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When wifi is enabled, and the phone is not connected to a wifi network, it scans for known networks. I always thought that networks send out their SSID (like StarbucksWifi) and if this SSID is in the list of known networks on the phone, the phone tries to connect.

Recently I found out that the phone actively tries to connect to known networks even if they are not available. Is it possible to block this last behavior?

See another discussion on Information Security about this.

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    This could be due to ability to have hidden networks, i.e. WiFi networks that don't broadcast their SSID. The phone would never be able to automatically connect to such networks. – Chahk Mar 26 '14 at 14:10
  • Yes I understand. But despite that, can I disable this functionality? – SPRBRN Mar 26 '14 at 14:14
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    I don't see of a way to do so in stock Android, nor have I seen such functionality in custom ROMs. It should be possible if you change the source code and compile a ROM yourself, unless this scanning is baked into a closed-source driver/kernel module. – Chahk Mar 26 '14 at 14:24
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I'm surprised by what you are saying, so let me recap.

Yes, most networks send out their SSID (it is called a beacon). If your wifi is enabled and you haven't disabled scanning then your phone will listen for those beacons and try to match the SSID in the beacon against your phone's list of SSID's. Depending on your settings it will also use the beacon to determine your current location and possibly even send the beacon's MAC address to google so google can keep its wifi location database up-to-date.

So far this is all passive - it is just listening for beacons. The phone doesn't need to actively try to connect to networks unless some of the networks don't send out beacons (hidden networks, as mentioned by @Chahk). But even if you have some hidden networks in your list, I thought that Android did not actively try to scan for those networks (send out probe requests). Actually, I remember people complaining about this, and 3rd party apps being written to address it, e.g. see "HiddenSSID Enabler" in the play store.

Has this changed? Can you tell me where you read about this?

If so, the only solution I can think of is to purge hidden networks from your list of known networks.

  • You're probably right. Recently I read an article in a newspaper about a security expert who had a device that could pick up all devices that were looking for a connection, and it could see the names of networks those devices had connected to. That made me a bit paranoid. – SPRBRN Mar 29 '14 at 14:21
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    Yes, someone with a wifi sniffer can see the probe requests that your device sends out if it is looking for hidden networks. On some systems you can go to the advanced configuration of an individual network and tell it to not connect automatically, and then it won't probe for it, but I don't think Android has that, so you need to purge and avoid using hidden networks if you don't others to able to determine that you have connected to that network. – Tom Mar 29 '14 at 15:12

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