buffer = ByteArray(UDP_BUFFER_SIZE)
+            val reply = DatagramPacket(buffer, buffer.size)
+            remote.receive(reply)
+
+            val response = buildUdpResponse(
+                reply.address,
+                reply.port,
+                reply.data.copyOfRange(reply.offset, reply.offset + reply.length)
+            )
+            relay.send(DatagramPacket(response, response.size, client.address, client.port))
+        } catch (_: SocketTimeoutException) {
+        } catch (_: IOException) {
+        } finally {
+            openDatagramSockets.remove(remote)
+            remote.close()
+        }
+    }
+
+    private fun parseUdpRequest(data: ByteArray): UdpRequest? {
+        if (data.size < 10 || data[0].toInt() != 0 || data[1].toInt() != 0 || data[2].toInt() != 0) {
+            return null
+        }
+
+        var offset = 3
+        val host = when (data[offset++].toInt() and 0xff) {
+            ADDRESS_IPV4 -> {
+                if (data.size < offset + 4 + 2) return null
+                InetAddress.getByAddress(data.copyOfRange(offset, offset + 4)).hostAddress.orEmpty()
+                    .also { offset += 4 }
+            }
+
+            ADDRESS_DOMAIN -> {
+                if (data.size < offset + 1) return null
+                val length = data[offset++].toInt() and 0xff
+                if (data.size < offset + length + 2) return null
+                String(data.copyOfRange(offset, offset + length), Charsets.UTF_8)
+                    .also { offset += length }
+            }
+
+            ADDRESS_IPV6 -> {
+                if (data.size < offset + 16 + 2) return null
+                InetAddress.getByAddress(data.copyOfRange(offset, offset + 16)).hostAddress.orEmpty()
+                    .also { offset += 16 }
+            }
+
+            else -> return null
+        }
+
+        val port = ((data[offset].toInt() and 0xff) shl 8) or (data[offset + 1].toInt() and 0xff)
+        offset += 2
+        return UdpRequest(host, port, data.copyOfRange(offset, data.size))
+    }
+
+    private fun buildUdpResponse(address: InetAddress, port: Int, payload: ByteArray): ByteArray {
+        val addressBytes = address.address
+        val addressType = if (addressBytes.size == 16) ADDRESS_IPV6 else ADDRESS_IPV4
+        return byteArrayOf(0, 0, 0, addressType.toByte()) +
+            addressBytes +
+            byteArrayOf(((port ushr 8) and 0xff).toByte(), (port and 0xff).toByte()) +
+            payload
+    }
+
+    private fun pipeBothWays(client: Socket, remote: Socket) {
+        val pool = executor ?: return
+        val first = pool.submit {
+            copyAndClose(client.getInputStream(), remote.getOutputStream(), client, remote)
+        }
+        val second = pool.submit {
+            copyAndClose(remote.getInputStream(), client.getOutputStream(), remote, client)
+        }
+        try {
+            first.get()
+            second.get()
+        } catch (_: Exception) {
+        }
+    }
+
+    private fun copyAndClose(input: InputStream, output: OutputStream, source: Socket, target: Socket) {
+        try {
+            input.copyTo(output)
+        } catch (_: IOException) {
+        } finally {
+            try {
+                source.shutdownInput()
+            } catch (_: IOException) {
+            }
+            try {
+                target.shutdownOutput()
+            } catch (_: IOException) {
+            }
+        }
+    }
+
+    private fun readAddress(input: InputStream): SocksAddress {
+        return when (val addressType = input.readByte()) {
+            ADDRESS_IPV4 -> {
+                val address = ByteArray(4)
+                input.readFully(address)
+                SocksAddress(InetAddress.getByAddress(address).hostAddress.orEmpty(), input.readPort())
+            }
+
+            ADDRESS_DOMAIN -> {
+                val length = input.readByte()
+                val domain = ByteArray(length)
+                input.readFully(domain)
+                SocksAddress(String(domain, Charsets.UTF_8), input.readPort())
+            }
+
+            ADDRESS_IPV6 -> {
+                val address = ByteArray(16)
+                input.readFully(address)
+                SocksAddress(InetAddress.getByAddress(address).hostAddress.orEmpty(), input.readPort())
+            }
+
+            else -> throw IOException("Unsupported address type: $addressType")
+        }
+    }
+
+    private fun skipAddress(input: InputStream) {
