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I have a Raspberry Pi that is hosting a simple website using nginx. The
RPi is acting as a Wireless Access Point - users can connect to its wireless
network, the RPi gives them an IP (it runs a DHCP server), and they can
access the site.

Because the RPi doesn't actually provide users with internet (only this one site), I have made it easier for users to find the site. Instead of knowing the exact URL for the site, I have told my dns server (dnsmasq) which the DHCP server tells clients to use, to resolve all queries to the LAN IP of my RPi (192.168.30.1).

At this point, my nginx has an entry in its config that says:

• if the host field of the user's request is not MyRPiServer.com, send a 302 redirect to MyRPiServer.com


• if the host is MyRPiServer.com, serve the local website

This works awesome.

I wanted to take it one step further. When Android connects to a wireless network,
it tries to connect to http://connectivitycheck.gstatic.com/generate_204 (or one of the other similar google pages) specifically to check if the request is being redirected. If it gets a 204 code, it assumes everything is fine. If it does not, it assumes it is behind a captive portal, and pops up a browser window that opens the captive portal login.

For some reason, when I tell nginx to respond to requests for the generate_204 page with either a 302 redirect, or even a 200 (with some text), Android doesn't popup the browser.

I have a mikrotik router with a built-in hotspot feature, which does indeed have android popup the browser with the captive portal login (on the same test phone). When I look at the traffic it sends my client, it is a simple HTTP 200, with some text, just like mine.

The one thing that does seem to work is if I disable my DNS server from resolving everything to 192.168.30.1, and use iptables to redirect port 80 to localhost on my RPi.

Does anyone know why redirecting port 80 works when it comes to Android's Captive Portal detection, but configuring the DNS server to resolve everything to the RPi local IP doesn't?

Looking at the code found here https://stackoverflow.com/a/14030276/4258196, it appears that the only thing Android cares about is if it can connect to the
host, and if it gets an HTTP 204 back. In my case, it's definitely connecting,
and it's definitely not getting a 204 back (nginx logs show it sending HTTP
302 and HTTP 200).

My phone is running Android 8, so the code linked might be different now I supposed.

The solution I found was to set my dnsmasq to continue to resolve everything to 192.168.30.1, but to have some exceptions for captive portal test servers:

10.45.12.1 clients3.google.com

10.45.12.1 clients.l.google.com

10.45.12.1 connectivitycheck.android.com

10.45.12.1 connectivitycheck.gstatic.com

10.45.12.1 play.googleapis.com

Basically if anything tries to resolve the above domains using our dns server, they get a reply of 10.45.12.1.

10.45.12.1 is a random IP that doesn't belong to anything. It just has to not be 192.168.30.1.

The list of domains came from here.

With this in place, as soon as you connect to the RPi's WiFi, it pops up the browser page showing my site.

This is a solution, but not really an answer to the question of why this happens. If anyone can explain it, I would appreciate it.

Edit:

With this solution, if I connect and disconnect from the WiFi on the device several times, sometimes Android pops up the login page, and sometimes it doesn't. For anyone doing something similar, in the end, for a better solution, I went with this:

• Have DNSmasq resolve everything to 10.45.12.1 (or anything outside of the 192.168.30.0/24 subnet)
  
  
  
  
  • It MUST be outside of the 192.168.30.0/24 subnet (or whatever your LAN subnet is) or the client will try to use ARP to figure out the MAC address of the given device, and will fail, because the device doesn't actually exist
  
  
  
  


• Have iptables forward port 80 coming from the wifi interface to localhost

This works for Android, OS X, and Windows. I don't have an iOS device to test this with. According to this, iOS devices may require some additional work.

I'm still curious why this was even necessary, and why resolving everything to 192.168.30.1 didn't work in the first place.

I spent a lot of time trying to figure this out and finally did it. You can see the code here https://github.com/tretos53/Captive-Portal

It's a combination of domains pointing to public IPs, iptables and nginx redirects.

There's a lot of different reasons for this, could be your specific hardware phone manufacturer as well. I currently built my Captive Portal library for Mongoose OS responding to all DNS queries with device IP and don't have any problems with devices besides Samsung devices which seem to require the 200 response with text.

Here's the library stack:
https://github.com/tripflex/captive-portal-wifi-stack

Specifically here's the Captive Portal handling, along with details of the endpoints I use, and details in the README on how it's handled:

https://github.com/tripflex/captive-portal

You will see the one solution I used for Samsung devices is to return a 200 with a generated HTML files using a meta refresh tag:

https://github.com/tripflex/captive-portal#cportalredirect_file-setting

<html>

   <head>

      <title>Redirecting to Captive Portal</title>

      <meta http-equiv='refresh' content='0; url=PORTAL_URL'>

   </head>

   <body>

      <p>Please wait, refreshing.  If page does not refresh, click <a href='PORTAL_URL'>here</a> to login.</p>

   </body>

</html>

But really the main thing was setting up endpoint handlings for each specific device:

https://github.com/tripflex/captive-portal#known-endpoints

• 
  /mobile/status.php Android 8.0 (Samsung s9+)


• 
  /generate_204 Android


• 
  /gen_204 Android


• 
  /ncsi.txt Windows


• 
  /hotspot-detect.html iOS/OSX


• 
  /hotspotdetect.html iOS/OSX


• 
  /library/test/success.html iOS


• 
  /success.txt OSX


• 
  /kindle-wifi/wifiredirect.html Kindle when requested with com.android.captiveportallogin


• 
  /kindle-wifi/wifistub.html Kindle before requesting with captive portal login window (maybe for detection?)