Comcast, Alternative DNS and CDNs

Recently I started noticing problems with our Comcast service that although speed tests were very speedy, as were certain downloads – primarily torrents of Linux distributions, other content downloads were pathetically slow.  These would include items such as iTunes downloads, streaming content and the like.  It wasn’t until I switched my router from using Google DNS (8.8.8.8, 8.8.4.4) back to using Comcast DNS and seeing a drastic speed increase did I realize why this slowdown was occurring.  The issue here is with Content Delivery Networks (CDNs) and alternative DNS services.

CDNs are great services that replicate data across a number of data centers to offer faster access to the content by allowing you to reach a center closer to your physical location.  This allows for lower latency as your connection has to go through fewer hops so you can get a better experience.  The problem comes with using alternative DNS services such as Google DNS.  These services tend to mess with the geo-location features of the CDNs and as such, they may route you to a center that is further away which would create a slower experience for the users.

This is the problem that we were seeing.  As mentioned, once I changed us back to using Comcast’s default DNS servers, everything from iTunes downloads and general page loading times increased dramatically.  Despite my disdain for Comcast DNS, I guess I’ll be leaving that alone for a while.