+        when (input.readByte()) {
+            ADDRESS_IPV4 -> input.skipFully(4)
+            ADDRESS_DOMAIN -> input.skipFully(input.readByte())
+            ADDRESS_IPV6 -> input.skipFully(16)
+            else -> throw IOException("Unsupported address type")
+        }
+        input.skipFully(2)
+    }
+
+    private fun sendReply(output: OutputStream, reply: Int, address: InetAddress, port: Int) {
+        val addressBytes = if (address.address.size == 16) {
+            InetAddress.getByName(LOCALHOST).address
+        } else {
+            address.address
+        }
+        output.write(
+            byteArrayOf(
+                SOCKS_VERSION.toByte(),
+                reply.toByte(),
+                0,
+                ADDRESS_IPV4.toByte(),
+                addressBytes[0],
+                addressBytes[1],
+                addressBytes[2],
+                addressBytes[3],
+                ((port ushr 8) and 0xff).toByte(),
+                (port and 0xff).toByte()
+            )
+        )
+        output.flush()
+    }
+
+    private fun protectAndBind(socket: Socket) {
+        if (!vpnService.protect(socket)) {
+            throw IOException("Failed to protect outbound TCP socket")
+        }
+        findUnderlyingNetwork()?.bindSocket(socket)
+    }
+
+    private fun createOutboundTcpSocket(): Socket {
+        val network = findUnderlyingNetwork()
+        val socket = network?.socketFactory?.createSocket() ?: Socket()
+        try {
+            if (!vpnService.protect(socket)) {
+                throw IOException("Failed to protect outbound TCP socket")
+            }
+            network?.bindSocket(socket)
+            return socket
+        } catch (error: IOException) {
+            try {
+                socket.close()
+            } catch (_: IOException) {
+            }
+            throw error
+        }
+    }
+
+    private fun protectAndBind(socket: DatagramSocket) {
+        if (!vpnService.protect(socket)) {
+            throw IOException("Failed to protect outbound UDP socket")
+        }
+        findUnderlyingNetwork()?.bindSocket(socket)
+    }
+
+    private fun findUnderlyingNetwork(): Network? {
+        val connectivityManager = vpnService.getSystemService(ConnectivityManager::class.java)
+            ?: return null
+        return connectivityManager.allNetworks.firstOrNull { network ->
+            val capabilities = connectivityManager.getNetworkCapabilities(network) ?: return@firstOrNull false
+            capabilities.hasCapability(NetworkCapabilities.NET_CAPABILITY_INTERNET) &&
+                !capabilities.hasTransport(NetworkCapabilities.TRANSPORT_VPN)
+        }
+    }
+
+    private fun InputStream.readByte(): Int {
+        val value = read()
+        if (value == -1) throw EOFException()
+        return value and 0xff
+    }
+
+    private fun InputStream.readFully(buffer: ByteArray) {
+        var offset = 0
+        while (offset < buffer.size) {
+            val read = read(buffer, offset, buffer.size - offset)
+            if (read == -1) throw EOFException()
+            offset += read
+        }
+    }
+
+    private fun InputStream.skipFully(length: Int) {
+        var remaining = length
+        while (remaining > 0) {
+            val skipped = skip(remaining.toLong()).toInt()
+            if (skipped <= 0) {
+                readByte()
+                remaining--
+            } else {
+                remaining -= skipped
+            }
+        }
+    }
+
+    private fun InputStream.readPort(): Int {
+        return (readByte() shl 8) or readByte()
+    }
+
+    private data class SocksAddress(val host: String, val port: Int)
+    private data class UdpRequest(val host: String, val port: Int, val payload: ByteArray)
+
+    private companion object {
+        const val LOCALHOST = "127.0.0.1"
+        const val SOCKS_VERSION = 5
+        const val METHOD_NO_AUTH = 0
+        const val METHOD_NOT_ACCEPTABLE = 0xff
+        const val COMMAND_CONNECT = 1
+        const val COMMAND_UDP_ASSOCIATE = 3
+        const val ADDRESS_IPV4 = 1
+        const val ADDRESS_DOMAIN = 3
+        const val ADDRESS_IPV6 = 4
+        const val REPLY_SUCCEEDED = 0
+        const val REPLY_COMMAND_NOT_SUPPORTED = 7
+        const val REPLY_HOST_UNREACHABLE = 4
+        const val CONNECT_TIMEOUT_MS = 10_000
+        const val UDP_REPLY_TIMEOUT_MS = 10_000
+        const val UDP_BUFFER_SIZE = 65_535
+        const val TAG = "LocalSocksProxy"
+    }
+}

