Add Android and Linux platform support (local gRPC working, tsnet WIP)

- Android: Dart FFI → Go c-shared (.so) in jniLibs (arm64-v8a + x86_64)
- Linux: Dart FFI → Go c-shared (.so) via Docker cross-compilation (amd64)
- Dart API: TsnetFlutter uses MethodChannel on iOS/macOS, FFI on Android/Linux
- Add ffi package dependency for native function bindings
- Build script: ./build_go.sh [ios|macos|android|linux|apple|all]
- Android RegisterInterfaceGetter to bypass netlink CAP_NET_ADMIN restriction
- Make TailscaleStart() idempotent and add GODEBUG=netdns=go for Android

Known: Android tsnet tunnel blocked by Go stdlib net.Interfaces() netlink
call — local gRPC works, Tailscale fallback needs libtailscale integration.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

android/build.gradle

@@ -0,0 +1,36 @@
+group 'io.svrnty.tsnet_flutter'
+version '1.0'
+
+buildscript {
+    repositories {
+        google()
+        mavenCentral()
+    }
+    dependencies {
+        classpath 'com.android.tools.build:gradle:8.1.0'
+    }
+}
+
+rootProject.allprojects {
+    repositories {
+        google()
+        mavenCentral()
+    }
+}
+
+apply plugin: 'com.android.library'
+
+android {
+    namespace 'io.svrnty.tsnet_flutter'
+    compileSdk 34
+
+    defaultConfig {
+        minSdk 21
+    }
+
+    sourceSets {
+        main {
+            jniLibs.srcDirs = ['src/main/jniLibs']
+        }
+    }
+}

android/src/main/AndroidManifest.xml

@@ -0,0 +1,4 @@
+<manifest xmlns:android="http://schemas.android.com/apk/res/android"
+    package="io.svrnty.tsnet_flutter">
+    <uses-permission android:name="android.permission.INTERNET" />
+</manifest>

