Ask user to refrain from further using NAT mode

src/main/res/values-zh/strings.xml

@@ -34,6 +34,7 @@
   <string name="auto_connect_summary">随系统启动后台服务</string>
   <string name="forward_success">后台服务已开始运行。</string>
   <string name="service_failed">无法连接远程服务器</string>
+  <string name="nat_deprecated">警告：在 Android 5.0 及更高版本中不建议使用 NAT 模式</string>
   <string name="nat_no_root">NAT 模式需要 ROOT 权限</string>
   <string name="switch_to_vpn">切换到 VPN 模式</string>
   <string name="stop">停止服务</string>

src/main/res/values/strings.xml

@@ -66,6 +66,7 @@
   <!-- notification category -->
   <string name="forward_success">Shadowsocks started.</string>
   <string name="service_failed">Failed to connect the remote server</string>
+  <string name="nat_deprecated">WARNING: NAT mode has been deprecated since Android 5.0</string>
   <string name="nat_no_root">NAT mode requires ROOT permission</string>
   <string name="switch_to_vpn">Switch to VPN mode</string>
   <string name="stop">Stop the service</string>

src/main/scala/com/github/shadowsocks/Shadowsocks.scala

@@ -168,6 +168,11 @@ class Shadowsocks
     // Update the UI
     if (fab != null) fab.setEnabled(true)
     updateState()
+    if (Build.VERSION.SDK_INT >= 21 && !ShadowsocksApplication.isVpnEnabled) {
+      val snackbar = Snackbar.make(findViewById(android.R.id.content), R.string.nat_deprecated, Snackbar.LENGTH_LONG)
+      snackbar.setAction(R.string.switch_to_vpn, (_ => preferences.natSwitch.setChecked(false)): View.OnClickListener)
+      snackbar.show
+    }
 
     if (!ShadowsocksApplication.settings.getBoolean(ShadowsocksApplication.getVersionName, false)) {
       ShadowsocksApplication.settings.edit.putBoolean(ShadowsocksApplication.getVersionName, true).apply()