edns-client-subnet (ECS) support

[FenghenHome]

AdGuardHome is a great dns, I really like it.
But I have a problem, I use my vps to create a public dns, AdGuardHome's edns-client-subnet has a problem, AdGuardHome will pass the vps ip address subnet to the upstream resolution, instead of passing my home ip address subnet to the upstream Parsing, so the ip parsed out is very far away from me.

[FenghenHome]

Of course, there is another problem. When AdGuardHome starts dns cache, ecs does not work.

[ameshkov]

Yeah, there are basically two different issues:

1. Set ECS extension to the client IP address
2. Handle ECS in the response and use it as a cache key

Both can be solved, of course.

[FenghenHome]

Thank you very much, I really look forward to it.

[AngelFalse]

I suggest adding a switch to let user choose whether to opt-in this feature. There are certain privacy-focusing users that dislike the idea of ECS

[ameshkov]

Sure, it will be disabled by default.