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DNS filtering rules syntax

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Qui ti mostriamo come scrivere regole di filtro DNS personalizzate da utilizzare nei prodotti AdGuard

Quick links: Download AdGuard Ad Blocker, Get AdGuard Home, Try AdGuard DNS

Introduction​

You can use AdGuard DNS filtering rules syntax to make the rules more flexible, so they can block content according to your preferences. AdGuard DNS filtering rules syntax can be used in different AdGuard products such as AdGuard Home, AdGuard DNS, AdGuard for Windows/Mac/Android.

There are three different approaches to writing hosts blocklists:

  • Adblock-style syntax: the modern approach to writing filtering rules based on using a subset of the Adblock-style rule syntax. This way blocklists are compatible with browser ad blockers.

  • /etc/hosts syntax: the old, tried-and-true approach that uses the same syntax that operating systems do for their hosts files.

  • Domains-only syntax: a simple list of domain names.

If you are creating a blocklist, we recommend using the Adblock-style syntax. It has a couple of important advantages over the old-style syntax:

  • Blocklists size. Using pattern matching allows you to have a single rule instead of hundreds of /etc/hosts entries.

  • Compatibility. Your blocklist will be compatible with browser ad blockers, and it will be easier to share rules with a browser filter list.

  • Extensibility. In the past decade, the Adblock-style syntax has greatly evolved, and we see no reason not to extend it even further and offer additional features for network-level blockers.

If you're maintaining either a /etc/hosts-style blocklist or multiple filtering lists (regardless of type), we provide a tool for blocklist compilation. We named it Hostlist compiler and we use it ourselves to create AdGuard DNS filter.

Basic examples​

  • ||example.org^: block access to the example.org domain and all its subdomains, like www.example.org.

  • @@||example.org^: unblock access to the example.org domain and all its subdomains.

  • 1.2.3.4 example.org: (attention, old /etc/hosts-style syntax) in AdGuard Home, respond with 1.2.3.4 to queries for the example.org domain but not its subdomains. In Private AdGuard DNS, block access to example.org. www.example.org remains allowed.

    In AdGuard Home, using the unspecified IP address (0.0.0.0) or a local address (127.0.0.1 and alike) for a host is basically the same as blocking that host.

    # Returns the IP address 1.2.3.4 for example.org.
    1.2.3.4 example.org
    # Blocks example.org by responding with 0.0.0.0.
    0.0.0.0 example.org

  • example.org: a simple domain rule. Blocks the example.org domain but not its subdomains. www.example.org remains allowed.

  • ! Here goes a comment and # Also a comment: comments.

  • /REGEX/: block access to the domains matching the specified regular expression.

Adblock-style syntax​

This is a subset of the traditional Adblock-style syntax which is used by browser ad blockers.

     rule = ["@@"] pattern [ "$" modifiers ]
modifiers = [modifier0, modifier1[, ...[, modifierN]]]

  • pattern: the hostname mask. Every hostname is matched against this mask. The pattern can also contain special characters, which are described below.

  • @@: the marker that is used in the exception rules. Start your rule with this marker if you want to turn off filtering for the matching hostnames.

  • modifiers: parameters that clarify the rule. They may limit the scope of the rule or even completely change the way it works.

Special characters​

  • *: the wildcard character. It is used to represent any set of characters. This can also be an empty string or a string of any length.

  • ||: matches the beginning of a hostname, including any subdomain. For instance, ||example.org matches example.org and test.example.org but not testexample.org.

  • ^: the separator character. Unlike browser ad blocking, there's nothing to separate in a hostname, so the only purpose of this character is to mark the end of the hostname.

  • |: a pointer to the beginning or the end of the hostname. The value depends on the character placement in the mask. For example, the rule ample.org| corresponds to example.org but not to example.org.com. |example corresponds to example.org but not to test.example.

Regular expressions​

If you want even more flexibility in making rules, you can use regular expressions instead of the default simplified matching syntax. If you want to use a regular expression, the pattern has to look like this:

pattern = "/" regexp "/"

Examples:

  • /example.*/ will block hosts matching the example.* regexp.

  • @@/example.*/$important will unblock hosts matching the example.* regexp. Note that this rule also implies the important modifier.

