Re: Is it possible to recognize clients not by their IP addresses?

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It might be in the future. dynamic-clients virtual server works just with Packet-Src-IP-Address now. There are plans to make NAS-Identifier available to it as well. You will be able to set different shared secrets then.

I'm waiting for such NAS-ID exactly and hope it might be based on the SSID of the AP.

Or it may be based on Calling-Station-Id I see the MAC address of the AP in this attribute attached to each message send to the radius server.