I am developing an extension using custom authentication. This custom authentication require to contact external server. I test it and it works fine. But when I check the fallback option, the behavior is not what I expected! So, I have two question:

1- When the authentication fallback come in action?
a. When the external server is down or unreachable.
b. If the user enter invalid password.
c. Both (a) and (b).

2- If the answer is (c) and I think so, how can I make the fallback only when the server is down or unreachable ?