app/src/main/java/jp/cyberrex/dummyvpn/MainActivity.kt

@@ -1,16 +1,49 @@
 package jp.cyberrex.dummyvpn
 
+import android.Manifest
+import android.app.Activity
+import android.content.Intent
+import android.net.ConnectivityManager
+import android.net.NetworkCapabilities
+import android.net.VpnService
+import android.os.Build
 import android.os.Bundle
 import androidx.activity.ComponentActivity
+import androidx.activity.compose.rememberLauncherForActivityResult
 import androidx.activity.compose.setContent
 import androidx.activity.enableEdgeToEdge
+import androidx.activity.result.contract.ActivityResultContracts
+import androidx.compose.animation.animateColorAsState
+import androidx.compose.foundation.background
+import androidx.compose.foundation.layout.Arrangement
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.Spacer
 import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.foundation.layout.height
 import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.layout.size
+import androidx.compose.foundation.shape.CircleShape
+import androidx.compose.material3.Button
+import androidx.compose.material3.ButtonDefaults
+import androidx.compose.material3.MaterialTheme
 import androidx.compose.material3.Scaffold
+import androidx.compose.material3.Surface
 import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
+import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.runtime.collectAsState
+import androidx.compose.runtime.getValue
+import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
+import androidx.compose.ui.draw.shadow
+import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.platform.LocalContext
+import androidx.compose.ui.text.font.FontWeight
+import androidx.compose.ui.text.style.TextAlign
 import androidx.compose.ui.tooling.preview.Preview
+import androidx.compose.ui.unit.dp
+import androidx.compose.ui.unit.sp
 import jp.cyberrex.dummyvpn.ui.theme.DummyVPNTheme
 
 class MainActivity : ComponentActivity() {
@@ -19,29 +52,188 @@ class MainActivity : ComponentActivity() {
         enableEdgeToEdge()
         setContent {
             DummyVPNTheme {
-                Scaffold(modifier = Modifier.fillMaxSize()) { innerPadding ->
+                DummyVpnApp()
-                    Greeting(
-                        name = "Android",
-                        modifier = Modifier.padding(innerPadding)
-                    )
-                }
             }
         }
     }
 }
 
