dhcp-server: implement captive portal option (RFC8910)

[herbetom]

Clients observing the URI value "urn:ietf:params:capport:unrestricted" may forego time-consuming forms of captive portal detection.

https://datatracker.ietf.org/doc/html/rfc8910

---

Another option is to provide an URL to an Captive Portal API (RFC8908). This can be static JSON. In the Freifunk context the venue-info-url is probably the most interesting part. When set Users of Android 11 an up are getting a Notifcation which links to the URL provided via venue-info-url. Also it's visible in the Details of the Wifi. For iOS the Link will also be visible there.

https://developer.android.com/about/versions/11/features/captive-portal?hl=de

Providing an actual Captive Portal API hoewer comes with the drawback of requiring a request to the Captive Portal API until a client knows it isn't restricted. Not worse then the current check most clients will presumably do currently but also not better.

For an example of an Captive Portal API see https://darmstadt.freifunk.net/captive

[DasSkelett]

Thanks, very interesting. Out of interest, do you have experience with the venue-info-url notifications, how often do they show up? Only at very first connect, or everytime the phone reconnects with the SSID?

But for now I also think the "no captive portal" hint is a good start.

[herbetom]

For Android my experience is that a notification shows up whenever you're connecting:

You can click it and open the link, the notification stays. You can swipe it away and it will return once you connect again. In addition in the Wifi Settings there is always a "Website Öffnen" Button:

Toggle to view Picture

But i've also heard reports for Android that the Notification is only shown once. But not my experience on multiple devices.

For iOS it's less aggresive. I think there is an initial notification the first time you connect but after that it's only shown as a Link in the Settings. But don't have much experience with iOS.