Listing linux users

Hi all. My scenario is: openldap/samba pdc server for authenticating Windows and Linux. I need to know when a user, from Linux clients, logs in and out from the system. If some problem occurs, i need to know who was logged, in particular computer, in that time. With windows computers, the "last" bash command (in server) gave me the information needed, but with linux clients i don't know what i have to do. They are not listed with "last" command. I don't want to redirect all auth logs from clients to my server. It probably will overload my server. There are some way to do this?