Add support for uBO's new "$cname" feature

[DandelionSprout]

Prerequisites

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• I am running the latest version (v0.101.0)
• I checked the documentation and found no answer
• I checked to make sure that this issue has not already been filed

Problem Description

Long story short: uBlock Origin introduced $cname in 1.25.3, which took me wildly by surprise as I thought I'd suggest they could create such a qualifier. But nevertheless, it seems like a pretty good fit for AdGuard Home that also serves an actual unique purpose, of ensuring that domains are only blocked through their URL address or their CNAME responses, instead of both.

That being said, their current implementation is whitelisting-only, for whatever reason. I see no immediate reason for why using $cname to only block by CNAME responses can't be done.

Proposed Solution

Add support for $cname to block based on CNAME only, and $~cname to block based on the main URL only.

Alternatives Considered

Add support for @@ + $cname in much the same way the uBO team did it; or maybe even convince the uBO team to give $cname the ability to block things. Either way to ensure cross-extension support.

Additional Information

Had I known any earlier about uBO adding it, I'd have submitted this feature request several weeks ago... 😬

[ameshkov]

I see no immediate reason for why using $cname to only block by CNAME responses can't be done.

Agreed

[ameshkov]

I am starting to think that $cname name might be misleading. AdGuard does not limit itself to CNAME scanning, it also checks A and AAAA responses so it's able to block domains by IP address.

Therefore, I suppose we should name this modifier $dnsresponse so that it was clear. $cname may be an alias kept just for compatibility.

I am also thinking about introducing a $dnstype modifier that would allow targeting specific DNS queries by type. For instance, ||example.org^$dnstype=A or ||example.org^$dnstype=TXT. It may be useful in conjunction with $dnsrewrite.

Also, I have to reassign this task to v0.105 as I don't have enough time to take care of it before v0.104 is released.

[gwarser]

That being said, their current implementation is whitelisting-only, for whatever reason. I see no immediate reason for why using $cname to only block by CNAME responses can't be done.

It's implemented like generichide. It stops blocking CNAME resolved requests for specified document. Not for specified request by itself.

[ameshkov]

This makes little sense to me. DNS replies are cached by the browser, and if it's blocked once for any website, it will be blocked on other websites regardless of any $cname exclusions.

[ameshkov]

Actually, this can be avoided if the domain name is resolved with bypass_cache. Although, there would be some other complications due to that if we want to avoid making unnecessary DNS queries.

Anyways, according to @gwarser's description, uBO's $cname is quite different from what @DandelionSprout requested and we simply cannot implement it that way in AdGuard Home.

[ameshkov]

Opened two separate tasks: $dnsrewrite and $dnstype:

• $dnstype modifier #2337
• $dnsrewrite modifier #2102