2009-11-10 23:20:05 -07:00
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Title: How DNS Works
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2008-10-27 15:57:45 -06:00
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When you request a URL like `http://goob.woozle.org/~neale/foo.html`,
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the first thing your browser does is send out a DNS query on
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"goob.woozle.org". Specifically, it asks for A records or CNAMEs. A
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records contain the name->IP mapping, and CNAMEs are like aliases.
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CNAMEs are a little out of vogue these days, so I'll focus on A records.
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Your browser sends the query to your recursive DNS resolver (the
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nameserver in /etc/resolv.conf). The resolver then pulls out the last
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part of the hostname (the .org), and looks for the server that can
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answer for the .org Top Level Domain (TLD). It does this by asking some
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big central nameservers that are listed by IP in its configuration. One
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of those big central nameservers will come back and say something like,
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".org is served by 1.2.3.4". Then your recursive resolver goes off to
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1.2.3.4 and asks it about "woozle.org". 1.2.3.4 will come back with
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another IP, in this case 216.39.146.229. Finally, the resolver connects
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to 216.39.146.229 and asks it about "goob.woozle.org". 216.39.146.229
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will come back with an answer of 216.39.146.229 (since 216.39.146.229 is
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what's listed as goob.woozle.org's IP address).
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The reason the .org domain said to go to 216.39.146.229 is because
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that's what I listed as the primary authoritative name server for the
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"woozle.org" domain with my host registrar (pacificroot.com). A lot of
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people use networksolutions.com as their host registrar. So on
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2009-11-10 23:20:05 -07:00
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216.39.146.229, I have an authoritative name server that knows about the
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woozle.org domain. Some examples of authoritative name server software
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are nsd, tinydns, and BIND.
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2008-10-27 15:57:45 -06:00
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Your recursive resolver has now obtained the mapping from
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"goob.woozle.org" to 216.39.146.229, so it returns that IP address to
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your web browser. If you're running a caching resolver, then the next
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time it's asked it won't bother querying the Internet again, it will
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just tell you the same thing it told you last time. That can make things
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a whole lot faster.
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Then your browser makes a TCP connection to 216.39.146.229, on port 80
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(the HTTP port). When it connects, it sends something like this:
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<pre>
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GET /~neale/foo.html
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Host: goob.woozle.org
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</pre>
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That's helpful, because I have a whole lot of hostnames all going to
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216.39.146.229. The web server looks at the host header and pulls up
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the appropriate page for that domain. My web server, thttpd, has an
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easy go of this: it just goes into a directory called "goob.woozle.org".
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Apache and other servers take a little more configuration, but in
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practice aren't much more difficult to run. This concept of many names
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pointing to the same IP address is called "virtual hosting".
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