Add Blocker to the external plugins list

Author: icyflame

content/explugins/blocker.md

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+title = "blocker"
+description = "*blocker* - Domain blocker plugin for CoreDNS."
+weight = 10
+tags = [  "plugin" , "dns" ]
+categories = [ "plugin", "external" ]
+date = "2025-04-13T11:58:24+09:00"
+repo = "https://github.com/icyflame/blocker"
+home = "https://github.com/icyflame/blocker/blob/master/README.org"
++++
+
+## Description
+
+The blocker plugin blocks a list of domains provided in a file written using the AdBlock Plus syntax
+format. The list of blocked domains is loaded into memory at start-up, and periodically after that.
+
+## Syntax
+
+~~~ txt
+blocker path-to-blocklist-file refresh-interval file-syntax empty-response-type
+~~~
+
+1. `path-to-blocklist-file`: Absolute path to a file that contains the list of blocked domains
+2. `refresh-interval`: Interval after which the file is read from disk and loaded into memory
+   periodically. Duration must be a string which can be parsed by Go's
+   [`time.ParseDuration`](https://pkg.go.dev/time#ParseDuration).
+3. `file-syntax`: One of either `hosts` or `abp`. `hosts` files have a list of blocked domains,
+   which will be blocked by exact match. `abp` syntax supports prefixes and multiple subdmoains
+   using a single line in the file
+4. `empty-response-type`: One of either `empty` or `nxdomain`. `empty` will return `0.0.0.0` or `::`
+   as the response when a DNS query for a blocked domain is made. `nxdomain` will return a
+   non-existent domain name response for blocked domains.
+
+## Metadata
+
+This plugin exports the metadata key `blocker/request-blocked`. The value of this key will be `YES`
+when a domain is blocked and `NO` in all other cases.
+
+If the `metadata` plugin is enabled, then this key can be added to the log line for each query using
+the `log` plugin:
+
+``` corefile
+.:53 {
+  metadata
+
+  log . "{common} {/blocker/request-blocked}"
+
+  blocker /home/user/blocklist_file 1h abp empty
+
+  forward . 1.1.1.1
+}
+```
+
+## Examples
+
+In this configuration, we block domains that are listed in the `/home/user/blocklist.abp` file and
+send an empty response (`0.0.0.0`) when domains in this list are quiered. This file will be reloaded
+from disk into memory every hour after start-up:
+
+``` corefile
+. {
+  blocker /home/user/blocklist.abp 1h abp empty
+
+  forward . 1.1.1.1
+}
+```