Checks a DNS server for the predictable-port recursion vulnerability. Predictable source ports can make a DNS server vulnerable to cache poisoning attacks (see CVE-2008-1447). The script works by querying porttest.dns-oarc.net (see https://www.dns- oarc.net/oarc/services/porttest). Be aware that any targets against which this script is run will be sent to and potentially recorded by one or more DNS servers and the porttest server. In addition your IP address will be sent along with the porttest query to the DNS server running on the target.