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Can somebody explain to me how a wireless AP denies access to the network if the client does not have the correct MAC Address.

Does it not send a probe response when a client has not got a correct MAC. Or when the client tries to authenticate, it gets denied.

please can you include reading sources as well, if you know of any.

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up vote 3 down vote accepted

It is really that simple, if the AP has a MAC filter, any device which attempts to authenticate that doesn't have a MAC address on the list will just be denied.

However the problem from a security perspective (which is why this should not be used as a security function) is that it is trivial to spoof a valid MAC address (by sniffing them wirelessly and waiting until the valid user has gone - this is a common technique for avoiding usage costs in airports)

Update to show where MAC address denial happens:

Client --------------Probe-------------> AP

AP -------Probe Response----------> Client

Client ---802.11 Auth Request-----> AP

MAC authentication happens in here. If the client MAC is not on the list the following responses will not get sent:

AP ------802.11 Auth Response---> Client

Client --802.11 Assoc Request----> AP

AP -----802.11 Assoc Respons----> Client

Which is then followed by EAP etc.

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am i right in thinking then that a probe response is sent from the AP to the client then the Client sends a authentication request and if the MAC does not exist on the ACL. Access is denied. –  Mintuz Dec 20 '11 at 14:56
    
@Mintuz - That is correct. What Rory has left slightly fuzzy though, is whether or not the Auth Response is sent to Clients not on the ACL. I believe the answer to that though, is "no". –  Iszi Dec 20 '11 at 17:26
    
sorry - fixed that now –  Rory Alsop Dec 20 '11 at 22:11
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