 @Composable
-fun Greeting(name: String, modifier: Modifier = Modifier) {
+private fun DummyVpnApp() {
-    Text(
+    val context = LocalContext.current
-        text = "Hello $name!",
+    val status by VpnStatusStore.status.collectAsState()
-        modifier = modifier
+
+    val notificationPermissionLauncher = rememberLauncherForActivityResult(
+        ActivityResultContracts.RequestPermission()
+    ) {}
+    val vpnPermissionLauncher = rememberLauncherForActivityResult(
+        ActivityResultContracts.StartActivityForResult()
+    ) { result ->
+        if (result.resultCode == Activity.RESULT_OK) {
+            DummyVpnService.start(context)
+        } else {
+            VpnStatusStore.set(VpnStatus.Off)
+        }
+    }
+
+    LaunchedEffect(Unit) {
+        if (isVpnNetworkActive(context)) {
+            VpnStatusStore.set(VpnStatus.On)
+        }
+
+        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.TIRAMISU) {
+            notificationPermissionLauncher.launch(Manifest.permission.POST_NOTIFICATIONS)
+        }
+    }
+
+    DummyVpnScreen(
+        status = status,
+        onToggle = {
+            when (status) {
+                VpnStatus.Off, is VpnStatus.Error -> {
+                    if (isVpnNetworkActive(context)) {
+                        DummyVpnService.stop(context)
+                    } else {
+                        VpnStatusStore.set(VpnStatus.Starting)
+                        val prepareIntent: Intent? = VpnService.prepare(context)
+                        if (prepareIntent != null) {
+                            vpnPermissionLauncher.launch(prepareIntent)
+                        } else {
+                            DummyVpnService.start(context)
+                        }
+                    }
+                }
+
+                VpnStatus.On -> DummyVpnService.stop(context)
+                VpnStatus.Starting, VpnStatus.Stopping -> Unit
+            }
+        }
     )
 }
 
+@Composable
+private fun DummyVpnScreen(
+    status: VpnStatus,
+    onToggle: () -> Unit,
+    modifier: Modifier = Modifier
+) {
+    val isOn = status == VpnStatus.On
+    val isBusy = status == VpnStatus.Starting || status == VpnStatus.Stopping
+    val buttonColor by animateColorAsState(
+        targetValue = when {
+            isOn -> Color(0xFF2F6F5E)
+            isBusy -> Color(0xFF6B7280)
+            else -> Color(0xFF31343A)
+        },
+        label = "vpnButtonColor"
+    )
+
+    Scaffold(modifier = modifier.fillMaxSize()) { innerPadding ->
+        Surface(
+            modifier = Modifier
+                .fillMaxSize()
+                .padding(innerPadding),
+            color = MaterialTheme.colorScheme.background
+        ) {
+            Column(
+                modifier = Modifier
+                    .fillMaxSize()
+                    .background(Color(0xFFF7F8FA))
+                    .padding(horizontal = 28.dp),
+                horizontalAlignment = Alignment.CenterHorizontally,
+                verticalArrangement = Arrangement.Center
+            ) {
+                Text(
+                    text = "DummyVPN",
+                    style = MaterialTheme.typography.headlineMedium,
+                    fontWeight = FontWeight.SemiBold,
+                    color = Color(0xFF202329)
+                )
+                Spacer(modifier = Modifier.height(12.dp))
