I am new to drupal and I am trying to override the core login functionality of drupal. To achieve this I did a lot of search on the net but found some solutions via theme which is not what i am looking for. I want to use the default form of core but at the same time want to bring a change in the authentication function. My requirement is that I need to validate with ldap and at the same I have a employee list data in another table. So first I need to check if the user who is trying to access that the right to login. Only if he has an access then only the ldap authentication need to be done.