class CustomAuthBackend(ModelBackend):

def authenticate(self, request, username=None, password=None, **kwargs):
    user = CustomUser.objects.using(request.POST['kundennr']).get(username=username)
    if user.check_password(password) and self.user_can_authenticate(user) :
         try:
            user = CustomUser.objects.using(request.POST['kundennr']).get(username=username)
            return user
         except User.DoesNotExist:
            return None

def get_user(self, user_id):
    try:
        return CustomUser.objects.get(pk=user_id)
    except User.DoesNotExist:
        return None