first working implementation

Gemfile

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+source 'https://rubygems.org'
+
+gem 'net-dns'
+gem 'prometheus-client'
+gem 'rack'
+gem 'puma'

Gemfile.lock

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+GEM
+  remote: https://rubygems.org/
+  specs:
+    net-dns (0.9.0)
+    prometheus-client (0.8.0)
+      quantile (~> 0.2.1)
+    puma (3.12.0)
+    quantile (0.2.1)
+    rack (2.0.6)
+
+PLATFORMS
+  ruby
+
+DEPENDENCIES
+  net-dns
+  prometheus-client
+  puma
+  rack
+
+BUNDLED WITH
+   1.13.6

config.ru

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+require 'rack'
+require './dnsbl_exporter'
+
+
+run lambda {|env|
+  begin
+    req = Rack::Request.new env
+    case req.path
+    when '/metrics'
+      collector = DnsblCollector.new
+      target =
+        begin
+          target = IPAddr.new req.params['target']
+        rescue IPAddr::AddressFamilyError
+          return [400, {"Content-Type" => "text/plain"}, ["Valid target-IP-Address expected.\n"]]
+        end
+      collector.collect target
+      [200, {"Content-Type" => "text/plain"}, [
+        Prometheus::Client::Formats::Text.marshal( collector.prometheus)
+      ]]
+    else
+      [404, {"Content-Type" => "text/plain"}, ["Not found.\nYou want to try /metrics?\n"]]
+    end
+  rescue
+    STDERR.puts "#$! (#{$!.class})", $!.backtrace
+    [500, {"Content-Type" => "text/plain"}, ["Server-error.\n"]]
+  end
+}

config.yml

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+# vim: set et sw=2 ts=2 sts=2:
+---
+resolver:
+  nameservers: 8.8.8.8
+blacklists:
+- all.s5h.net
+- b.barracudacentral.org
+- bl.emailbasura.org
+- bl.spamcannibal.org
+- bl.spamcop.net
+- blacklist.woody.ch
+- bogons.cymru.com
+- cbl.abuseat.org
+- cdl.anti-spam.org.cn
+- combined.abuse.ch
+- db.wpbl.info
+- dnsbl-1.uceprotect.net
+- dnsbl-2.uceprotect.net
+- dnsbl-3.uceprotect.net
+- dnsbl.anticaptcha.net
+- dnsbl.dronebl.org
+- dnsbl.inps.de
+- dnsbl.sorbs.net
+- dnsbl.spfbl.net
+- drone.abuse.ch
+- duinv.aupads.org
+- dul.dnsbl.sorbs.net
+- dyna.spamrats.com
+- dynip.rothen.com
+- http.dnsbl.sorbs.net
+- ips.backscatterer.org
+- ix.dnsbl.manitu.net
+- korea.services.net
+- misc.dnsbl.sorbs.net
+- noptr.spamrats.com
+- orvedb.aupads.org
+- pbl.spamhaus.org
+- proxy.bl.gweep.ca
+- psbl.surriel.com
+- relays.bl.gweep.ca
+- relays.nether.net
+- sbl.spamhaus.org
+- short.rbl.jp
+- singular.ttk.pte.hu
+- smtp.dnsbl.sorbs.net
+- socks.dnsbl.sorbs.net
+- spam.abuse.ch
+- spam.dnsbl.anonmails.de
+- spam.dnsbl.sorbs.net
+- spam.spamrats.com
+- spambot.bls.digibase.ca
+- spamrbl.imp.ch
+- spamsources.fabel.dk
+- ubl.lashback.com
+- ubl.unsubscore.com
+- virus.rbl.jp
+- web.dnsbl.sorbs.net
+- wormrbl.imp.ch
+- xbl.spamhaus.org
+- z.mailspike.net
+- zen.spamhaus.org
+- zombie.dnsbl.sorbs.net

dnsbl_exporter.rb

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+require 'prometheus/client'
+require 'prometheus/client/formats/text'
+require 'net/dns'
+require 'yaml'
+
+class	DnsblCollector
+	attr_reader :lists, :prometheus, :configfile, :resolver, :metrics
+
+	def initialize prometheus: nil, configfile: nil
+		@configfile = configfile || 'config.yml'
+		@prometheus = prometheus || Prometheus::Client.registry
+		%i[dnsbl_listed dnsbl_query_error].each {|m| @prometheus.unregister m }
+		@metrics = {
+			dnsbl_listed: @prometheus.gauge( :dnsbl_listed, 'IP listed currently on Blacklist'),
+			dnsbl_query_error: @prometheus.counter( :dnsbl_query_error, 'Errors while query for blacklist'),
+		}
+		load_config
+	end
+
+	def load_config
+		config = YAML.load_file @configfile
+		@resolver = config['resolver']
+		@lists =
+			config['blacklists'].map do |bl|
+				case bl
+				when String then {blacklist: bl}
+				when Hash then bl
+				else raise ConfigError, "Unexpected Element in blacklists: #{bl}"
+				end
+			end
+	end
+
+	def collect ip
+		prefix = ip.send :_reverse
+		ip = ip.to_s
+		dns = Net::DNS::Resolver.new @resolver
+		todo = @lists.sort{|_|rand}
+		durs = {}
+		10.times.map do
+			Thread.new do
+				while bl = @lists.pop
+					b = Time.now
+					begin
+						r = dns.search( "#{prefix}.#{bl[:blacklist]}", Net::DNS::A).answer.empty?
+						@metrics[:dnsbl_listed].set( {blacklist: bl[:blacklist], target: ip}, r ? 0 : 1)
+					rescue Net::DNS::Resolver::NoResponseError
+						@metrics[:dnsbl_query_error].increment( {blacklist: bl[:blacklist], target: ip})
+					end
+					durs[bl[:blacklist]] = Time.now-b
+				end
+			end
+		end.each &:join
+	end
+end