code: public function onRegister(Registry $registry)
  	{
  		parent::onRegister($registry);
  
  		// Register this model as being the fallback page for a given dns
  		if ($this->fallback && $this->type == 'root' && !$registry->isRegisteredAlias($this, 'contao.dns-fallback', $this->dns))
  		{
  			$registry->registerAlias($this, 'contao.dns-fallback', $this->dns);
  		}
  	}
docs: Register the contao.dns-fallback alias when the model is attached to the registry
  
  @param Registry $registry The model registry