Comments​

Any line that starts with an exclamation mark or a hash sign is a comment and it will be ignored by the filtering engine. Comments are usually placed above rules and used to describe what a rule does.

Example:

! This is a comment.
# This is also a comment.

Rule modifiers​

You can change the behavior of a rule by adding modifiers. Modifiers must be located at the end of the rule after the $ character and be separated by commas.

Examples:


  • ||example.org^ is the matching pattern. $ is the delimiter, which signals that the rest of the rule are modifiers. important is the modifier.

  • You may want to use multiple modifiers in a rule. In that case, separate them by commas:

    ||example.org^$client=127.0.0.1,dnstype=A

    ||example.org^ is the matching pattern. $ is the delimiter, which signals that the rest of the rule are modifiers. client=127.0.0.1 is the client modifier with its value, 127.0.0.1. , is the delimiter between modifiers. And finally, dnstype=A is the dnstype modifier with its value, A.

NOTE: If a rule contains a modifier not listed in this document, the whole rule must be ignored. This way we avoid false-positives when people are trying to use unmodified browser ad blockers' filter lists like EasyList or EasyPrivacy.

client​

The client modifier allows specifying clients this rule is applied to. There are two main ways to identify a client:

  • By their IP address or CIDR prefix. This way works for all kinds of clients.

  • By their name. This way only works for persistent clients (in AdGuard Home) and devices (in Private AdGuard DNS), which you have manually added.

    NOTE: In AdGuard Home, ClientIDs are not currently supported, only names are. If you have added a client with the name “My Client” and ClientID my-client spell your modifier as $client='My Client' as opposed to $client=my-client.

The syntax is:

$client=value1|value2|...

You can also exclude clients by adding a ~ character before the value. In this case, the rule is not be applied to this client's DNS requests.

$client=~value1

Client names usually contain spaces or other special characters, which is why you should enclose the name in quotes. Both single and double ASCII quotes are supported. Use the backslash (\) to escape quotes (" and '), commas (,), and pipes (|).

NOTE: When excluding a client, you must place ~ outside the quotes.

Examples:

  • @@||*^$client=127.0.0.1: unblock everything for localhost.

  • ||example.org^$client='Frank\'s laptop': block example.org for the client named Frank's laptop only. Note that quote (') in the name must be escaped.

  • ||example.org^$client=~'Mary\'s\, John\'s\, and Boris\'s laptops': block example.org for everyone except for the client named Mary's, John's, and Boris's laptops. Note that comma (,) must be escaped as well.

  • ||example.org^$client=~Mom|~Dad|Kids: block example.org for Kids, but not for Mom and Dad. This example demonstrates how to specify multiple clients in one rule.

  • ||example.org^$client=192.168.0.0/24: block example.org for all clients with IP addresses in the range from 192.168.0.0 to 192.168.0.255.

denyallow​

You can use the denyallow modifier to exclude domains from the blocking rule. To add multiple domains to one rule, use the | character as a separator.

The syntax is:

$denyallow=domain1|domain2|...

This modifier allows avoiding creating unnecessary exception rules when our blocking rule covers too many domains. You may want to block everything except for a couple of TLD domains. You could use the standard approach, i.e. rules like this:

! Block everything.
/.*/

! Unblock a couple of TLDs.
@@||com^
@@||net^

The problem with this approach is that this way you will also unblock tracking domains that are located on those TLDs (i.e. google-analytics.com). Here's how to solve this with denyallow:

*$denyallow=com|net

Examples:

  • *$denyallow=com|net: block everything except for *.com and *.net.

  • @@*$denyallow=com|net: unblock everything except for *.com and *.net.

  • ||example.org^$denyallow=sub.example.org. block example.org and *.example.org but don't block sub.example.org.

dnstype​

The dnstype modifier allows specifying DNS request or response type on which this rule will be triggered.

The syntax is:

$dnstype=value1|value2|...
$dnstype=~value1|~value2|~...

The names of the types are case-insensitive, but are validated against a set of actual DNS resource record (RR) types.

Do not combine exclusion rules with inclusion ones. This:

$dnstype=~value1|value2

is equivalent to this:

$dnstype=value2

Examples:

  • ||example.org^$dnstype=AAAA: block DNS queries for the IPv6 addresses of example.org.

