core-bundle/src/Resources/contao/models/PageModel.php
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);
		}
	}
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);
		}
	}
Register the contao.dns-fallback alias when the model is attached to the registry

@param Registry $registry The model registry