code: function isBlacklisted(ip, callback) {
      var host_lookup = reverseIp(ip) + bl_zones[current_bl];
  
      dns.resolve4(host_lookup, function(err, domain) {
          if (err) {
              // Not blacklisted
              callback(false);
          } else {
              // It is blacklisted
              callback(true);
          }
      });
  }
docs: The actual checking against the DNS blacklist