  • ||example.org^$dnstype=~A|~CNAME: only allow A and CNAME DNS queries for example.org, block out the rest.

NOTE: Before version v0.108.0, AdGuard Home would use the type of the request to filter the response records, as opposed to the type of the response record itself. That caused issues, since that meant that you could not write rules that would allow certain CNAME records in responses in A and AAAA requests. In v0.108.0 that behaviour was changed, so now this:

||canon.example.com^$dnstype=~CNAME

allows you to avoid filtering of the following response:

ANSWERS:
->  example.com
    canonical name = canon.example.com.
    ttl = 60
->  canon.example.com
    internet address = 1.2.3.4
    ttl = 60

dnsrewrite​

The dnsrewrite response modifier allows replacing the content of the response to the DNS request for the matching hosts. Note that this modifier in AdGuard Home works in all rules, but in Private AdGuard DNS — only in custom ones.

Rules with the dnsrewrite response modifier have higher priority than other rules in AdGuard Home.

Responses to all requests for a host matching a dnsrewrite rule will be replaced. The answer section of the replacement response will only contain RRs that match the request's query type and, possibly, CNAME RRs. Note that this means that responses to some requests may become empty (NODATA) if the host matches a dnsrewrite rule.

The shorthand syntax is:

$dnsrewrite=1.2.3.4
$dnsrewrite=abcd::1234
$dnsrewrite=example.net
$dnsrewrite=REFUSED

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.

The full syntax is of the form RCODE;RRTYPE;VALUE:

$dnsrewrite=NOERROR;A;1.2.3.4
$dnsrewrite=NOERROR;AAAA;abcd::1234
$dnsrewrite=NOERROR;CNAME;example.net
$dnsrewrite=REFUSED;;

The $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:

||example.com^$dnsrewrite=example.net
! Or:
||example.com^$dnsrewrite=NOERROR;CNAME;example.net

then the response will be something like:

nslookup example.com my.adguard.local
Server: my.adguard.local
Address: 127.0.0.1#53

Non-authoritative answer:
example.com canonical name = example.net.
Name: example.net
Address: 1.2.3.4

Next, the CNAME rewrite. After that, all other records' values are summed as one response, so this:

||example.com^$dnsrewrite=NOERROR;A;1.2.3.4
||example.com^$dnsrewrite=NOERROR;A;1.2.3.5

will result in a response with two A records.

Currently supported RR types with examples:

  • ||4.3.2.1.in-addr.arpa^$dnsrewrite=NOERROR;PTR;example.net. adds a PTR record for reverse DNS. Reverse DNS requests for 1.2.3.4 to the DNS server will result in example.net.

    NOTE: the IP MUST be in reverse order. See RFC 1035.

  • ||example.com^$dnsrewrite=NOERROR;A;1.2.3.4 adds an A record with the value 1.2.3.4.

  • ||example.com^$dnsrewrite=NOERROR;AAAA;abcd::1234 adds an AAAA record with the value abcd::1234.

  • ||example.com^$dnsrewrite=NOERROR;CNAME;example.org adds a CNAME record. See explanation above.

  • ||example.com^$dnsrewrite=NOERROR;HTTPS;32 example.com alpn=h3 adds an HTTPS record. Only a subset of parameter values is supported: values must be contiguous and, where a value-list is expected, only one value is currently supported:

    ipv4hint=127.0.0.1             // Supported.
    ipv4hint="127.0.0.1"           // Unsupported.
    ipv4hint=127.0.0.1,127.0.0.2   // Unsupported.
    ipv4hint="127.0.0.1,127.0.0.2" // Unsupported.

    This will be changed in the future.

  • ||example.com^$dnsrewrite=NOERROR;MX;32 example.mail adds an MX record with precedence value 32 and exchange value example.mail.

  • ||example.com^$dnsrewrite=NOERROR;SVCB;32 example.com alpn=h3 adds a SVCB value. See the HTTPS example above.

  • ||example.com^$dnsrewrite=NOERROR;TXT;hello_world adds a TXT record with the value hello_world.

  • ||_svctype._tcp.example.com^$dnsrewrite=NOERROR;SRV;10 60 8080 example.com adds an SRV record with priority value 10, weight value 60, port8080, and target value example.com.

