North American Network Operators Group
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Re: New Outage Hits Comcast Subscribers

  • From: Peter John Hill
  • Date: Thu Apr 14 15:06:32 2005

Do you understand anycast? Do you understand how different operating systems react to failures of configured dns servers?

You really need to look into anycast and see why it is used. Perhaps the comcast people are as naive as you about dns... Check out:
http://www.net.cmu.edu/pres/anycast/

or my favorite: http://www.net.cmu.edu/pres/lisa03/

This excellent presentation will help you with your understanding:

"In configuring multiple hosts to respond to the same address, stateless protocols such as DNS can be easily scaled. Servers can be located in closer proximity to clients, providing faster responses to queries. In the event of a single host failure, routes can quickly be withdrawn and servers in other locations handle the request traffic, all without any changes to client configurations.

Recursive DNS clients built into many of today's operating systems deal rather poorly with a failure of their primary recursive server. Of eight operating systems evaluated in a recent survey, seven kept no history of failed servers, trying each DNS query against the first server and waiting for a response before moving to secondary servers. Using anycast, service is maintained even in the face of a single or multiple host failure. This substantially reduces resolution delays due to server failure."

Peter Hill


On Apr 14, 2005, at 11:24 AM, Daniel Senie wrote:

At 02:00 PM 4/14/2005, Peter John Hill wrote:
I have completely given up on relying on Comcast for dns service... For now I will continue to use them for "transit"

If they are unwilling to implement anycast dns then I cannot trust them...
It's unclear why anycast would be required. Most or all of their customers use DHCP to obtain address information, including DNS information. It would be just as reasonable for them to install a few small DNS servers along-side the router at the cable head-end at every town. Now it might be simpler for them to manage if they placed those same servers but used Anycast, but the effect should be the same.

The point is, anycast is not the issue. Reliable service is the issue. DNS isn't their only issue, of course (that they're single-homed to AT&T adds to their unreliability, not that they can fix that at present).

Dan