login: Show a note if the only active external auth backend is not available

refs #8274

application/controllers/AuthenticationController.php

@@ -39,6 +39,7 @@ class AuthenticationController extends ActionController
             $this->redirectNow(Url::fromPath('setup'));
         }
 
+        $triedOnlyExternalAuth = null;
         $auth = $this->Auth();
         $this->view->form = $form = new LoginForm();
         $this->view->title = $this->translate('Icingaweb Login');
@@ -126,6 +127,7 @@ class AuthenticationController extends ActionController
             } elseif ($request->isGet()) {
                 $user = new User('');
                 foreach ($chain as $backend) {
+                    $triedOnlyExternalAuth = $triedOnlyExternalAuth === null;
                     if ($backend instanceof ExternalBackend) {
                         $authenticated  = $backend->authenticate($user);
                         if ($authenticated === true) {
@@ -134,6 +136,8 @@ class AuthenticationController extends ActionController
                                 Url::fromPath(Url::fromRequest()->getParam('redirect', 'dashboard'))
                             );
                         }
+                    } else {
+                        $triedOnlyExternalAuth = false;
                     }
                 }
             }
@@ -141,6 +145,7 @@ class AuthenticationController extends ActionController
             $this->view->errorInfo = $e->getMessage();
         }
 
+        $this->view->requiresExternalAuth = $triedOnlyExternalAuth && !$auth->isAuthenticated();
         $this->view->requiresSetup = Icinga::app()->requiresSetup();
     }
 

application/views/scripts/authentication/login.phtml

@@ -16,6 +16,15 @@
       '<a href="' . $this->href('setup') . '" title="' . $this->translate('Icinga Web 2 Setup-Wizard') . '">',
       '</a>'
     ); ?></p>
+    <?php elseif ($requiresExternalAuth): ?>
+    <p class="icon-info info-box"><?= sprintf(
+      $this->translate(
+        'You\'re currently not authenticated using any of the web server\'s authentication mechanisms.'
+          . ' Make sure you\'ll configure such either by using the %s or by setting it up manually,'
+          . ' otherwise you\'ll not be able to login.'
+      ),
+      '<em title="icingacli help users">IcingaCLI</em>'
+    ); ?></p>
     <?php endif ?>
     <h1><?= $this->translate('Welcome to Icinga Web 2'); ?></h1>
    <?php

public/css/icinga/login.less

@@ -126,6 +126,15 @@
       font-weight: bold;
     }
   }
+
+  p.info-box {
+    width: 50%;
+    margin: 0 auto 2.5em;
+
+    em {
+      text-decoration: underline;
+    }
+  }
 }
 
   /* make keyframes that tell the start state and the end state of our object */