Comment by cesarb | Hacker News Reader

Comment by cesarb | Hacker News Reader

From what I read about Android elsewhere, if you originally connected to the network by typing its name, it will broadcast the network name when searching. If, however, you originally connected to the network by selecting it on the list of nearby networks, it won't broadcast the network name when searching.

There's an app called "WiFi Advanced Config Editor" (https://play.google.com/store/apps/details?id=org.marcus905....) which shows in detail how a saved wifi network is configured. I believe that whether it will broadcast the network name or not is displayed as the "Hidden SSID" checkbox.

Edit: after trawling through the Android source code, I found the code which apparently sets the scan_ssid= value in the wpa_supplicant config file. It seems to be this one: http://androidxref.com/5.1.0_r1/xref/frameworks/opt/net/wifi...

If I'm reading that code correctly, it sets the "scan_ssid" variable (WifiConfiguration.hiddenSSIDVarName is a public static final String containing "scan_ssid") according to the value of the WifiConfiguration.hiddenSSID variable. As documented in the WifiConfiguration class (http://androidxref.com/5.1.0_r1/xref/frameworks/base/wifi/ja...):

"This is a network that does not broadcast its SSID, so an SSID-specific probe request must be used for scans."

I've observed (just looked at the log to make sure) devices running Android KitKat sending probe requests for non-hidden networks.

But were these networks added by selecting from the list, or by typing their name? From what I read, that's what makes it set the internal "hidden SSID" flag, not whether the network actually has a hidden SSID or not.

See for instance the post at http://forum.xda-developers.com/showthread.php?t=2634042 ("Any network added manually with the '+' icon will have scan_ssid=1. Any network picked from the list of scan results will not have a scan_ssid property.")

It does this for networks picked from the scan results - I might have been mistaken about one or two networks, but I see the probes for multiple networks I'm sure I picked from the list.

This is worrying. When I tested some time ago (at home, so next to my home SSID), I saw only a broadcast probe quickly followed by it connecting to the network. Perhaps I should try again, but powering down the AP first.

Did you try the "WiFi Advanced Config Editor" or similar to see what Android thinks about these networks, to see if there's a pattern?

I looked at my wpa_supplicant.conf. None of the networks have scan_ssid set. I'm unsure how often this happens. I have a probe logger running 24/7 at my apartment with four radios, but it only logs the first probe it sees for a given SSID for each client radio MAC address. The only time related piece of data I have is one instance where I know there were about three days between the first time I connected to a network and when it showed up in my log, but that really doesn't say a whole log.

Forget Android. What does wpa_supplicant do? It's not clear from your links that any scan_ssid value achieves hiding the network names.

From the wpa_supplicant.conf man page:

      scan_ssid
	     SSID scan technique; 0 (default) or 1.  Technique 0 scans for the
	     SSID using	a broadcast Probe Request frame	while 1	uses a
	     directed Probe Request frame.  Access points that cloak them-
	     selves by not broadcasting	their SSID require technique 1,	but
	     beware that this scheme can cause scanning	to take	longer to com-
	     plete.

So presumably a (default) broadcast Probe Request would not disclose saved network names but somehow this doesn't appear to be true? Hence my question?

This may be a bug in wpa_supplicant, I'm not sure. I looked at the code, and it seems to be trying to avoid using the SSID in a probe unless this value is set to 1, but the code is structured such that the check needs to be done in many places so one of them may have omitted it. Should be in scan.c.

Turns out it's not a bug, it's a feature called Preferred Network Offload. Some useful links can be found here: http://www.reddit.com/r/androidapps/comments/2u2ww0/dev_wifi...

> Preferred Network Offload

Looks like you found the bug, thanks: https://www.eff.org/deeplinks/2014/07/your-android-device-te...

So it's a bug after all.

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