build_go.sh

@@ -187,12 +187,85 @@ build_macos() {
   du -sh "$OUTPUT"
 }
 
+# ============================================================
+# Android (c-shared → .so in jniLibs)
+# ============================================================
+build_android() {
+  local NDK_HOME="${ANDROID_NDK_HOME:-$HOME/Library/Android/sdk/ndk/$(ls $HOME/Library/Android/sdk/ndk/ 2>/dev/null | sort -V | tail -1)}"
+
+  if [ ! -d "$NDK_HOME" ]; then
+    echo "Error: Android NDK not found. Set ANDROID_NDK_HOME."
+    exit 1
+  fi
+
+  local TOOLCHAIN="$NDK_HOME/toolchains/llvm/prebuilt/darwin-x86_64"
+  local OUTPUT_DIR="$SCRIPT_DIR/android/src/main/jniLibs"
+  mkdir -p "$OUTPUT_DIR/arm64-v8a" "$OUTPUT_DIR/x86_64"
+
+  # Android needs the "android" build tag for proper platform detection
+  # (e.g., netmon_polling.go instead of netmon_linux.go which requires CAP_NET_ADMIN)
+  local ANDROID_TAGS="android,$OMIT_TAGS"
+
+  # Android arm64
+  echo "==> [Android] Building for arm64-v8a..."
+  CGO_ENABLED=1 GOOS=android GOARCH=arm64 \
+    CC="$TOOLCHAIN/bin/aarch64-linux-android21-clang" \
+    go build -buildmode=c-shared -tags "$ANDROID_TAGS" -ldflags="-s -w" \
+    -o "$OUTPUT_DIR/arm64-v8a/libtailscale.so" .
+
+  # Android x86_64 (emulator)
+  echo "==> [Android] Building for x86_64..."
+  CGO_ENABLED=1 GOOS=android GOARCH=amd64 \
+    CC="$TOOLCHAIN/bin/x86_64-linux-android21-clang" \
+    go build -buildmode=c-shared -tags "$ANDROID_TAGS" -ldflags="-s -w" \
+    -o "$OUTPUT_DIR/x86_64/libtailscale.so" .
+
+  # Clean up generated headers (not needed for .so)
+  rm -f "$OUTPUT_DIR"/*/libtailscale.h
+
+  echo "==> [Android] Done:"
+  ls -lh "$OUTPUT_DIR/arm64-v8a/libtailscale.so"
+  ls -lh "$OUTPUT_DIR/x86_64/libtailscale.so"
+}
+
+# ============================================================
+# Linux (c-shared → .so)
+# ============================================================
+build_linux() {
+  local OUTPUT_DIR="$SCRIPT_DIR/linux"
+
+  # Linux amd64 — requires a Linux cross-compiler or building on Linux.
+  # On macOS, use Docker: docker run --rm -v $PWD:/src -w /src golang:1.23 ./build_go.sh linux
+  echo "==> [Linux] Building for amd64..."
+  if [ "$(uname)" = "Linux" ]; then
+    CGO_ENABLED=1 GOOS=linux GOARCH=amd64 \
+      go build -buildmode=c-shared -tags "$OMIT_TAGS" -ldflags="-s -w" \
+      -o "$OUTPUT_DIR/libtailscale.so" .
+  else
+    echo "    Cross-compiling Linux from $(uname) via Docker..."
+    docker run --rm --platform linux/amd64 -v "$SCRIPT_DIR:/src" -w /src/ios/Go \
+      golang:latest bash -c "
+        CGO_ENABLED=1 GOOS=linux GOARCH=amd64 \
+        go build -buildmode=c-shared -tags '$OMIT_TAGS' -ldflags='-s -w' \
+        -o /src/linux/libtailscale.so .
+      "
+  fi
+
+  rm -f "$OUTPUT_DIR/libtailscale.h"
+
+  echo "==> [Linux] Done:"
+  ls -lh "$OUTPUT_DIR/libtailscale.so"
+}
+
 # ============================================================
 # Main
 # ============================================================
 case "$PLATFORM" in
-  ios)   build_ios ;;
-  macos) build_macos ;;
-  all)   build_ios; build_macos ;;
-  *)     echo "Usage: $0 [ios|macos|all]"; exit 1 ;;
+  ios)     build_ios ;;
+  macos)   build_macos ;;
+  android) build_android ;;
+  linux)   build_linux ;;
+  all)     build_ios; build_macos; build_android; build_linux ;;
+  apple)   build_ios; build_macos ;;
+  *)       echo "Usage: $0 [ios|macos|android|linux|apple|all]"; exit 1 ;;
 esac

ios/Go/android_interfaces.go

@@ -0,0 +1,39 @@
+//go:build android
+
+package main
+
+import (
+	"net"
+
+	"tailscale.com/net/netmon"
+)
+
+func init() {
+	// Register an alternate interface getter for Android.
+	// Go's net.Interfaces() uses netlink which requires CAP_NET_ADMIN,
+	// and /sys/class/net is restricted on modern Android.
+	// Provide a minimal loopback interface to satisfy tsnet's startup.
+	// tsnet uses userspace netstack and doesn't need real interface data.
+	netmon.RegisterInterfaceGetter(func() ([]netmon.Interface, error) {
+		return []netmon.Interface{
+			{
+				Interface: &net.Interface{
+					Index:        1,
+					MTU:          65536,
+					Name:         "lo",
+					HardwareAddr: nil,
+					Flags:        net.FlagUp | net.FlagLoopback | net.FlagRunning,
+				},
+			},
+			{
+				Interface: &net.Interface{
+					Index:        2,
+					MTU:          1500,
+					Name:         "wlan0",
+					HardwareAddr: net.HardwareAddr{0x02, 0x00, 0x00, 0x00, 0x00, 0x01},
+					Flags:        net.FlagUp | net.FlagRunning | net.FlagMulticast,
+				},
+			},
+		}, nil
+	})
+}

ios/Go/bridge.go

@@ -20,6 +20,8 @@ import (
 	"fmt"
 	"io"
 	"net"
+	"os"
+	"runtime"
 	"sync"
 	"unsafe"
 
@@ -50,6 +52,13 @@ func TailscaleStart(stateDir, authKey, hostname *C.char) *C.char {
 		return nil // already started — idempotent
 	}
 
