I'm using several Android devices that have Android 6.0.x and 6.0.1, and that I want to use in an IPv6 island ("private") network. The Android devices are either attached to a WiFi-to-Ethernet bridge, or some tablets even accept USB 2.0 Ethernet dongles using OTG cables. The IPv6 network has RAs signalling the IPv6 prefix to use, as well as DNS IPv6 server address, and optionally some more-specific IPv6 routes. There are neither DHCPv6 nor DHCPv4 servers available. Well, there are DHCPv6 servers in this network, but they only respond with delegated prefixes.
Unfortunately, I cannot reliably connect my Android devices to this network. For cross-checking, an ordinary Linux Raspbian-equipped Pi does immediately become connected and works correctly, using DNS and getting proper addresses.
Even when I set a static IPv4 address, my Android devices often don't correctly connect in that they don't use IPv6 DNS, albeit it is getting advertised (RFC 5006). I have to set fake IPv4 DNS server addresses to be able to set the static IPv4 parameters. But even then, the Android devices aren't properly working, either not correctly using DNS or even getting disconnected after some seconds.
What kind of IPv4 connectivity do Android devices expect?