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<An Engineers Guide to DNS ⁄ >

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The Domain Name System (DNS) is part of the "dark matter" of the internet. It's hard to observe the DNS directly yet it exerts an obscure, pervasive influence without which everything would fly apart. Because it's so difficult to probe people tend to take it for granted, which I think is a mistake. DNS problems can hurt the speed and reliability of your applications without you even noticing. In this article we'll take a look at the behavior of the DNS and walk through some experiments you can run to gather valuable data about your users' network performance.

A Clever Shambles

Before two computers can talk to each other on the 'net, one of them has to know the numeric IP address of the other. Using the DNS is often compared to looking up a number in the phone book. But that can give the impression the information is in one place, close to hand.

Instead, imagine it's 1982. You live in Tucson and you want to call a hotel in Toronto. You don't have a Toronto phone book so you call your local library. They don't have one either. Life is boring in Tucson, so the librarian uses her New York phone book to call another library. The nice lady in New York looks up the hotel's number in her copy of the Toronto phone book, tells it to your local librarian, who then calls back to give it to you. Doing all this is a hassle, so everyone in the chain writes down the number just in case the question ever comes up again.

The DNS is even more complex because of the hierarchy of internet domains. Consider the host name foo.bar.example.net. To look it up your computer will have to look up every part of the name, in reverse order. That means resolving ".", then "net.", then "example.net.", "bar.example.net.", and finally "foo.bar.example.net."[0]. It's not just a matter of finding the Toronto book. It's looking up someone who knows someone who has the Canada book and from there who has the Ontario book, then the Toronto book, and so on.

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