The module allows to import users from the AD, but also allows or authentication.
see: https://docs.mendix.com/appstore/modules/ldap-synchronization
The DN will not be exposed with the login but is available during import. You should be able to find the login method in the java classes that come with the module.
An old blog but you might be able to get some information from it that provides an idea on how to proceed overwriting the login function:
https://bartgroot.nl/mendix/custom-checks-on-login/
else just search on google for: overwrite login mendix