This blog post is a follow-up on the outage that occurred on September 8th. Just before 8:00 PM PDT that day, we became aware of a Domain Name Service (DNS) issue that was causing a service interruption for some some Microsoft services, including Windows Live services such as Hotmail and SkyDrive. No customer data was lost or compromised during this outage. The team has investigated the root cause and has taken immediate steps to improve.
So, what happened? A tool that helps balance network traffic was being updated and the update did not work correctly. As a result, configuration settings were corrupted, which caused a service disruption.
At 10:23 PM PDT we began to see service restoration. We confirmed that the incident was resolved by 11:35 PM PDT, although it took some time for the changes to replicate around the world and reach all our customers.
We determined the cause to be a corrupted file in Microsoft’s DNS service. The file corruption was a result of two rare conditions occurring at the same time. The first condition is related to how the load balancing devices in the DNS service respond to a malformed input string (i.e., the software was unable to parse an incorrectly constructed line in the configuration file). The second condition was related to how the configuration is synchronized across the DNS service to ensure all client requests return the same response regardless of the connection location of the client. Each of these conditions was tracked to the networking device firmware used in the Microsoft DNS service.
After restoring service, we have identified two streams of work to drive specific service improvements around monitoring, problem identification, and recovery. Along with these service improvements, Microsoft is focused on further hardening the DNS service to improve its overall redundancy and fail-over capability.
We are also developing an additional recovery process that will allow a specific property the ability to fail over to restore service and then fail back when the DNS service is restored. In addition, we are reviewing the recovery tools to see if we can make more improvements that will decrease the time it takes to resolve outages.
We are determined to deliver the very best possible service to our customers and regret any inconvenience caused by this outage.
Arthur de Haan
Vice President Windows Live Test and Service Engineering
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