AuthenticateAdmin.php

<?php

namespace App\Http\Middleware;

use Closure;
use Illuminate\Support\Facades\Auth;

class AuthenticateAdmin
{
  public function handle($request, Closure $next, $guard = null)
  {
    if (Auth::guard($guard)->guest() || !Auth::guard($guard)->user()->is_admin) {

      if ($request->ajax() || $request->wantsJson()) {
        return response('Unauthorized.', 401);
      } else {
//        return abort('401');
        return redirect('/');
      }
    }
    return $next($request);
  }
}

AuthServiceProvider.php

<?php

namespace App\Providers;

use Illuminate\Support\Facades\Gate;
use Illuminate\Support\Facades\Auth;
use Illuminate\Auth\EloquentUserProvider;
use Illuminate\Foundation\Support\Providers\AuthServiceProvider as ServiceProvider;

class AuthServiceProvider extends ServiceProvider
{
    /**
     * The policy mappings for the application.
     *
     * @var array
     */
    protected $policies = [
        'App\Model' => 'App\Policies\ModelPolicy',
    ];

    /**
     * Register any authentication / authorization services.
     *
     * @return void
     */
    public function boot()
    {
        $this->registerPolicies();

        // Binding eloquent.admin to our EloquentAdminUserProvider
        Auth::provider('eloquent.admin', function($app, array $config) {
          return new EloquentAdminUserProvider($app['hash'], $config['model']);
        });
    }
}

EloquentAdminUserProvider.php

<?php

namespace App\Providers;

use Illuminate\Auth\EloquentUserProvider;
use Illuminate\Support\Str;

class EloquentAdminUserProvider extends EloquentUserProvider
{

  public function retrieveByCredentials(array $credentials)
  {
    // Of course here, you could perform the query yourself with the is_admin comparison, but
    // I think it's best to avoid as much duplication as possible
    $user = parent::retrieveByCredentials($credentials);

    return $user && $user->is_admin === false
      ? null
      : $user;
  }
}

Kernel.php

<?php

namespace App\Http;

use Illuminate\Foundation\Http\Kernel as HttpKernel;

class Kernel extends HttpKernel
{
    /**
     * The application's global HTTP middleware stack.
     *
     * These middleware are run during every request to your application.
     *
     * @var array
     */
    protected $middleware = [
        \Illuminate\Foundation\Http\Middleware\CheckForMaintenanceMode::class,
    ];

    /**
     * The application's route middleware groups.
     *
     * @var array
     */
    protected $middlewareGroups = [
        'web' => [
            \App\Http\Middleware\EncryptCookies::class,
            \Illuminate\Cookie\Middleware\AddQueuedCookiesToResponse::class,
            \Illuminate\Session\Middleware\StartSession::class,
            \Illuminate\View\Middleware\ShareErrorsFromSession::class,
            \App\Http\Middleware\VerifyCsrfToken::class,
            \Illuminate\Routing\Middleware\SubstituteBindings::class,
        ],

        'api' => [
            'throttle:60,1',
            'bindings',
        ],
    ];

    /**
     * The application's route middleware.
     *
     * These middleware may be assigned to groups or used individually.
     *
     * @var array
     */
    protected $routeMiddleware = [
        'auth' => \Illuminate\Auth\Middleware\Authenticate::class,
        'auth.admin' => \App\Http\Middleware\AuthenticateAdmin::class,
        'auth.basic' => \Illuminate\Auth\Middleware\AuthenticateWithBasicAuth::class,
        'bindings' => \Illuminate\Routing\Middleware\SubstituteBindings::class,
        'can' => \Illuminate\Auth\Middleware\Authorize::class,
        'guest' => \App\Http\Middleware\RedirectIfAuthenticated::class,
        'throttle' => \Illuminate\Routing\Middleware\ThrottleRequests::class,
    ];
}

web.php

<?php

Route::group(['middleware' => ['auth.admin:admin, web']], function()
  {
    Route::get('/dashboard', function () {
      return 'Admin Home';
    });
  });