Posts Tagged 'Open Resolvers'

November 2, 2012

The Trouble with Open DNS Resolvers

In the last couple of days, there's been a bit of buzz about "open DNS resolvers" and DNS amplification DDoS attacks, and SoftLayer's name has been brought up a few times. In a blog post on October 30, CloudFlare explained DNS Amplification DDoS attacks and reported the geographic and network sources of open DNS resolvers that were contributing to a 20Gbps attack on their network. SoftLayer's AS numbers (SOFTLAYER and the legacy THEPLANET-AS number) show up on the top ten "worst offenders" list, and Dan Goodin contacted us to get a comment for a follow-up piece on Ars Technica — Meet the network operators helping to fuel the spike in big DDoS attacks.

While the content of that article is less sensationalized than the title, there are still a few gaps to fill about when it comes to how SoftLayer is actually involved in the big picture (*SPOILER ALERT* We aren't "helping to fuel the spike in big DDoS attacks"). The CloudFlare blog and the Ars Technica post presuppose that the presence of open recursive DNS resolvers is a sign of negligence on the part of the network provider at best and maliciousness at worst, and that's not the case.

The majority of SoftLayer's infrastructure is made up of self-managed dedicated and cloud servers. Customers who rent those servers on a monthly basis have unrestricted access to operate their servers in any way they'd like as long as that activity meets our acceptable use policy. Some of our largest customers are hosting resellers who provide that control to their customers who can then provide that control to their own customers. And if 23 million hostnames reside on the SoftLayer network, you can bet that we've got a lot of users hosting their DNS on SoftLayer infrastructure. Unfortunately, it's easier for those customers and customers-of-customers and customers-of-customers-of-customers to use "defaults" instead of looking for, learning and implementing "best practices."

It's all too common to find those DNS resolvers open and ultimately vulnerable to DNS amplification attacks, and whenever our team is alerted to that vulnerability on our network, we make our customers aware of it. In turn, they may pass the word down the customer-of-customer chain to get to the DNS owner. It's usually not a philosophical question about whether DNS resolvers should be open for the greater good of the Internet ... It's a question of whether the DNS owner has any idea that their "configuration" is vulnerable to be abused in this way.

SoftLayer's network operations, abuse and support teams have tools that flag irregular and potentially abusive traffic coming from any server on our network, and we take immediate action when we find a problem or are alerted to one by someone who sends details to The challenge we run into is that flagging obvious abusive behavior from an active DNS server is a bit of a cat-and-mouse game ... Attackers cloak their activity in normal traffic. Instead of sending a huge amount of traffic from a single domain, they send a marginal amount of traffic from a large number of machines, and the "abusive" traffic is nearly impossible for even the DNS owner to differentiate from "regular" traffic.

CloudFlare effectively became a honeypot, and they caught a distributed DNS amplification DoS attack. The results they gathered are extremely valuable to teams like mine at SoftLayer, so if they go the next step to actively contact the abuse channel for each of the network providers in their list, I hope that each of the other providers will jump on that information as I know my team will.

If you have a DNS server on the SoftLayer network, and you're not sure whether it's configured to prevent it from being used for these types of attacks, our support team is happy to help you out. For those of you interested in doing a little DNS homework to learn more, Google's Developer Network has an awesome overview of DNS security threats and mitigations which gives an overview of potential attacks and preventative measures you can take. If you're just looking for an easy way to close an open recursor, scroll to the bottom of CloudFlare's post, and follow their quick guide.

If, on the other hand, you have your own DNS server and you don't want to worry about all of this configuration or administration, SoftLayer operates private DNS resolvers that are limited to our announced IP space. Feel free to use ours instead!


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