Hands up: who knows what an android device does when it sees a WiFi network coming up?
Exactly, since Lollipo (Android 5) your phone or tablet leaks a quick HTTP request to check if it has internet access.
This check is, for example, done with
clients3.google.com/generate_204, a “webpage” that always returns an HTTP status code
204 No Content.
Thus, if the phone receives a
204 it is connected to the internet, otherwise it assumes that this network does not provide proper internet access or is just a captive portal.
However, that way Google of course always knows when you connect from where. And how often. And which device you’re using. etc… :(
How to prevent the leak
Even if people may like that feature, that is of course a privacy issue – so how can we counter that?
I already maintain a convenient configuration for AdAway at stuff.lesscomplex.org/adaway.txt, which blocks Google’s captive portal detection.
However, blocking that “feature” also comes with some drawbacks…
The downside of blocking captive portal detection
The consequences of blocking all request of the captive portal detection are obvious: your phone assumes that no network hat internet access. And therefore, it wouldn’t connect automatically, saying
No Internet Access Detected, won’t automatically reconnect. see image on top
That will probably increase your mobile data usage, as you always need (to remember) to do connect manually. And even if you manually connect to a network “without internet” the WiFi icon will get an exclamation mark and the phone says
Connected, no Internet. see second image
What can we do about it?
Disable captive portal detection
Changed as of Android 7, see update below!
One small drawback of that approach: you need to execute that again after flashing a new image… However, I guess you’ll anyway have a small workflow for re-flashing your phone – just add that tiny bit to it ;-)
Another drawback is that you loose the captive portal detection… Of course, that’s what you intended, but sometimes it may be useful to have that feature in hotels etc..
Change the server for captive portal detection with the Android API
You can also change the URL to the captive portal server to a server under your control.
Let’s say you have a site running at
scratch.binfalse.de/generate_204 that simulates a captive portal detection server backend(!?) and always returns
204, no matter what request.
Then you can use that URL for captive portal detection!
Override the captive portal server on a root-shell (adb or SSH etc) by calling:
Changed as of Android 7, see update below!
This way you retain the captive portal detection without leaking data to Google. However, you will again loose the setting when flashing the phone again..
Change the server for captive portal detection using AdAway
Another option for changing the captive portal detection server is to change its IP address to one that’s under your control.
You can do that with AdAway, for example.
Let’s say your captive portal detection server has the IP address
18.104.22.168, then you may add the following to your AdAway configuration:
The webserver at
22.214.171.124 should then of course accept requests for the foreign domains.
This way, you also don’t leak the data to Google and you will also keep the settings after flashing the phone (as long as you leave AdAway installed).
However, there are also some things to keep in mind:
First, I could imagine that Google may be a bit upset if you redirect their domains to a different server?
And second, you don’t know if those are the only servers used for captive portal detection.
If Google at some point comes up with another domain for captive portal detection, such as
captive.google.com, you’re screwed.
See also the CaptivePortal description at the android reference.
Create captive portal detection server with Nginx
Just add the following to your Nginx configuration:
Create captive portal detection server with Apache
Create captive portal detection server with PHP
A simple PHP script will also do the trick:
As of Android 7 the settings have changes.
To enable/disable captive portal detection you need to set
captive_portal_mode to either
0Don’t attempt to detect captive portals, see CAPTIVE_PORTAL_MODE_IGNORE.
1When detecting a captive portal, display a notification that prompts the user to sign in, see CAPTIVE_PORTAL_MODE_PROMPT.
2When detecting a captive portal, immediately disconnect from the network and do not reconnect to that network in the future, see CAPTIVE_PORTAL_MODE_AVOID.
To define the captive portal server you actually have three settings:
captive_portal_use_httpsshould the phone use HTTPS for captive portal detection? (
captive_portal_http_urlURL to the captive portal w/o HTTPS.
captive_portal_https_urlURL to the captive portal when using HTTPS.
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