[lambda]

There is the UDP source port, which is where the reply will be sent, and a 16 bit ID number associated with each request. Randomizing both of these gives you 32 bits of entropy, which would makes spoofing attacks a lot harder. There were a lot of systems vulnerable to such attacks before source port randomization and ID randomization became common to mitigate such attacks.

https://en.wikipedia.org/wiki/DNS_spoofing

Of course, for this attack, you may only need to get the source port to match, since if it's the DNS resolver that has the bug, it may parse the whole response before noticing that the ID doesn't match. And some NATs may break source port randomizaiton, since they allocate their own source ports to keep a table of their source port to the internal IP and source port; if they do so in a more predictable manner, it may be relatively easy to spam them with packets, or you could just spam the whole 65536 bit range with bad packets.