Before a user gets to the network there is usually some form of machine authentication, this probably verifies and configures the system for some basic level of access. Short of mapping a user to a MAC address prior or during this process (802.1x) it is not simple to have users authenticate at this point. It is more usual for a user to attempt to authenticate once the system processes (daemons) are started, and this may well require the network configuration to have already been performed.