When the domain name system (DNS) was first designed, security was an afterthought. Threats simply weren't a consideration at a time when merely carrying out a function — routing Internet users to websites — was the core objective. As the weaknesses of the protocol became evident, engineers began to apply a patchwork of fixes. After several decades, it is now apparent that this reactive approach to DNS security has caused some unintended consequences and challenges.
Steps to Secure DNS Data
The first improvements to DNS security aimed to make the data safer, such as early efforts to add more information about data sources. Later, DNSSEC (DNS security extensions) was developed to make forging data much more difficult. One unintended consequence, which wasn't a concern at the time, responses to DNS queries now contains more data than ever before. The gain in response size, versus query size, became considerable.
Steps to Secure the System
While initial steps were being taken to secure DNS, the system gained greater capacity. That is, it was over-provisioned to secure availability and resiliency. The cost of computers and bandwidth fell, enabling organizations to deploy more and more servers, plus the servers themselves were becoming more powerful. This helped to prevent servers from being flooded with queries, both from legitimate users and illegitimate users launching DDoS attacks.
Progress was being made. But…
The Solutions Become a Problem
Recently, the DDoS attackers have capitalized on the way DNSSEC amplifies the size of query responses and has used the incredible capacity to focus packet floods on targets. In these attacks, which have been labeled reflection attacks, a computer sends a small DNS query to a server that responds with a large DNSSEC-fueled response, which is not sent to the computer but instead to the falsified return address. This process is repeated to many DNS servers in parallel, resulting in a traffic pattern that is barely noticable by the DNS servers but has severe consequences for the victim. The victim's servers are overwhelmed and websites go offline, causing revenues, customer service and brand reputations to suffer.
No Good Deed Goes Unpunished
To recap, while the DNS has been armed for security, it has also become a global, high-capacity, very reliable utility for generating attack traffic. Much like an electric utility generates power for cities; the DNS can generate immense amounts of traffic to flood victims.
So, What's Next?
We can't undo the improvements to DNS security. If we did, the DNS would once again be an untrustworthy and unreliable source for data. To move forward, we must figure out a way to eliminate reflection attacks and continue on with our security strategies. Or rethink these strategies overall.
On the horizon: expanding the role TCP (transmission control protocol, one of the core protocols of the Internet) plays in the DNS. Historically, expansion has been a taboo subject because TCP doesn't make much sense for DNS, but it removes the effectiveness of reflection attacks, for the most part. While there are reasons not to utilize TCP — another topic for another blog post — the fact is that DNS is already defined to operate over TCP, despite years of building devices that assume it doesn't. While this is a subject to experiment with, it's not a certain solution.
Other options exist, but it is too early to even begin to describe them. New topologies and new arrangements are in the works. Other changes to the protocol are being considered. Where we go next is anyone's guess.
By Edward Lewis, Director and Member of the Technical Staff at Neustar
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