Stop senseless mass-queries on DNS-server [duplicate]

Question

I have a disturbing problem for a few weeks now ... One of my three DNS-servers (running bind9 as slave of a Plesk DNS master) is getting spammed with senseless DNS requests. The origin IPs of those requests are from all over the world, so I can't just drop an IP range or a whole country by iptables.

Here are a few log lines so you can see the scheme of those "attacks":

10-May-2015 11:15:54.714 client 120.87.202.161#47441: query: sjgzincrmxobabst.www.luzhiye.com IN A + (my-ip)
10-May-2015 11:15:56.252 client 111.187.196.51#41387: query: mrcf.www.330dp.com IN A + (my-ip)
10-May-2015 11:15:56.806 client 89.90.44.173#56673: query: qzwp.www.330dp.com IN A + (my-ip)
10-May-2015 11:15:57.891 client 116.93.242.237#55721: query: srapafupglkxaver.www.330dp.com IN A + (my-ip)
10-May-2015 11:15:59.611 client 123.153.92.59#20847: query: yj.www.330dp.com IN A + (my-ip)

Those requests are not really attacks because the host server is very relaxed. I tried to block the requests with iptables, but after a few minutes/hours the requested domains are changing, so I have no chance to do so.

Chain INPUT (policy ACCEPT)
target     prot opt source               destination
DROP       all  --  anywhere             anywhere             STRING match  "888fy.com" ALGO name bm TO 65535
DROP       all  --  anywhere             anywhere             STRING match  "taohua.me" ALGO name bm TO 65535
DROP       all  --  anywhere             anywhere             STRING match  "yymo.us" ALGO name bm TO 65535
DROP       all  --  anywhere             anywhere             STRING match  "taohuazu.cc" ALGO name bm TO 65535
DROP       all  --  anywhere             anywhere             STRING match  "taohua.me" ALGO name bm TO 65535
DROP       all  --  anywhere             anywhere             STRING match  "227x.com" ALGO name bm TO 65535

Does anyone have experience with these kind of "attack"? How to stop it? I tried to shutdown the whole server for 48 hours. After the restart there were no attacks anymore, but a few hours later they started again.

Configuration (named.conf.local / named.conf.options):

dnssec-validation auto;
allow-new-zones yes;
notify master-only;

auth-nxdomain no;    # conform to RFC1035
listen-on-v6 { any; };

statistics-file "/var/cache/bind/named.stats";
zone-statistics yes;

controls {
        inet * port 953 allow { plesk-ip; 127.0.0.1; } keys {"rndc-key"; };
};

logging {
        channel b_query {
                file "/var/log/bind9/query.log" versions 2 size 1m;
                print-time yes;
                severity info;
        };
        category queries { b_query; };
};

I ran a few online tests to verify again that my DNS server is not an open resolver. Successfully: No open resolver found.

Help is really appreciated!