+                Text(
+                    text = statusText(status),
+                    style = MaterialTheme.typography.bodyLarge,
+                    textAlign = TextAlign.Center,
+                    color = Color(0xFF5F6673)
+                )
+                Spacer(modifier = Modifier.height(48.dp))
+                Button(
+                    onClick = onToggle,
+                    enabled = !isBusy,
+                    modifier = Modifier
+                        .size(210.dp)
+                        .shadow(18.dp, CircleShape),
+                    shape = CircleShape,
+                    colors = ButtonDefaults.buttonColors(
+                        containerColor = buttonColor,
+                        contentColor = Color.White,
+                        disabledContainerColor = buttonColor,
+                        disabledContentColor = Color.White
+                    )
+                ) {
+                    Box(contentAlignment = Alignment.Center) {
+                        Text(
+                            text = buttonText(status),
+                            fontSize = 44.sp,
+                            fontWeight = FontWeight.Bold,
+                            letterSpacing = 0.sp
+                        )
+                    }
+                }
+                Spacer(modifier = Modifier.height(36.dp))
+                Text(
+                    text = detailText(status),
+                    style = MaterialTheme.typography.bodyMedium,
+                    textAlign = TextAlign.Center,
+                    color = if (status is VpnStatus.Error) Color(0xFF9B2F2F) else Color(0xFF69717E)
+                )
+            }
+        }
+    }
+}
+
+private fun buttonText(status: VpnStatus): String {
+    return when (status) {
+        VpnStatus.On -> "ON"
+        VpnStatus.Off, is VpnStatus.Error -> "OFF"
+        VpnStatus.Starting -> "..."
+        VpnStatus.Stopping -> "..."
+    }
+}
+
+private fun statusText(status: VpnStatus): String {
+    return when (status) {
+        VpnStatus.On -> "VPN is active"
+        VpnStatus.Off -> "VPN is inactive"
+        VpnStatus.Starting -> "Starting VPN"
+        VpnStatus.Stopping -> "Stopping VPN"
+        is VpnStatus.Error -> "VPN could not start"
+    }
+}
+
+private fun detailText(status: VpnStatus): String {
+    return when (status) {
+        VpnStatus.On -> "All apps are routed through the local pass-through VPN."
+        VpnStatus.Off -> "Tap the button to start the pass-through VPN."
+        VpnStatus.Starting -> "Preparing the VPN interface and local proxy."
+        VpnStatus.Stopping -> "Closing the VPN interface and local proxy."
+        is VpnStatus.Error -> status.message
+    }
+}
+
+private fun isVpnNetworkActive(context: android.content.Context): Boolean {
+    val connectivityManager = context.getSystemService(ConnectivityManager::class.java)
+        ?: return false
+    return connectivityManager.allNetworks.any { network ->
+        connectivityManager.getNetworkCapabilities(network)
+            ?.hasTransport(NetworkCapabilities.TRANSPORT_VPN) == true
+    }
+}
+
 @Preview(showBackground = true)
 @Composable
-fun GreetingPreview() {
+private fun DummyVpnScreenPreview() {
     DummyVPNTheme {
-        Greeting("Android")
+        DummyVpnScreen(status = VpnStatus.Off, onToggle = {})
     }
-}
+}

