The DNS (the domain name system) is one of those Internet services that everybody uses; but most don’t even know it exists. That is partially a good thing – it is supposed to be invisible in the sense that it just works rather than causing problems. But everything – Internet Explorer, Firefox, Chrome, and anything that uses the network – uses the DNS.
But What Does It Do?
The DNS in a very simple sense is the way that applications such as Chrome (or any web browser) finds out what network address a name points to. When we visit a web page such as http://www.bbc.co.uk/, the web browser needs to know what network address to make a connection to. So the web browser asks the DNS “what network address does www.bbc.co.uk point to” and the DNS answers “212.58.246.92 and 212.58.246.93” (as of the time of writing). The DNS does quite a bit more than that – even ignoring the details of how the servers operate – as it can answer other kinds of questions than just what the network address of a name is. But the process works pretty much the same way whatever kind of question is asked, so we’ll concentrate on the name questions. Technically the name www.bbc.co.uk is a fully-qualified domain name, and the network address is either an IPv4 address or an IPv6 address which can be seen if we perform a lookup on www.google.co.uk instead of www.bbc.co.uk (as the BBC doesn’t have an IPv6 address as yet) :-
# host www.google.co.uk www.google.co.uk has address 74.125.132.94 www.google.co.uk has IPv6 address 2a00:1450:400c:c06::5e
That’s a command-line way of performing a DNS lookup, which is rather irrelevant to this discussion except that it shows just the DNS answer.
So How Does It Work?
When you perform a DNS lookup (or more usually an application performs a DNS lookup on your behalf), it makes use of a piece of software on your computer called the resolver. This is more complex than is described here, and can use mechanisms other than the DNS. But ignoring all of that, the resolver composes a question in terms that a DNS server would understand. It then sends the question to all of the DNS servers it knows about.
Hopefully one or more of those DNS servers will answer the question, and the application can get on with whatever it is doing.
If an answer is not returned, the question is sent again, and this carries on until the resolver decides that enough is enough and returns an error to the application. Which of course results in an unexpected error such as a web browser saying that Google doesn’t exist!
There’s a fair bit more to it than this of course – particularly how the DNS servers find out the answer to your question, but this is enough for now.