TYPO3 - Weiche für 404 oder FE-Login

localconf.php

$TYPO3_CONF_VARS['SYS']['curlUse'] = '1';
$TYPO3_CONF_VARS['FE']['pageNotFound_handling'] = 'USER_FUNCTION:fileadmin/scripts/pageNotFoundHandling.php:user_pageNotFound->pageNotFound';

pageNotFoundHandling.php

<?php
define(LOGIN_URL, 'http://www.example.com/login');
define(NOTFOUND_URL, 'http://www.example.com/page-not-found');

class user_pageNotFound {
  function pageNotFound($param, $ref) {
    if ($param['pageAccessFailureReasons']['fe_group'] != array(''=>0)) {
      header('HTTP/1.0 403 Forbidden');
      $url = LOGIN_URL.'?redirect_url=' . $param['currentUrl'];
    } else {
      $url = NOTFOUND_URL;
    }

    session_start();
    $strCookie = 'PHPSESSID=' . $_COOKIE['PHPSESSID'] . '; path=/';
    session_write_close();

    $c = curl_init();
    curl_setopt($c, CURLOPT_URL, $url);
    curl_setopt($c, CURLOPT_COOKIE, $strCookie);
    $contents = curl_exec($c);
    curl_close($c);

    if ($contents) return $contents;
        else return FALSE;
  }
}
?>

The author of this source is Michael Cannon:
http://typo3vagabond.com/typo3/redirect-to-login-for-access-denied-typo3-users/

Important

1. Your need CURL
2. Use GET/POST for felogin, i.e, redirectMode = getpost
3. Don't use "link to first subpage", set a direct link the subpage.