+	// On Android, Go's net.Interfaces() uses netlink which requires
+	// CAP_NET_ADMIN. Set GODEBUG to use Go's pure DNS resolver and
+	// suppress network interface enumeration errors.
+	if runtime.GOOS == "android" {
+		os.Setenv("GODEBUG", "netdns=go")
+	}
+
 	s := &tsnet.Server{
 		Dir:       C.GoString(stateDir),
 		Hostname:  C.GoString(hostname),

lib/tsnet_ffi.dart

@@ -0,0 +1,125 @@
+import 'dart:ffi';
+import 'dart:io';
+
+import 'package:ffi/ffi.dart';
+import 'package:flutter/services.dart';
+
+// C function typedefs from bridge.go
+typedef _StartNative = Pointer<Utf8> Function(Pointer<Utf8>, Pointer<Utf8>, Pointer<Utf8>);
+typedef _StartProxyNative = Pointer<Utf8> Function(Pointer<Utf8>, Int32, Pointer<Int32>);
+typedef _StartProxyDart = Pointer<Utf8> Function(Pointer<Utf8>, int, Pointer<Int32>);
+typedef _SimpleNative = Pointer<Utf8> Function();
+typedef _OutParamNative = Pointer<Utf8> Function(Pointer<Pointer<Utf8>>);
+typedef _FreeNative = Void Function(Pointer<Utf8>);
+typedef _FreeDart = void Function(Pointer<Utf8>);
+
+/// FFI-based implementation for Android and Linux.
+/// Loads the Go shared library and calls C functions directly.
+class TsnetFfi {
+  late final DynamicLibrary _lib;
+  late final _StartNative _start;
+  late final _StartProxyDart _startProxy;
+  late final _SimpleNative _stopProxy;
+  late final _SimpleNative _stop;
+  late final _OutParamNative _status;
+  late final _OutParamNative _tailscaleIP;
+  late final _FreeDart _free;
+
+  TsnetFfi() {
+    _lib = _loadLibrary();
+    _start = _lib.lookupFunction<_StartNative, _StartNative>('TailscaleStart');
+    _startProxy = _lib.lookupFunction<_StartProxyNative, _StartProxyDart>('TailscaleStartProxy');
+    _stopProxy = _lib.lookupFunction<_SimpleNative, _SimpleNative>('TailscaleStopProxy');
+    _stop = _lib.lookupFunction<_SimpleNative, _SimpleNative>('TailscaleStop');
+    _status = _lib.lookupFunction<_OutParamNative, _OutParamNative>('TailscaleStatus');
+    _tailscaleIP = _lib.lookupFunction<_OutParamNative, _OutParamNative>('TailscaleIP');
+    _free = _lib.lookupFunction<_FreeNative, _FreeDart>('TailscaleFreeString');
+  }
+
+  static DynamicLibrary _loadLibrary() {
+    if (Platform.isAndroid) {
+      return DynamicLibrary.open('libtailscale.so');
+    } else if (Platform.isLinux) {
+      // Look in the app's lib directory (where Flutter bundles native libs)
+      return DynamicLibrary.open('libtailscale.so');
+    }
+    throw UnsupportedError('FFI not supported on ${Platform.operatingSystem}');
+  }
+
+  String? _consumeString(Pointer<Utf8> ptr) {
+    if (ptr == nullptr) return null;
+    final str = ptr.toDartString();
+    _free(ptr);
+    return str;
+  }
+
+  void _checkError(Pointer<Utf8> errPtr, String code) {
+    final err = _consumeString(errPtr);
+    if (err != null) {
+      throw PlatformException(code: code, message: err);
+    }
+  }
+
+  Future<void> start(String stateDir, String authKey, String hostname) async {
+    final dirC = stateDir.toNativeUtf8();
+    final keyC = authKey.toNativeUtf8();
+    final hostC = hostname.toNativeUtf8();
+    try {
+      final err = _start(dirC, keyC, hostC);
+      _checkError(err, 'START_FAILED');
+    } finally {
+      malloc.free(dirC);
+      malloc.free(keyC);
+      malloc.free(hostC);
+    }
+  }
+
+  Future<int> startProxy(String remoteIP, int remotePort) async {
+    final ipC = remoteIP.toNativeUtf8();
+    final portOut = malloc<Int32>();
+    try {
+      final err = _startProxy(ipC, remotePort, portOut);
+      _checkError(err, 'PROXY_FAILED');
+      return portOut.value;
+    } finally {
+      malloc.free(ipC);
+      malloc.free(portOut);
+    }
+  }
+
+  Future<void> stopProxy() async {
+    final err = _stopProxy();
+    _checkError(err, 'STOP_PROXY_FAILED');
+  }
+
+  Future<void> stop() async {
+    final err = _stop();
+    _checkError(err, 'STOP_FAILED');
+  }
+
+  Future<String?> status() async {
+    final errOut = malloc<Pointer<Utf8>>();
+    try {
+      errOut.value = nullptr;
+      final json = _status(errOut);
+      final err = _consumeString(errOut.value);
+      if (err != null) throw PlatformException(code: 'STATUS_FAILED', message: err);
+      return _consumeString(json);
+    } finally {
+      malloc.free(errOut);
+    }
+  }
+
+  Future<String> tailscaleIP() async {
+    final errOut = malloc<Pointer<Utf8>>();
+    try {
+      errOut.value = nullptr;
+      final ip = _tailscaleIP(errOut);
+      final err = _consumeString(errOut.value);
+      if (err != null) throw PlatformException(code: 'IP_FAILED', message: err);
+      return _consumeString(ip) ?? '';
+    } finally {
+      malloc.free(errOut);
+    }
+  }
+}

