Add support for DNS64/NAT64 networks

[ameshkov]

The problem is that using a custom DNS server in the IPv6-only T-Mobile network is impossible as it relies on their own default DNS in the following way: when a domain is hosted on an IPv4 address, their DNS returns a private (?) IPv6-mapped address instead.

1. For instance, example.org address is 93.184.216.34.
2. T-Mobile's DNS server will resolve it to something like this instead: ::FFFF:93.184.216.34 (and this address is reachable in their IPv6-only network).

If we figure what exact prefix they use for IPv6 mapping (most likely it is ::FFFF: but I need a confirmation), we will be able to convert IPv4 addresses returned by a custom DNS server to their IPv6-mapped counterpart.

Please note, that this is just an assumption.

We need help from users who are on T-Mobile to figure if that is so. If you are a T-Mobile user, please collect AdGuard logs and send them to devteam at adguard.com.

[ameshkov]

Thank you for the logs!

So here's a prefix used by T-Mobile:
2607:7700:0:28::

Also, we can actually do it in a smart way and detect the DNS64 prefix on-the-fly:
AdguardTeam/AdguardForAndroid#1884 (comment)

@ameshkov

This might be extremely stupid question but when I choose ipv6 on AdGuard dns my network doesn’t work, looks like my cellular data only works with ipv4.

[ameshkov]

This might be extremely stupid question but when I choose ipv6 on AdGuard dns my network doesn’t work, looks like my cellular data only works with ipv4.

Why not, it just means that you have an IPv4-only network.

[ameshkov]

@IvanIin take a look at how it is implemented in Android: AdguardTeam/AdguardForAndroid#1884