Use python script to get Hex Dump of a DNS query
$ python

>>> import binascii
>>> def dns_hex(hex):
...     a = [binascii.hexlify(i) for i in hex.split(".")]
...     print "06".join(a[0:-3])+"07".join(a[-3:-1])+"02"+a[-1]
... 
>>> dns_hex("attack.attack.attack")
61747461636b0761747461636b0261747461636b

Here we are going to use the dump in the iptable rule

Create the iptable rule

$ sudo iptables -I INPUT 1 -p udp --dport 53 -m string --algo bm --from 30 --to 60 --hex-string "|61747461636b0761747461636b0261747461636b|" -j DROP