I am configuring a photobooth which I (and some-one else) can control via a local http server, but when I do that I do not have any internet on my phone .
Is it possible to have multiple cell phones connect to a wifi network (without a wan connection), and let the cell phones use their own 3g internet?
This is a function of the phone and it’s operating system. A WiFi network without internet connectivity is not what most phones expect, but obviously has utility in these types of situations. It is up to the phone to understand that the internet is only available via the cellular connection.
Thanks for the suggestion.
Just tested it, however I had to change the dns servers for that wifi on my iPhone to 18.104.22.168 & 22.214.171.124 . Now my iPhone recognizes that it does not have internet and provides an option to use the cell data as backup. While keeping the wifi , so that solved my problem .
In my experience, which is limited to iOS, phones work like any other TCP device when making routing decisions. They will choose the highest priority gateway available. So, if they don't think WiFi has a connection to the internet, they won't send packets intended for the internet over WiFi and will instead use their cellular connection.
Phones discover whether a WiFi network has a connection to the internet via various means, but most often on small networks by way of the default gateway reported by the DHCP server. If you don't have a default gateway configured for an interface then the DHCP server running on that interface doesn't report a default gateway to clients making DHCP requests on that network. Under these circumstances the phone will communicate with devices on the LAN, but will send all other traffic to the Internet at large over their cellular connection.
I think denying DNS traffic (UDP and TCP/53) on wifi for those phone clients will cause their connectivity checks to fail and force them to revert to cellular for internet connectivity.
Afair both iOS and Android phones try to load some kind of hardcoded https:// URL to check if they can reach the internet. Denying DNS should prevent them from reaching that URL.
This is often true, but not universal. Some phones/OS's will disconnect from wifi when they determine that it doesn't have internet connectivity. And other phones/OS's will 'prefer' wifi and avoid sending data via cellular when the wifi connection is established. That is why some wifi AP systems actually have a mode where they can turn off the radio when the upstream connection is lost -- this forces mobile devices to use cellular when they might otherwise keep trying to use wifi (if you've had an internet outage at your home or business, you may have experienced a mobile device not smartly failing over to cellular). I think this has been improved over the last few years with both iOS and Android, but it isn't always a given.
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