Logout of MAC Address Authentication Users

When users go offline but the access device and RADIUS server do not detect that the offline events, the following problems may occur:

The RADIUS server still performs accounting for the users, causing incorrect accounting.
Unauthorized users may spoof IP addresses and MAC addresses of authorized users to access the network.
If there are many offline users, these users are still counted as access users of the device. As a result, other users may fail to access the network.

The access device needs to detect user logout immediately, delete the user entry, and notify the RADIUS server to stop accounting.

The Access Device Controls User Logout

The access device controls user logout in either of the following ways:

Run the cut access-user command to force a user to go offline.
Configure user detection to check whether a user is online. If the user does not respond within a specified period, the access device considers the user to be offline and deletes the user entry.

If an administrator detects that an unauthorized user is online or wants a user to go offline and then go online again during a test, the administrator can run the cut access-user command on the access device to force the user to go offline. For a user in normal access state, the access device checks the online status of the user through ARP probing. If the access device detects that the user goes offline, it logs the user out and deletes the user entry.

Figure 1 User logout detection process

Assume that the handshake period of a user is 3T, which can be set by running the authentication timer handshake-period handshake-period command. Here, T=handshake-period/3.

The user sends any packet to trigger MAC address authentication, and the detection timer starts.
Within several T periods, the access device receives traffic from the client and the user keeps online.
The user sends the last packet. When the current T period expires, the access device determines that the user is online because traffic is still received from the client and resets the detection timer.
The access device does not receive traffic from the client within a T period, and sends the first ARP request packet. The client does not respond.
The access device does not receive traffic from the client within another T period, and sends the second ARP request packet. The client does not respond.
The access device does not receive traffic from the client within a third T period. The access device determines that ARP probing fails and deletes the user entry.

The RADIUS Server Forces a User to Log Out

The RADIUS server controls user logout in either of the following methods:

Sends a Disconnect Message (DM) to an access device to force a user to go offline.
Uses the standard RADIUS attributes Session-Timeout and Termination-Action. The Session-Timeout attribute specifies the online duration timer of user. The value of Termination-Action is set to 0, indicating that the user is disconnected by the RADIUS server when the online duration timer expires.