app/src/main/java/jp/cyberrex/dummyvpn/Tun2SocksProcess.kt

@@ -0,0 +1,153 @@
+package jp.cyberrex.dummyvpn
+
+import android.content.Context
+import android.os.ParcelFileDescriptor
+import android.system.Os
+import android.system.OsConstants
+import android.util.Log
+import java.io.File
+import java.io.FileNotFoundException
+import java.util.concurrent.TimeUnit
+
+class Tun2SocksProcess(private val context: Context) {
+    private var process: Process? = null
+    private var logThread: Thread? = null
+    private var watchThread: Thread? = null
+
+    fun start(tunInterface: ParcelFileDescriptor, socksPort: Int) {
+        stop()
+
+        val binary = findBinary()
+        val tunForProcess = ParcelFileDescriptor.dup(tunInterface.fileDescriptor)
+        val tunFd = tunForProcess.detachFd()
+        val command = listOf(
+            binary.absolutePath,
+            "-device",
+            "fd://0",
+            "-mtu",
+            "1500",
+            "-proxy",
+            "socks5://127.0.0.1:$socksPort",
+            "-loglevel",
+            "info"
+        )
+
+        val stdinBackup = Os.dup(STDIN)
+        try {
+            Os.dup2(fileDescriptorOf(tunFd), STDIN_FILENO)
+            process = ProcessBuilder(command)
+                .redirectErrorStream(true)
+                .redirectInput(ProcessBuilder.Redirect.INHERIT)
+                .start()
+            val startedProcess = process ?: return
+            startLogging(startedProcess)
+            startWatching(startedProcess)
+        } catch (error: Exception) {
+            throw error
+        } finally {
+            Os.dup2(stdinBackup, STDIN_FILENO)
+            closeFileDescriptor(stdinBackup)
+            closeDetachedFd(tunFd)
+        }
+    }
+
+    fun stop() {
+        logThread?.interrupt()
+        logThread = null
+        watchThread?.interrupt()
+        watchThread = null
+
+        process?.destroy()
+        try {
+            if (process?.waitFor(750, TimeUnit.MILLISECONDS) == false) {
+                process?.destroyForcibly()
+                process?.waitFor(750, TimeUnit.MILLISECONDS)
+            }
+        } catch (_: InterruptedException) {
+            Thread.currentThread().interrupt()
+            process?.destroyForcibly()
+        }
+        process = null
+    }
+
+    private fun startLogging(startedProcess: Process) {
+        logThread = Thread {
+            try {
+                startedProcess.inputStream.bufferedReader().useLines { lines ->
+                    lines.forEach { line ->
+                        Log.i(TAG, line)
+                    }
+                }
+            } catch (_: Exception) {
+            }
+        }.apply {
+            name = "tun2socks-log"
+            isDaemon = true
+            start()
+        }
+    }
+
+    private fun startWatching(startedProcess: Process) {
+        watchThread = Thread {
+            try {
+                val exitCode = startedProcess.waitFor()
+                if (process == startedProcess) {
+                    Log.e(TAG, "tun2socks exited with code $exitCode")
+                    VpnStatusStore.set(VpnStatus.Error("tun2socks exited with code $exitCode"))
+                }
+            } catch (_: InterruptedException) {
+                Thread.currentThread().interrupt()
+            }
+        }.apply {
+            name = "tun2socks-watch"
+            isDaemon = true
+            start()
+        }
+    }
+
+    private fun findBinary(): File {
+        val binary = File(context.applicationInfo.nativeLibraryDir, BINARY_NAME)
+        if (binary.exists() && binary.canExecute()) {
+            return binary
+        }
+
+        throw FileNotFoundException(
+            "tun2socks binary not found. Place it at app/src/main/jniLibs/<abi>/$BINARY_NAME"
+        )
+    }
+
+    private fun closeDetachedFd(fd: Int) {
+        try {
+            ParcelFileDescriptor.adoptFd(fd).close()
+        } catch (_: Exception) {
+        }
+    }
+
+    private fun closeFileDescriptor(fd: java.io.FileDescriptor) {
+        try {
+            Os.close(fd)
+        } catch (_: Exception) {
+        }
+    }
+
+    private fun fileDescriptorOf(fd: Int): java.io.FileDescriptor {
+        return java.io.FileDescriptor().also { descriptor ->
+            setFileDescriptorInt(descriptor, fd)
+            val flags = Os.fcntlInt(descriptor, OsConstants.F_GETFD, 0)
+            Os.fcntlInt(descriptor, OsConstants.F_SETFD, flags and OsConstants.FD_CLOEXEC.inv())
+        }
+    }
+
+    private fun setFileDescriptorInt(descriptor: java.io.FileDescriptor, fd: Int) {
+        val field = java.io.FileDescriptor::class.java.getDeclaredField("descriptor")
+        field.isAccessible = true
+        field.setInt(descriptor, fd)
+    }
+
+    private companion object {
+        const val TAG = "Tun2SocksProcess"
+        const val BINARY_NAME = "libtun2socks.so"
+        const val STDIN_FILENO = 0
+        val STDIN: java.io.FileDescriptor = java.io.FileDescriptor.`in`
+    }
+}

app/src/main/java/jp/cyberrex/dummyvpn/VpnStatus.kt

@@ -0,0 +1,22 @@
+package jp.cyberrex.dummyvpn
+
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.flow.asStateFlow
+
+sealed interface VpnStatus {
+    data object Off : VpnStatus
+    data object Starting : VpnStatus
+    data object On : VpnStatus
+    data object Stopping : VpnStatus
+    data class Error(val message: String) : VpnStatus
+}
+
+object VpnStatusStore {
+    private val mutableStatus = MutableStateFlow<VpnStatus>(VpnStatus.Off)
+    val status: StateFlow<VpnStatus> = mutableStatus.asStateFlow()
+
+    fun set(status: VpnStatus) {
+        mutableStatus.value = status
+    }
+}

