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Have you ever wondered: What are DNS Servers?
Jon Richardson takes up the issue of DNS servers in this plain english description of the role of DNS servers in modern networking. In case you really want to know, here is a short answer to the DNS mystery... read on at your own risk! Domain Name (DNS) Servers are the address books that our computers rely on to find their way around the Internet. They also allow our ISP's (or company's) mail server to find the mail server of a user somewhere else. If you've ever looked at the network settings on your computer, you may have seen a few DNS servers listed - but always by their IP address. The friendly names we humans like to use, like "jon@mycompany.net" mean nothing to a computer unless it can figure out where the domain "mycompany.net" is located. The only thing our computers care about (and the web browser or mail client we run on them) is the Internet Protocol (IP) address of the machine we're trying to connect to. I may type in "www.google.com" but my computer has to ask a DNS server to give it the numeric IP address: 216.239.37.101... then my computer can connect to the web server at Google. If you watch the bottom margin of your browser, sometimes you can catch a glimpse of this conversation between the browser and the DNS server. If you have only one DNS server identified in your network settings, and that DNS server crashes, you will not be able to use the Internet until the server is brought back online. Most computers have room for at least two, and often three DNS servers. This way if one breaks, you have a backup server to use. Most ISP's give you a primary and a secondary DNS server to enter in your computer. Let's say you want to check out the schedule for some flights from Denver to San Jose on United. You open up your favorite Web Browsers and type in www.ual.com. Now, for the sake of this discussion, suppose that in the last 3 days nobody has asked your DNS servers to resolve www.ual.com for them... so they don't have the IP address handy for www.ual.com. Your request generates an upstream request to the root DNS server for the ".com" domain (all servers whose address ends in .com). The root server will have a table which indicates what ISP provides connections to the ual.com domain. Somewhere in that network is a DNS server or two that are "Authoritative" for www.ual.com. The root server knows which servers are Authoritative and refers our DNS server to them. Our DNS server contacts one of those servers and gets the IP address for www.ual.com, we get the IP address, and our browser is able to display the schedule pages for United flights. Once our DNS server resolves this name to an IP address, it saves the entry for 3 days. Often, ISP's will tell you it takes 72 (or more) hours for DNS changes to "converge." If you make an IP address change on a DNS server that's authoritative for www.richardsontech.net, all the other DNS servers in the world who have already resolved and cached the old IP address won't even check for the new address until their cache time to live expires for that address. If you plan on moving from one ISP to another and need help making sure your DNS transfer goes smoothly, please ask the new ISP lots of questions. Try to set up your website and mailboxes ahead of time. If you manage your own website and you want information on "The Google Dance", there's a pretty good description here: Google Dance. If you want to list your website with DMOZ.ORG, here is a link to the directory: Open Directory.
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© 2007 Jon Richardson
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