Saturday, 1 November 2014

How to Launch a 65Gbps DDoS, and How to Stop One

Yesterday I posted a post mortem on an outage we had Saturday. The outage was caused when we applied an overly aggressive rate limit to traffic on our network while battling a determined DDoS attacker. In the process of writing it I mentioned that we'd seen a 65Gbps DDoS earlier on Saturday. I've received several questions since that all go something like: "65Gbps DDoS!? Who launches such an attack and how do you defend yourself against it?!" So I thought I'd give a bit more detail.
What Constitutes a Big DDoS?
A 65Gbps DDoS is a big attack, easily in the top 5% of the biggest attacks we see. The graph below shows the volume of the attack hitting our EU data centers (the green line represents inbound traffic). When an attack is 65Gbps that means every second 65 Gigabits of data is sent to our network. That's the equivalent data volume of watching 3,400 HD TV channels all at the same time. It's a ton of data. Most network connections are measured in 100Mbps, 1Gbps or 10Gbps so attacks like this would quickly saturate even a large Internet connection.
At CloudFlare, an attack needs to get over about 5Gbps to set off alarms with our ops team. Even then, our automated network defenses usually stop attacks without the need of any manual intervention. When an attack gets up in the tens of Gigabits of data per second, our ops team starts monitoring the attack: applying filters and shifting traffic to ensure the attacked customer's site stays online and none of the rest of our network is affected.
So You Want to Launch a DDoS
So how does an attacker generate 65Gbps of traffic? It is highly unlikely that the attacker has a single machine with a big enough Internet connection to generate that much traffic on its own. One way to generate that much traffic is through a botnet. A botnet is a collection of PCs that have been compromised with a virus and can be controlled by what is known as a botnet herder.
Botnet herders will often rent out access to their botnets, often billing in 15 minute increments (just like lawyers). Rental prices depend on the size of the botnets. Traditionally, email spammers purchased time on botnets in order to send their messages to appear to come from a large number of sources. As email spam has become less profitable with the rise of better spam filters, botnet herders have increasingly turned to renting out their networks of compromised machines to attackers wanting to launch a DDoS attack.
To launch a 65Gbps attack, you'd need a botnet with at least 65,000 compromised machines each capable of sending 1Mbps of upstream data. Given that many of these compromised computers are in the developing world where connections are slower, and many of the machines that make up part of a botnet may not be online at any given time, the actual size of the botnet necessary to launch that attack would likely need to be at least 10x that size. While by no means unheard of, that's a large botnet and using all its resources to launch a DDoS risks ISPs detecting many of the compromised machines and taking them offline.
Amplifying the Attacks
Since renting a large botnet can be expensive and unwieldy, attackers typically look for additional ways to amplify the size of their attacks. The attack on Saturday used one such amplification technique called DNS reflection. To understand how these work, you need to understand a bit about how DNS works.
When you first sign up for an Internet connection, your ISP will provide you with a recursive DNS server, also known as a DNS resolver. When you click on a link, your computer sends a lookup to your ISP's DNS resolver. The lookup is asking a question, like: what is the IP address of the server for If the DNS resolver you query knows the answer, because someone has already asked it recently and the answer is cached, it responds. If it doesn't, it passes the request on to the authoritative DNS for the domain.
Typically, an ISP's DNS resolvers are setup to only answer requests from the ISP's clients. Unfortunately, there are a large number of misconfigured DNS resolvers that will accept queries from anyone on the Internet. These are known as "open resolvers" and they are a sort of latent landmine on the Internet just waiting to explode when misused.
DNS queries are usually sent via the UDP protocol. UDP is a fire-and-forget protocol, meaning that there is no handshake to establish that where a packet says it is coming from actually is where it is coming from. This means, if you're an attacker, you can forge the header of a UDP packet to say it is coming from a particular IP you want to attack and send that forged packet to an open DNS resolver. The DNS resolver will reply back with a response to the forged IP address with an answer to whatever question was asked.
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