Any attack on the internet itself would most likely attack either DNS or routers.

DNS is the protocol and servers which translate names to IP addresses. It has been attacked before. There are a number of "root servers" around the world, which are the core of the entire DNS system. If you can take down a large proportion of them, the internet would effectively become unusable.

Routers are usually custom hardware devices made by (e.g.) Cisco. These are vulnerable because they need to securely communicate with each other details of every route which exists to every network which forms the internet (in principle at least). If this communication fails or is subverted, then packets will end up going to the wrong place, around in circle etc, and the entire system will fail.

Both of these things have been successfully attacked before (either deliberately or accidentally), and resulted in several hours when the internet was mostly not usable for most people.

However, they get gradually stronger.

A single software bug affecting them all would be unlikely. The top-level DNS mostly run different versions of different software on different OSs on different hardware.

Any attack would likely be most serious if a flaw was found in a protocol used by these core servers, to speak to each other, which allowed someone to attack them all at once or confuse them all. I *think* this happened with the protocol which routers use to communicate routes once (Not sure what that protocol was).

Most of the routers on the internet are doing very simple routing, and have only a few, mostly static routes, or trust a relatively small number of other routers. But the "backbone" ones which are owned by major ISPs need to communicate with each other a great deal. This is considered by some as a weakness.

One of the things which IPv6 fixes, is apparently, it makes routing much easier.

Slarty