Probe Request/Response

     Probe Requests and Responses are key for active scanning. This allows a client station to find out information about the networks around them. When a client transmits a probe request, the client is requesting information about either a specific SSID that they know of or are requesting information from any APs in the area. This can be done by changing the value of the "SSID parameter set". When changing this to a specific SSID, the client is probing for that specific network. When this value is changed to a wildcard value, the client is asking any AP that hears it to respond. If the AP is configured to hide its SSID in beacon frames, the client must send a probe request with the SSID parameter set to the SSID of the network for it to receive a probe response back.

Probe Request:

    Probe requests are sent to a broadcast address (ff:ff:ff:ff:ff:ff). During active scanning the client sends probe requests to a specific channel, waits for a response, and then moves to the next channel if nothing is received. If a client sends a probe request with a specific SSID in the SSID field, it is called a directed probe, if the SSID field is set to wildcard it is considered a Null Probe Request.

    Looking at a probe request frame:



    Note the capabilities listed in the probe request. Keep in mind this is an 802.11b client, so the HT capabilities field is not present


Probe Response:

    Probe responses look very similar to beacon frames, with the exception that they do not contain the TIM and QoS capabilities. Also, probe response frames contain the requested information elements that have been requested by the client station. After the probe response is received the client can begin the association process.

Looking at the Probe Response frame:












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