What Is a Distributed Denial-of-Service Attack and How Did It Break Twitter?

A close-up image of a finger pushing a red key titled "DDoS," which stands for distributed denial-of-service attacks, on a white keyboard.
Photo: iStockphoto

On Friday, multiple distributed denial-of-service (DDoS) attacks hit the Internet services company Dyn. The cyberattack prevented many users on the U.S. East Coast from navigating to the most popular websites of Dyn customers, which include Twitter, Reddit, and Netflix.

Dyn detected the first attack at 7:10 a.m. Eastern time on Friday and restored normal service about two hours later. Then at 11:52 a.m. ET, Dyn began investigating a second attack. By 2:00 p.m., the company said it was still working to resolve “several attacks” at once.

The interruptions inconvenienced many Internet users, and the daily operation of Internet giants in entertainment, e-commerce, and social media. There still aren’t many details available about Dyn’s predicament, and the company did not immediately respond to an interview request. But we do know from Dyn’s posts that the first two assaults on its network were DDoS attacks. Its customers’ outages again show that major Internet companies remain vulnerable to this common hacker scheme—one that has plagued networks since 2000.

A denial-of-service attack aims to slow or stop users from accessing content or services by impeding the ability of a network or server to respond to their requests. The word “distributed” means that hackers executed the Dyn attacks by infecting and controlling a large network of computers called a botnet, rather than running it from a single machine that they own.

Hackers can assemble a botnet by spreading malware, which is often done by prompting unsuspecting users to click a link or download a file. That malware can be programmed to periodically check with a host computer owned by hackers for further instructions. To launch an attack, the hackers, or bot-herders, send a message through this “command and control” channel, prompting infected computers to send many requests for a particular website, server, or service all at once. Some of the biggest botnets in history have boasted 2 million computers, capable of sending up to 74 billion spam emails a day.

The sudden onslaught of requests quickly gobbles up all the network's bandwidth, disk space, or processing power. That means real users can’t get their requests through because the system is too busy trying to respond to all the bots. In the worst cases, a DDoS can crash a system, taking it completely offline.

Both of Friday’s attacks targeted Dyn’s Managed Domain Name System. Through this system, Dyn provides a routing service that translates Web addresses that users type into a browser, such as spectrum.ieee.org. Users who type in a Web address are first sent through a Dyn server that looks up the IP address for a server that hosts the content the user is trying to reach. The Dyn server passes this information on to the user's browser.

To disrupt this process, says Sanjay Goel, a professor of information technology at the State University of New York (SUNY) at Albany, the bot-herders probably sent tons of translation requests directly to Dyn’s servers by looking up the servers’ IP addresses. They could have also simply asked the bots to send requests for Amazon.com and Twitter.com to cause similar issues. Attacking a DNS or a content delivery provider such as Dyn or Akamai in this manner gives hackers the ability to interrupt many more companies than they could by directly attacking corporate servers, because several companies share Dyn's network.

In Dyn’s case, it has built its Managed DNS on an architecture called Anycast, in which any particular IP address for a server in its system can actually be routed through servers in more than a dozen data centers. So, if the IP address of one server is targeted, 10 others may still be able to handle the normal traffic while it's beseiged with bot requests. Art Manion, a technical manager at Carnegie Mellon University’s Software Engineering Institute, says this system should make Dyn more resilient to DDoS attacks, and the company has touted it as highly secure.

Dyn said on Friday in an update to its website that the first attack mainly impacted services in the “US East.” The Anycast network includes data centers in Washington, D.C., Miami, and Newark, N.J., as well as in Dallas and Chicago, though it’s not clear whether these locations were specifically targeted.    

Even in the affected region, only certain users experienced issues. One reason could be that other users' browsers had previously used Dyn to locate the specific server they needed to recover, say, Twitter.com. Because that information is now cached in their browsers, those users can bypass Dyn to fetch the desired content, so long as the servers that store Twitter’s website are still functioning.

Another reason for the inconsistent impacts could be that a common mechanism for handling DDoS attacks is to simply drop every fifth request from the queue in order to relieve the network of traffic. The result: Some requests from legitimate users wind up being dropped along with those from bots.

Once an attack begins, companies can bring backup servers online to manage the blizzard of requests. Victims can also work with Internet service providers to block the IP addresses of devices generating the most traffic, which means that they're likely part of the botnet. "You start blocking the different addresses where it's coming from, so depending on how massive the botnet is, it may take some time," says SUNY Albany's Goel.

Increasingly, bot-herders have recruited Internet of Things devices, which often have poor security, to their ranks. This allows them to launch ever more powerful attacks because of the sheer numbers of such devices. Two of the largest DDoS attacks on record have occurred within the past two months: first, a 620-gigabit-per-second attack targeting independent security reporter Brian Krebs of KrebsonSecurity.com, and then a 1,100-Gb/s siege on the French hosting company OVH.

Even with state-of-the-art protections and mitigation strategies, companies are limited by the amount of bandwidth they have to handle such sudden onslaughts. “Ultimately, Akamai has total x amount of bandwidth, and if the attacker is sending x-plus-10 traffic, the attacker still wins,” says Carnegie Mellon's Manion. “It mathematically favors whoever has more bandwidth or more traffic, and the attackers today can have more traffic.”

Dyn’s global network manages over 500 billion queries a month, so the culprits would have had to send many millions or even billions of requests simultaneously in order to stall it. Manion says that to prevent DDoS attacks, companies must address root causes such as poor IoT security, rather than scrambling to stop them once they’ve begun.

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