app/src/main/java/jp/cyberrex/dummyvpn/ui/theme/Color.kt

@@ -2,10 +2,10 @@ package jp.cyberrex.dummyvpn.ui.theme
 
 import androidx.compose.ui.graphics.Color
 
-val Purple80 = Color(0xFFD0BCFF)
+val DeepGreen80 = Color(0xFFA7D7C5)
-val PurpleGrey80 = Color(0xFFCCC2DC)
+val Slate80 = Color(0xFFC8CDD4)
-val Pink80 = Color(0xFFEFB8C8)
+val SoftBlue80 = Color(0xFFB8D3E6)
 
-val Purple40 = Color(0xFF6650a4)
+val DeepGreen40 = Color(0xFF2F6F5E)
-val PurpleGrey40 = Color(0xFF625b71)
+val Slate40 = Color(0xFF4D5663)
-val Pink40 = Color(0xFF7D5260)
+val SoftBlue40 = Color(0xFF406C86)

app/src/main/java/jp/cyberrex/dummyvpn/ui/theme/Theme.kt

@@ -12,15 +12,15 @@ import androidx.compose.runtime.Composable
 import androidx.compose.ui.platform.LocalContext
 
 private val DarkColorScheme = darkColorScheme(
-    primary = Purple80,
+    primary = DeepGreen80,
-    secondary = PurpleGrey80,
+    secondary = Slate80,
-    tertiary = Pink80
+    tertiary = SoftBlue80
 )
 
 private val LightColorScheme = lightColorScheme(
-    primary = Purple40,
+    primary = DeepGreen40,
-    secondary = PurpleGrey40,
+    secondary = Slate40,
-    tertiary = Pink40
+    tertiary = SoftBlue40
 
     /* Other default colors to override
     background = Color(0xFFFFFBFE),
@@ -37,7 +37,7 @@ private val LightColorScheme = lightColorScheme(
 fun DummyVPNTheme(
     darkTheme: Boolean = isSystemInDarkTheme(),
     // Dynamic color is available on Android 12+
-    dynamicColor: Boolean = true,
+    dynamicColor: Boolean = false,
     content: @Composable () -> Unit
 ) {
     val colorScheme = when {
@@ -55,4 +55,4 @@ fun DummyVPNTheme(
         typography = Typography,
         content = content
     )
-}
+}

app/src/main/jniLibs/README.md

@@ -0,0 +1,12 @@
+Place ABI-specific tun2socks binaries here and name each executable `libtun2socks.so`.
+
+Expected paths:
+
+- `arm64-v8a/libtun2socks.so`
+- `armeabi-v7a/libtun2socks.so`
+- `x86/libtun2socks.so`
+- `x86_64/libtun2socks.so`
+
+Android does not allow executing files copied into app data directories on modern
+versions. Packaging the executable as a native library keeps it in an executable
+location exposed by `applicationInfo.nativeLibraryDir`.

app/src/main/res/values/strings.xml

@@ -1,3 +1,8 @@
 <resources>
     <string name="app_name">DummyVPN</string>
-</resources>
+    <string name="vpn_notification_channel">VPN status</string>
+    <string name="vpn_notification_title">DummyVPN is active</string>
+    <string name="vpn_notification_text">Traffic is being passed through the local VPN.</string>
+    <string name="vpn_notification_stop">Stop</string>
+    <string name="vpn_error_unknown">Unknown VPN error</string>
+</resources>

build.gradle.kts

@@ -2,4 +2,4 @@
 plugins {
     alias(libs.plugins.android.application) apply false
     alias(libs.plugins.kotlin.compose) apply false
-}
+}