How to insert custom redirect script in index.php without losing it on update
If you want to run code before the Drupal kernel is started use a middleware.
Use the drush code generator drush gen middleware and put your code in handle() of the generated middleware class:
MymoduleMiddleware.php
&lt;?php

namespace Drupal\mymodule;

use Drupal\Core\StringTranslation\StringTranslationTrait;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\HttpKernel\HttpKernelInterface;

/**
 * MymoduleMiddleware middleware.
 */
class MymoduleMiddleware implements HttpKernelInterface {

  use StringTranslationTrait;

  /**
   * The kernel.
   *
   * @var \Symfony\Component\HttpKernel\HttpKernelInterface
   */
  protected $httpKernel;

  /**
   * Constructs the MymoduleMiddleware object.
   *
   * @param \Symfony\Component\HttpKernel\HttpKernelInterface $http_kernel
   *   The decorated kernel.
   */
  public function __construct(HttpKernelInterface $http_kernel) {
    $this-&gt;httpKernel = $http_kernel;
  }

  /**
   * {@inheritdoc}
   */
  public function handle(Request $request, $type = self::MASTER_REQUEST, $catch = TRUE) {

    if ($request-&gt;getClientIp() == '127.0.0.10') {
      return new Response($this-&gt;t('Bye!'), 403);
    }

    return $this-&gt;httpKernel-&gt;handle($request, $type, $catch);
  }

}