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When the device tries to connect to an access point or router, it has to sign in on the captive portal page with the message saying

"Sign in to Wi-Fi network".

How does the access point/router identify that a sign in is required for the device and what is the process behind it?

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3 Answers 3

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35

The detection url differs between different Android versions.

For Marshmallow, it uses connectivitycheck.gstatic.com/generate_204, check the code here AndroidXRef - Marshmallow 6.0.0_r1

For Kitkat, it uses clients3.google.com/generate_204 check the code AndroidXRef - KitKat 4.4.4_r1

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2
  • I didn't want to hijack the answer by editing it, so I'll just leave a comment. It took me a while to realize that the detection URL triggers the notification if it cannot be reached. In addition, I needed my access point to respond to this request with some kind of content - in my case I returned a 200 code as well as the index page of my web server when /generate_204 is requested. Without content in the response, regardless of the return code, I would get a "Wi-Fi has no Internet access" notification.
    – ubomb
    Commented Jan 19, 2017 at 23:29
  • @ubomb Seems odd, since http://connectivitycheck.gstatic.com/generate_204 actually returns a 204 with no content, and obviously works. Maybe you just left out the Content-Length: 0 header.
    – Tamir Daniely
    Commented Jun 17, 2018 at 6:22
10

This technology is called "Captive portal". There are several implementations. As I know, Android tries to reach a file named generate_204 on one of the Google servers and if it is not possible, a notification in generated.

Further reading on Wikipedia: Captive_portal

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6
  • Actually i know about captive portal. But i know the specific URL for check it. I want to know How it is work. Not what it is.
    – Sagar Vaghela
    Commented Sep 15, 2015 at 9:07
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    What do you mean - "how it is work"? Android sends a request, and if there is no positive response, it means that there is no connection to the internet, and notification is generated. That's how it works.
    – Άνδρας
    Commented Sep 15, 2015 at 9:11
  • Which is the specific URL on which android request. I have mention in question.
    – Sagar Vaghela
    Commented Sep 15, 2015 at 9:15
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    In your previous comment you said that you know the specific URL. I think it's https://www.gstatic.com/generate_204
    – Άνδρας
    Commented Sep 15, 2015 at 9:28
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    The URL seems to be clients3.google.com/generate_204 . Information taken from this site for chromium which is probably the same for android. chromium.org/chromium-os/chromiumos-design-docs/…
    – benjamin
    Commented Sep 15, 2015 at 9:34
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A captive portal is generally implemented by directing all traffic to a valid HTTP response (200). This HTTP response is what appears in the captive portal.

This can be done with tools like iptables or dnsmasq.

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