  • ||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.

Exception rules unblock one or all rules:

  • @@||example.com^$dnsrewrite unblocks all DNS rewrite rules.

  • @@||example.com^$dnsrewrite=1.2.3.4 unblocks the DNS rewrite rule that adds an A record with the value 1.2.3.4.

important​

The important modifier applied to a rule increases its priority over any other rule without the modifier. Even over basic exception rules.

Examples:

  • In this example:

    ||example.org^$important
    @@||example.org^

    ||example.org^$important will block all requests to *.example.org despite the exception rule.

  • In this example:

    ||example.org^$important
    @@||example.org^$important

    the exception rule also has the important modifier, so it will work.

badfilter​

The rules with the badfilter modifier disable other basic rules to which they refer. It means that the text of the disabled rule should match the text of the badfilter rule (without the badfilter modifier).

Examples:

  • ||example.com$badfilter disables ||example.com.

  • @@||example.org^$badfilter disables @@||example.org^.

    NOTE: The badfilter modifier currently doesn't work with /etc/hosts-style rules. 127.0.0.1 example.org$badfilter will not disable the original 127.0.0.1 example.org rule.

ctag​

The ctag modifier can only be used in AdGuard Home.

It allows to block domains only for specific types of DNS client tags. You can assign tags to clients in the AdGuard Home UI. In the future, we plan to assign tags automatically by analyzing the behavior of each client.

The syntax is:

$ctag=value1|value2|...

If one of client's tags matches the ctag values, this rule applies to the client. The syntax for exclusion is:

$ctag=~value1|~value2|...

If one of client's tags matches the exclusion ctag values, this rule doesn't apply to the client.

Examples:

  • ||example.org^$ctag=device_pc|device_phone: block example.org for clients tagged as device_pc or device_phone.

  • ||example.org^$ctag=~device_phone: block example.org for all clients except those tagged as device_phone.

The list of allowed tags:

  • By device type:

    • device_audio: audio devices.
    • device_camera: cameras.
    • device_gameconsole: game consoles.
    • device_laptop: laptops.
    • device_nas: NAS (Network-attached Storages).
    • device_pc: PCs.
    • device_phone: phones.
    • device_printer: printers.
    • device_securityalarm: security alarms.
    • device_tablet: tablets.
    • device_tv: TVs.
    • device_other: other devices.

  • By operating system:

    • os_android: Android.
    • os_ios: iOS.
    • os_linux: Linux.
    • os_macos: macOS.
    • os_windows: Windows.
    • os_other: other OSes.

  • By user group:

    • user_admin: administrators.
    • user_regular: regular users.
    • user_child: children.

/etc/hosts-style syntax​

For each host a single line should be present with the following information:

IP_address canonical_hostname [aliases...]

Fields of the entries are separated by any number of space or tab characters. Text from the # character until the end of the line is a comment and is ignored.

Hostnames may contain only alphanumeric characters, hyphen-minus signs (-), and periods (.). They must begin with an alphabetic character and end with an alphanumeric character. Optional aliases provide for name changes, alternate spellings, shorter hostnames, or generic hostnames (for example, localhost).

Example:

# This is a comment
127.0.0.1 example.org example.info
127.0.0.1 example.com
127.0.0.1 example.net # this is also a comment

In AdGuard Home, the IP addresses are used to respond to DNS queries for these domains. In Private AdGuard DNS, these addresses are simply blocked.

Domains-only syntax​

A simple list of domain names, one name per line.

Example:

# This is a comment
example.com
example.org
example.net # this is also a comment

If a string is not a valid domain (e.g. *.example.org), AdGuard Home will consider it to be an Adblock-style syntax rule.

Hostlist compiler​

If you are maintaining a blocklist and use different sources in it, Hostlist compiler may be useful to you. It is a simple tool that makes it easier to compile a hosts blocklist compatible with AdGuard Home, Private AdGuard DNS or any other AdGuard product with DNS filtering.

What it's capable of:

  1. Compile a single blocklist from multiple sources.

  2. Exclude the rules you don't need.

  3. Cleanup the resulting list: deduplicate, remove invalid rules, and compress the list.