lib/tsnet_flutter.dart

@@ -1,13 +1,16 @@
 import 'dart:convert';
+import 'dart:io';
 
 import 'package:flutter/services.dart';
+import 'package:tsnet_flutter/tsnet_ffi.dart';
 
 /// Embedded Tailscale tsnet client for Flutter.
 ///
 /// Provides a userspace WireGuard tunnel via Tailscale's tsnet library,
 /// without requiring the user to install the Tailscale app or a VPN entitlement.
 ///
-/// Architecture: Go static library (tsnet) ↔ Swift C bridge ↔ MethodChannel ↔ Dart
+/// - iOS/macOS: Swift MethodChannel → C bridge → Go static library
+/// - Android/Linux: Dart FFI → Go shared library (.so)
 ///
 /// Usage:
 /// ```dart
@@ -19,52 +22,93 @@ import 'package:flutter/services.dart';
 class TsnetFlutter {
   static const _channel = MethodChannel('tsnet_flutter');
 
-  /// Join the Tailnet with the given auth key.
-  ///
-  /// [authKey] is a Tailscale auth key (reusable, tag-scoped).
-  /// [hostname] is the name this device will appear as in the Tailnet.
-  ///
-  /// The auth key is typically received from the heater during BLE provisioning.
-  /// State is persisted to the app's documents directory so the device
-  /// identity survives app restarts.
-  Future<void> start({
-    required String authKey,
-    String hostname = 'constellation-phone',
-  }) async {
-    await _channel.invokeMethod('start', {
-      'authKey': authKey,
-      'hostname': hostname,
-    });
+  /// FFI backend for Android/Linux — initialized lazily.
+  TsnetFfi? _ffi;
+
+  bool get _useFfi => Platform.isAndroid || Platform.isLinux;
+
+  TsnetFfi get _ensureFfi => _ffi ??= TsnetFfi();
+
+  /// State directory for persisting Tailscale identity.
+  String? _stateDir;
+
+  Future<String> _getStateDir() async {
+    if (_stateDir != null) return _stateDir!;
+
+    // Use a directory relative to the current working directory or temp
+    // For proper production use, pass a path from path_provider
+    late final Directory dir;
+    if (Platform.isAndroid) {
+      // Android: use the app's internal storage via the temp directory parent
+      final temp = Directory.systemTemp;
+      dir = Directory('${temp.parent.path}/tailscale_state');
+    } else if (Platform.isLinux) {
+      final home = Platform.environment['HOME'] ?? '/tmp';
+      dir = Directory('$home/.local/share/tailscale_state');
+    } else {
+      dir = Directory('${Directory.systemTemp.path}/tailscale_state');
+    }
+    await dir.create(recursive: true);
+    _stateDir = dir.path;
+    return _stateDir!;
   }
 
