This isn’t necessarily software, this can also be hardware, or a combination of both.
However there’s almost alway a hardware aspect to a DoS or DDoS attack, as it has to do with the speed and capacity of the connection between what is called the system’s Network Interface (Ethernet, Fiber Optic, etc) and the outside world.
Secondarily, in a Web context, it has to do with how fast the webserver software on a system can serve up Web pages.
This can be a bottleneck because of CPU capacity/speed, memory, system tuning, or a number of other factors.
It is too bad one cannot have certain web pages on one server using a separate software as well as a separate system network interface that also allows users to cross over to the other site once passing a set of certain tasks. That would help a lot at in essence keeping these DDoS or Dos attack away from the bulk of the site and won't tax the rest of the system that serves up pages etc. It would be sort of like a 'front porch' for the site. If they can't get through the door they can only destroy the front porch, but nothing inside.
I assume going forward we will be seeing a lot more of these sort of attacks. I am surprised there is not more offered to accommodate this new reality. I guess they figure if a page can tie into the larger site then so can a hacker so why work so hard and spend so much money on maintaining basically two systems, but at least it will make the attackers work harder to pull off an effective attack.