What exactly do you mean with rDNS resolver? We do not want the reverse lookup name. For instance, if you look up a google.com name with dig, you get an IP add

What exactly do you mean with rDNS resolver? We do not want the reverse lookup name. For instance, if you look up a google.com name with dig, you get an IP address. If you then do the reverse lookup with dig -x, you get a 1e100.net name. That's as good as the IP address for our purpose. Plus: We need to respond with a DROP or ALLOW verdict to a network packet without the ability to do any blocking requests. So we can only use information already available in the kernel to decide.