How to do really simple adblocking with the PowerDNS Recursor 4.x

simple-adblock-powerdns.md

First, clone the Mozilla focus project and make it fetch its list:

$ git clone https://github.com/mozilla/focus.git
$ cd focus
$ ./checkout.sh
$ cd Lists

This delivers several JSON formatted files, of which we are going to use disconnect-advertising.json. We'll filter out the good bits using jq, and create a Lua representation:

(
echo 'return{'
for a in $(jq '.[].trigger["url-filter"]' disconnect-advertising.json  | cut -f3 -d? | sed 's:\\\\.:.:g' | sed s:\"::)
do 
   echo \"$a\", 
done 
echo '}'
) > blocklist.lua    # ends up as return{"dom1.com", "dom2.com", .... "dom3.com"}

Next, we use this small file adblock.lua to tell the PowerDNS Recursor 4.x what to do:

adservers=newDS()

function preresolve(dq)
	if(not adservers:check(dq.qname)) then
		return false
	end
	
	if(dq.qtype == pdns.A) then
		dq:addAnswer(dq.qtype, "127.0.0.1")
	elseif(dq.qtype == pdns.AAAA) then
		dq:addAnswer(dq.qtype, "::1")
	end
	return true
end

adservers:add(dofile("blocklist.lua"))

Next add to recursor.conf:

lua-dns-script=adblock.lua

Now fire up the PowerDNS Recursor and access to the domain names from the Mozilla focus project will be replaced by a link to 127.0.0.1. Note: the actual blocking strategy used by Mozilla is a lot smarter, and includes knowledge of the website containing the ad!

That error can happen if you tried to load the lua file using lua-config-file instead of lua-dns-script