Added DNS recursion amplification scanner

This auxiliary module will test a DNS server for recursion and determine the size of the UDP reply. Based on this the DoS amplifier is calculated.
To easily test this in a private environment, setup a CentOS 5 VM and use the following steps:

- yum -y install caching-nameserver
- Edit the /etc/named.caching-nameserver.conf file and make it look like this:
options {
	listen-on port 53 { any; };
	directory 	"/var/named";
};

- service named restart

msf > use auxiliary/scanner/dns/dns_amp
msf auxiliary(dns_amp) > set RHOSTS 192.168.2.132
RHOSTS => 192.168.2.132
msf auxiliary(dns_amp) > run
[*] Sending DNS probes to 192.168.2.132->192.168.2.132 (1 hosts)
[*] Sending 67 bytes to each host using the IN ANY isc.org request
[+] 192.168.2.132:53 - Response is 471 bytes [7.03x Amplification]
[*] Scanned 1 of 1 hosts (100% complete)
[*] Auxiliary module execution completed

With wireshark you can easily verify if the request/response bytes etc. are correct (This is what I did:))
It's also possible to change "IN ANY" request for "isc.org" to something else by using other query types and/or different domain:

msf auxiliary(dns_amp) > show options

Module options (auxiliary/scanner/dns/dns_amp):

   Name        Current Setting  Required  Description
   ----        ---------------  --------  -----------
   BATCHSIZE   256              yes       The number of hosts to probe in each set
   CHOST                        no        The local client address
   DOMAINNAME  isc.org          yes       Domain to use for the DNS request
   QUERYTYPE   ANY              yes       Query type(A, NS, SOA, MX, TXT, AAAA, RRSIG, DNSKEY, ANY)
   RHOSTS      192.168.2.132    yes       The target address range or CIDR identifier
   RPORT       53               yes       The target port
   THREADS     1                yes       The number of concurrent threads