Centric IT

In the last 6 months or so it seems to have become very popular to use open DNS providers such as Google’s Public DNS or OpenDNS. I personally use Google all the time for testing as it’s simple to remember! However these should not be used for daily consumption because they break an important internet technology called CDNs (Content Distribution Networks). I’ve recently found people complaining of slow page loading times, poor YouTube performance, and just generally slow response times around the Internet.

What CDNs do

To understand how public DNS like Google’s breaks things, I need to explain a little about what CDNs do for websites. Let’s use Akamai as an example:

Akamai have over 70,000 servers deployed in over 70 countries. These servers are different to traditional web hosting in that they aren’t installed in the large well-known data centres, instead they are located inside your ISPs networks. If you run a traceroute to a website you will see the path go through your ISP first then out to the Internet. If you run a traceroute to an Akamai server you will likely see a lot less hops and it stop inside your ISP.

When you talk to most major websites you aren’t talking to their web servers directly, instead you are communicating with these Akamai servers who then respond with most of the assets quickly from the closest location to you. Over 80% of most websites rarely change – the JavaScript, furniture images and CSS. That means that if this data is stored in a CDN server close to you, it can be served up much faster as well as freeing up the web servers to do the data crunching work they’re designed for.

For the changing data, this still routes through Akamai’s servers, only it doesn’t take the cheapest route available like regular web traffic. Instead the Akamai servers have direct links into major data centers rather than bouncing around ISPs taking the cheapest route possible. Think of it like taking a plane journey over driving somewhere. The car is cheaper, but a lot slower!

How these new public DNS services ruin it all

If you don’t already know how DNS works, head over here and check it out for some background..

To deliver the best experience, Akamai needs to select the very closest Edge server for your location. The trick to doing this, is to knowing where you are, before you first connect to an Edge Web-server. If you connect to an Edge server on the other side of the world, your experience is going to be much slower than one that is 2 hops away, directly on your ISP’s network.

Their way to do this is to determine your location based on the DNS request you’ve made, which means the Edge server selected is based on what DNS server you use. If you use a DNS server from your local ISP, then you’re going to get a result that is close to those DNS servers. If you use Google, you’re going to get a result that is close to those servers – not one that is necessarily close to you.

DNS services such as OpenDNS and Google’s Public DNS have some advantages.  Unfortunately, using them affects your experience on the huge number of sites using Akamai and other CDN networks. Sometimes, they won’t work at all!

To compound the issue, the DNS server is the first server your computer hits when trying to look up a website. If that server is the other side of the world, as opposed to in your ISP, for the same reasons as described above your initial DNS requests will be slower in the first place.