you could solve this in your routes with a filter, that will be executed first. it checks then for the available subdomains and if it doesn't find it, it redirects to a default subdomain.
Route::filter('subdomain', function()
{
$subdomain = current(explode('.', Request::url()));
if (!in_array($subdomain, Config::get('app.countries.available'))) {
return Redirect::to(Config::get('app.default_subdomain') . '.' . Config::get('app.base_address'));
}
});
Route::group(['before' => 'subdomain'], function()
{
...
}