How to enter username at login?

[Pierre Girard]

Hello,
     I have an ubuntu machine configured with network authentification. 
When I get to the login screen the first time, I can enter a username, 
then a password and I connect successfully.

However, the next time I get to the login screen the only option I have 
is to select from the user list which only contains the user I logged on 
with already. I can't get back the « username » / « password » behavior. 
I don't mind selecting from the list, but anyone should be able to 
connect to the system.

Any idea how to change that behavior?

Thanks.