--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mod_register_dnsbl_warn/README.markdown	Fri Feb 23 21:50:47 2018 +0100
@@ -0,0 +1,14 @@
+Introduction
+============
+
+This module checks the IP address of newly registered users against a
+DNS block list. If a positive match is found, it gets logged.
+
+Configuration
+=============
+
+  Option              Type     Default
+  ------------------- -------- ------------
+  registration\_rbl   string   *Required*
+
+

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mod_register_dnsbl_warn/mod_register_dnsbl_warn.lua	Fri Feb 23 21:50:47 2018 +0100
@@ -0,0 +1,25 @@
+local adns = require "net.adns";
+local rbl = module:get_option_string("registration_rbl");
+
+local function reverse(ip, suffix)
+	local a,b,c,d = ip:match("^(%d+).(%d+).(%d+).(%d+)$");
+	if not a then return end
+	return ("%d.%d.%d.%d.%s"):format(d,c,b,a, suffix);
+end
+
+-- TODO async
+-- module:hook("user-registering", function (event) end);
+
+module:hook("user-registered", function (event)
+	local session = event.session;
+	local ip = session and session.ip;
+	local rbl_ip = ip and reverse(ip, rbl);
+	if rbl_ip then
+		local log = session.log;
+		adns.lookup(function (reply)
+			if reply and reply[1] then
+				log("warn", "Account %s@%s registered from IP %s found in RBL (%s)", event.username, event.host or module.host, ip, reply[1].a);
+			end
+		end, rbl_ip);
+	end
+end);