-  /// Create a localhost TCP proxy to a heater's Tailscale IP.
-  ///
-  /// Returns the local port number. Connect gRPC to `127.0.0.1:<port>`.
-  /// Traffic is forwarded through the Tailscale WireGuard tunnel to
-  /// [heaterTailscaleIP]:[port] on the heater.
-  ///
-  /// Uses userspace netstack — no VPN entitlement needed on iOS.
-  Future<int> startProxy(String heaterTailscaleIP, {int port = 5050}) async {
-    final localPort = await _channel.invokeMethod<int>('startProxy', {
-      'ip': heaterTailscaleIP,
-      'port': port,
-    });
-    return localPort!;
+  /// Join the Tailnet with the given auth key.
+  Future<void> start({
+    required String authKey,
+    String hostname = 'tsnet-flutter',
+  }) async {
+    if (_useFfi) {
+      final dir = await _getStateDir();
+      await _ensureFfi.start(dir, authKey, hostname);
+    } else {
+      await _channel.invokeMethod('start', {
+        'authKey': authKey,
+        'hostname': hostname,
+      });
+    }
+  }
+
+  /// Create a localhost TCP proxy to a remote Tailscale IP.
+  /// Returns the local port number.
+  Future<int> startProxy(String remoteIP, {int port = 5050}) async {
+    if (_useFfi) {
+      return await _ensureFfi.startProxy(remoteIP, port);
+    } else {
+      final localPort = await _channel.invokeMethod<int>('startProxy', {
+        'ip': remoteIP,
+        'port': port,
+      });
+      return localPort!;
+    }
   }
 
   /// Stop the local TCP proxy.
   Future<void> stopProxy() async {
-    await _channel.invokeMethod('stopProxy');
+    if (_useFfi) {
+      await _ensureFfi.stopProxy();
+    } else {
+      await _channel.invokeMethod('stopProxy');
+    }
   }
 
   /// Disconnect from the Tailnet and clean up.
   Future<void> stop() async {
-    await _channel.invokeMethod('stop');
+    if (_useFfi) {
+      await _ensureFfi.stop();
+    } else {
+      await _channel.invokeMethod('stop');
+    }
   }
 
   /// Get the current Tailscale connection status.
   Future<TailscaleStatus> status() async {
-    final json = await _channel.invokeMethod<String>('status');
+    String? json;
+    if (_useFfi) {
+      json = await _ensureFfi.status();
+    } else {
+      json = await _channel.invokeMethod<String>('status');
+    }
     if (json == null) {
       return TailscaleStatus(state: TailscaleState.stopped);
     }
@@ -73,8 +117,12 @@ class TsnetFlutter {
 
   /// Get this device's Tailscale IPv4 address (100.x.x.x).
   Future<String> tailscaleIP() async {
-    final ip = await _channel.invokeMethod<String>('tailscaleIP');
-    return ip!;
+    if (_useFfi) {
+      return await _ensureFfi.tailscaleIP();
+    } else {
+      final ip = await _channel.invokeMethod<String>('tailscaleIP');
+      return ip!;
+    }
   }
 }
 

linux/CMakeLists.txt

@@ -0,0 +1,8 @@
+cmake_minimum_required(VERSION 3.10)
+project(tsnet_flutter_library VERSION 0.2.0 LANGUAGES C)
+
+# Bundle the pre-built Go shared library with the Flutter app.
+# The .so is built by build_go.sh and placed in this directory.
+install(FILES "${CMAKE_CURRENT_SOURCE_DIR}/libtailscale.so"
+  DESTINATION "${CMAKE_INSTALL_PREFIX}/lib"
+  COMPONENT Runtime)

pubspec.lock

@@ -49,6 +49,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.3.3"
+  ffi:
+    dependency: "direct main"
+    description:
+      name: ffi
+      sha256: "6d7fd89431262d8f3125e81b50d3847a091d846eafcd4fdb88dd06f36d705a45"
+      url: "https://pub.dev"
+    source: hosted
+    version: "2.2.0"
   flutter:
     dependency: "direct main"
     description: flutter

pubspec.yaml

@@ -12,6 +12,7 @@ environment:
 dependencies:
   flutter:
     sdk: flutter
+  ffi: ^2.1.0
 
 dev_dependencies:
   flutter_test:
@@ -25,3 +26,8 @@ flutter:
         pluginClass: TsnetFlutterPlugin
       macos:
         pluginClass: TsnetFlutterPlugin
+      android:
+        package: io.svrnty.tsnet_flutter
+        ffiPlugin: true
+      linux:
+        ffiPlugin: true