I may not have the technical expertise to understand this fully but right now I'm doing adblocking by using adguard's DNS IPs in my router (1).
It kinda works everywhere but for some apps like Chromecast I have to null route two IP addresses (8.8.8.8 and 8.8.4.4) otherwise it doesn't work. Those are both Google's IPs afaik.
So my question is: will I be able to keep doing it after this? I am asking because I am extremely suspicious of Google these days and wondering if they have an ulterior motive to prevent users from doing such host based adblocking in future?
Yes, 8.8.8.8 and 8.8.4.4 are Google DNS resolvers. Sounds like your Chromecast has them hard-coded instead of respecting your locally configured DNS.
In comparison to your current position, where you're black-holing the hard-coded addresses and the app is falling back to your configured DNS: yes you will be able to continue doing that, assuming the Chromecast will maintain the same fallback behaviour. The exact addresses you need to block and the protocol/ports you need to block may change (eg. to port 443 tcp instead of port 53 udp).
The big thing that DoH will prevent is something that you aren't currently doing, which is: instead of null-routing 8.8.8.8, you could be intercepting DNS requests to 8.8.8.8 and responding to them yourself. You would need to do this if the chromecast didn't fall back to using your proper DNS server. DoH will prevent this kind of interception, so your only choices are to allow it through or block it. And if the Chromecast then refuses to fall back, then blocking it will make the device not work, with no viable workaround short of replacing the firmware.
The goal of DoH is ultimately to prevent your ISP doing the exact same thing to you, ie. intercepting (or just listening in on) your DNS requests even when you want them to go elsewhere. Unfortunately, there's no way to prevent the same protections from extending to a malicious local device trying to circumvent you on your local network.
The reason for 8^4 being used for "can I connect to the internet" is valid since the DHCP DNS might be an internal router that's up even when the internet is unreachable; why it's being used for all DNS queries is less defensible. How long till it uses DoH to https://dns.google ?
If using Firefox you’ll need to manually change the DNS settings, as it will by default bypass your local DNS and send it to Cloudflare using DoH. You can easily disable that though.
It kinda works everywhere but for some apps like Chromecast I have to null route two IP addresses (8.8.8.8 and 8.8.4.4) otherwise it doesn't work. Those are both Google's IPs afaik.
So my question is: will I be able to keep doing it after this? I am asking because I am extremely suspicious of Google these days and wondering if they have an ulterior motive to prevent users from doing such host based adblocking in future?
(1) https://adguard.com/en/adguard-dns/overview.html