The device keeps its USB default route after the host drops the link, and then has no internet at all

scope: generic · severity: trap · confidence: proven · subsystem: network

Symptom — the phone is reachable over ssh and ip addr looks perfect, but nothing on it can reach the internet. In a browser this is a blank or “Problem Displaying Page”; in a shell it is wget: can't connect to remote host. Every arm that loads a page is silently void, and the numbers it prints describe an error page.

Cause — the USB gadget’s default route is installed with a lower metric than wifi (100 against DHCP’s 600), so it wins. It is a static route nothing withdraws: when the HOST stops configuring its end – unplugged, or the NM profile set to autoconnect no, which is the recommended fix for the-usb-gadget-can-steal-the-hosts-default-route – the phone keeps routing every packet at a gateway that is no longer there.

The two fixes therefore collide: making the host safe makes the device internet-less, and neither end says so.

What to do — when you move a device onto wifi, delete the gadget default route on the DEVICE as well:

sudo ip route del default via <host ip> dev usb0
ip route            # exactly one default, via wlan0

Check the route before believing any browser arm, and force IPv4 when you check reachability: DNS may answer AAAA and succeed over IPv6 while every IPv4 path is black-holed, which is how this hid for a whole session (wget -qO- https://example.com succeeded while Epiphany showed an error page). document.title is the cheapest witness that a page is the page.