code: def _authenticate_user_dn(self, password):
          
          if self.dn is None:
              raise self.AuthenticationFailed("failed to map the username to a DN.")
  
          try:
              sticky = self.settings.BIND_AS_AUTHENTICATING_USER
  
              self._bind_as(self.dn, password, sticky=sticky)
          except ldap.INVALID_CREDENTIALS:
              raise self.AuthenticationFailed("user DN/password rejected by LDAP server.")
docs: Binds to the LDAP server with the user's DN and password. Raises
          AuthenticationFailed on failure.