They're already "networked."
Since every name server has a list of valid root name servers, and it proceeds down that list for any name it doesn't have cached, you would have to take down all root name servers to effectively cripple DNS. Since those are located all over the world, this is a losing battle. By the time you got a few taken down, someone else would have new ones set up and get their IPs distributed to all remaining name servers.
Since DNS is a distributed hierarchy, you would have to nail a massive number of name servers at once, and even then the effects would be temporary.
I think this is less about shutting down DNS servers (which I'd think any hacker would be going by IP address anyway), and more about shutting down the internet backbone to isolate the US from an attack. I mean literally cutting the hardlines. That's far more feasible. While the US and Canada are pretty tightly integrated with fiber (a legacy of our shared telephone systems), there's only a few undersea cables connecting us to the rest of the world (and the rest of the world to itself, a massive amount of purely international traffic flows through the US) and a few through to Mexico and Latin America. All could be cut in short order.
While the control systems are distributed, the actual cabling of the internet is hierarchical, with the longer distance, higher capacity lines being exponentially less numerous than shorter distance connections. Even the Tier 1 operators each have roughly half dozen international cables going in/out of the continent.
Of course, as I said before, we're not the only one using the wires. Any president contemplating this would have to deal with the consequences of shutting down a large part of the world for some time. Yes, rewirings would be made, but you're still looking at widespread outages for days. Nobody wants to rob the world of $1 trillion of GDP without damn good cause.

2005 image of global data flows over the internet backbone.
Last edited: