Do Android devices scan *actively* for WiFi networks?

i am prog开发者_开发技巧ramming for google android and i wonder if the scans for wireless 802.11 networks are done passively or actively? i mean: does the device actually emit a beacon request signal on WifiManager.startScan() or does it just listen for beacons sent periodically by the access points?

Android does passive scans, it listens for beacons. Doing an active scan can create a lot of congestion on the network by sending out probe requests and waiting for probe responses.

Even though this question is 11 years old, it still has relevance today and so I thought I'd give it an up-to-date answer:

Yes, Android devices do search for APs actively. Passive scanning requires for the device to listen for beacons for a longer time, which is energy consuming see this paper by Freudiger for more details

Active scanning only requires sending probe requests and then waiting for the reply on the corresponding channels. This takes less energy as it requires for the radio to be turned on for a much shorter time span. Unfortunately, active scans require for the device to identify itself somehow, and this makes it easily trackable. Mitigations in place are e.g. MAC address randomisation with every burst, randomised MACs during connection with an AP, randomised Sequence Numbers, omission of unnecessary fields and more. It all doesn't seem to be enough as people still find way to optimise their algorithms to improve tracking via Probe Requests. ;-)

Rain Ma replied that Probe Requests contain SSIDs, this is technically not true for all devices anymore. The only reason for SSIDs in Probe Requests is to find hidden networks. In Android devices, newer OS versions don't transmit SSIDs at all anymore, save for when scanning for hidden networks. Up until Android 8, all manually added networks were automatically assumed to be hidden networks. Starting with Android 9, one has to explicitly mark a network to be "hidden" for the device to probe with SSID. iOS does automatic detection of hidden networks, and they say they don't probe with SSID at all unless they detect a network to be hidden.

The answer is YES

• Most Android devices always try to send Probe Request Frame every few minutes. They keep a list of SSID you used to connect, and always try to send probe request frame with SSID to see if they are available now. I guess one of the reason is 802.11 pre-authencation , and some other reason may be: enhance the wifi list display speed.
• They are working in a active.
• iOS devices seldom do active scan

If you wanna konw more about the rules, you'd better read "802.11 Wireless Networks The Guide"

You can refer to the pciture : Packages captured by omnipeek