A visual explanation of how DNS lookups work
Most reasonably technical Internet users have a pretty good idea what DNS is, but what actually happens when you look up a domain name is not always so clear. For those of you who are a bit uncertain of how it works (or just like geeky server charts), we found an excellent picture describing the chain of events of a DNS lookup.
The image below is from Verisign, and to simplify matters a bit it ignores the effect of caching (normally results are cached at various points along the chain):
Image source: Verisign Domain Name Industry Brief, June 2007 (PDF), last page.
The chain of events to get the IP address for www.abc.com:
First your computer queries the name server (DNS server) it is set up to use. This is the recursive name server shown above.
The name server doesn’t know the IP address for www.abc.com, so it will start the following chain of queries before it can report back the IP address to your computer (the numbers below correspond to the numbers in the image).
- Query the Internet root servers to get the name servers for the .com TLD.
- Query the .com TLD name servers to get the authoritative name servers for abc.com.
- Query the authoritative name servers for abc.com to finally get the IP address for the host www.abc.com, then return that IP address to your computer.
- Done! Now that your computer has the IP address for www.abc.com, it can access that host.
This won’t be news to some of you, but you have to admit it’s a good picture! Definitely worth sharing.