Laravel post request with error "TokenMismatchException

我在Laravel 4.2中得到了这个TokenMismatchException。当我试图发布请求时，TokenMismatchException会出现。例如登录页面。如果我提交该表单，就会显示TokenMismatchException。有什么方法可以验证所有提交的帖子请求吗
错误如下：

Route::filter('csrf', function()
{
    if (Session::token() != Input::get('_token'))
    {
        throw new Illuminate'Session'TokenMismatchException;
    }
});

这是我的代码：
route.php

Route::get('login',array('as' => 'login', 'uses' => 'AuthController@getLogin'))->before('guest');
Route::post('login',array('uses' => 'AuthController@postLogin'))->before('csrf');
Route::group(array('before' => 'auth'), function(){
   Route::get('dashboard', array('as' => 'panel', 'uses' => 'DashboardController@view_dashboard'));
});

AuthController.php

class AuthController extends Controller {
public function getLogin(){
    return View::make('users.login');
}
public function postLogin(){
    $rules = array('username' => 'required', 'password' => 'required');
    $validator = Validator::make(Input::all(), $rules);
    if($validator->fails()){
        return Redirect::route('login')->withErrors($validator);
    }
    $auth = Auth::attempt(array(
        'username' => Input::get('username'),
        'password' => Input::get('password')
    ), false);
    if(!$auth){
        return Redirect::route('login')->withErrors(array(
        'Maaf anda bukan sebagai admin..'
        ));
    }
    //return "Success";
    return Redirect::to('dashboard');
}   
}

DashboardController.php

class DashboardController extends Controller {
    public function view_dashboard(){
        return View::make('dashboard.view_home_admin');
    }
}