Pull request: 4133 empty rewrite

Merge in DNS/adguard-home-wiki from 4133-empty-rewrite to master

Squashed commit of the following:

commit 11deec72902b9231cde12bee4a7e5d6391a0cd3f
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Tue Jan 18 22:02:28 2022 +0300

    Hosts-Blocklists: imp rewrites description
Eugene Burkov 2022-01-19 14:19:24 +03:00
parent 9f172570d0
commit e9aab3c2a5

@ -345,7 +345,14 @@ $dnsrewrite=example.net
$dnsrewrite=REFUSED
```
The keywords, like `REFUSED`, MUST be in all caps.
The keywords MUST be in all caps (e.g. `NOERROR`). Keyword rewrites take
precedence over the other and will result in an empty response with an
appropriate response code.
Before **v0.107.3** the only possible keyword is `REFUSED`.
Since **v0.107.3** response codes `NOERROR`, `NXDOMAIN`, and `SERVFAIL` are also
supported.
The full syntax is of the form `RCODE;RRTYPE;VALUE`:
@ -356,6 +363,9 @@ $dnsrewrite=NOERROR;CNAME;example.net
$dnsrewrite=REFUSED;;
```
Since **v0.107.3** a `$dnsrewrite` modifier with the `NOERROR` response code may
also has empty `RRTYPE` and `VALUE` fields.
The `CNAME` one is special because AdGuard Home will resolve the host and add
its info to the response. That is, if `example.net` has IP `1.2.3.4`, and the
user has this in their filter rules:
@ -382,9 +392,8 @@ Name: example.net
Address: 1.2.3.4
```
Keyword rewrites (for example, `REFUSED`) take precedence over the other. Next,
the `CNAME` rewrite. After that, all other records's values are summed as one
response, so this:
Next, the `CNAME` rewrite. After that, all other records's values are summed as
one response, so this:
```none
||example.com^$dnsrewrite=NOERROR;A;1.2.3.4
@ -441,6 +450,12 @@ Currently supported RR types with examples:
* `||example.com^$dnsrewrite=NXDOMAIN;;` responds with an `NXDOMAIN` code.
* `$dnstype=AAAA,denyallow=example.org,dnsrewrite=NOERROR;;` responds with an
empty `NOERROR` answers for all `AAAA` requests except the ones for
`example.org`.
**NOTE:** this is available since **v0.107.3**.
Exception rules remove one or all rules:
* `@@||example.com^$dnsrewrite` removes all DNS rewrite rules.