So then it's just a vhost and html page!
This would be when you are using some form of single sign on and you have configured Moodle to use an alternative login url to bypass the discovery phase of SSO. (i.e. entering moodle.example.com auto redirects to shibboleth IDP or openID etc.) This essentially blocks any non domain users from logging in and there is also no way of logging in with a manual account if things go wrong with SSO