Simultaneous Use

Recall from Chapter 1 that RADIUS is a stateless protocol. Additionally, because of the way RADIUS accounting works, it’s entirely possible and even probable that a RADIUS server will have an internal list of who is currently logged on that is different than the actual state of the RADIUS client ports—in other words, your RADIUS server may think users are logged on when they really aren’t, and vice versa. Fortunately, most NAS equipment includes some mechanism by which the administrator (or the RADIUS daemon servicing authentication requests) can query it to find out which user is assigned to what port. This could be done through Telnet, the deprecated finger protocol, or even the Simple Network Monitoring Protocol (SNMP).

This ability is especially important when attempting to control multiple logins at the same time from the same user. There exists a utility to tell FreeRADIUS to check on the terminal server first to see if a user is already logged on before denying his request to log on, thereby compensating for the RADIUS accounting discrepancies. The best way to do this is by installing two modules—the SNMP_Session and BER modules—from the popular traffic-monitoring program MRTG. (These are core Perl modules, actually.) Having those modules installed lets a utility included in FreeRADIUS, the checkrad script, communicate with the terminal server equipment directly using the SNMP protocol. You can obtain more information and download these modules from the ...

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