Defending Against DDoS: Attacks

As we discussed in our Introduction to Defending Against Network-based Distributed Denial of Service Attacks, DDoS is a blunt force instrument for many adversaries. So organizations need to remain vigilant against these attacks. There is not much elegance in a volumetric attack – adversaries impact network availability by consuming all the bandwidth into a site and/or by knocking down network and security devices, overwhelming their ability to handle the traffic onslaught. Today’s traditional network and security devices (routers, firewalls, IPS, etc.) were not designed to handle these attacks. Nor were network architectures built to easily decipher attack traffic and keep legitimate traffic flowing. So an additional layer of products and services has emerged to protect networks from DDoS attacks. But first things first. Before we dig into ways to deal with these attacks let’s understand the types of attacks and how attackers assemble resources to blast networks to virtual oblivion. The Attacks The first category of DDoS attacks is the straightforward flood. Attackers use tools that send requests using specific protocols or packets (SYN, ICMP, UDP, and NTP are the most popular) but don’t acknowledge the responses. If enough attack computers send requests to a site, its bandwidth can quickly be exhausted. Even if bandwidth is sufficient, on-site network and security devices need to maintain session state while continuing to handle additional (legitimate) inbound session requests. Despite the simplicity of the problem floods continue to be a very effective tactic for overwhelming targets. Increasingly we see the DNS infrastructure targeted by DDoS attacks. This prevents the network from successfully routing traffic from point A to point B, because the map is gone. As with floods, attackers can overwhelm the DNS by blasting it with traffic, especially because DNS infrastructure has not scaled to keep pace with overall Internet traffic growth. DNS has other frailties which make it an easy target for DDoS. Like the shopping cart and search attacks we highlighted for Application DoS, legitimate DNS queries can also overwhelm the DNS service and knock down a site. The attacks target weaknesses in the DNS system, where a single request for resolution can trigger 4-5 additional DNS requests. This leverage can overwhelm domain name servers. We will dig into magnification tactics later in this series. Similarly, attackers may request addresses for hosts that do not exist, causing the targeted servers to waste resources passing on the requests and polluting caches with garbage to further impair performance. Finally, HTTP continues to be a popular target for floods and other application-oriented attacks, taking advantage of the inherent protocol weaknesses. We discussed slow HTTP attacks in our discussion of Application Denial of Service, so we won’t rehash the details here, but any remediations for volumetric attacks should alleviate slow HTTP attacks as well. Assembling the Army To launch a volumetric attack an adversary needs devices across the Internet to pound the victim with traffic. Where do these devices come from? If you were playing Jeopardy the correct response would be “What is a bot network, Alex?” Consumer devices continue to be compromised and monetized at an increasing rate, driving by increasingly sophisticated malware and the lack of innovation in consumer endpoint protection. These compromised devices generate the bulk of DDoS traffic. Of course attackers need to careful – Internet Service Providers are increasingly sensitive to consumer devices streaming huge amounts of traffic at arbitrary sites, and take devices off the network when they find violations of their terms of service. Bot masters use increasingly sophisticated algorithms to control their compromised devices, to protect them from detection and remediation. Another limitation of consumer devices is their limited bandwidth, particularly upstream. Bandwidth continues to grow around the world, but DDoS attackers hit capacity constraints. DDoS attackers like to work around these limitations of consumer devices by instead compromising servers to blast targets. Given the millions of businesses with vulnerable Internet-facing devices, it tends to be unfortunately trivial for attackers to compromise some. Servers tend to have much higher upstream bandwidth so they are better at serving up malware, commanding and controlling bot nodes, and launching direct attacks. Attackers are currently moving a step beyond conventional servers, capitalizing on cloud services to change their economics. Cloud servers – particularly Infrastructure as a Service (IaaS) servers are inherently Internet-facing and often poorly configured. And of course cloud servers have substantial bandwidth. For network attacks, a cloud server is like a conventional server on steroids – DDoS attackers see major gains in both efficiency and leverage. To be fair, the better-established cloud providers take great pains to identify compromised devices and notify customers when they notice something remiss. You can check out Rich’s story for how Amazon proactively notified us of a different kind of issue, but they do watch for traffic patterns that indicate misuse. Unfortunately by the time misuse is detected by a cloud provider, server owner, or other server host, it may be too late. It doesn’t take long to knock a site offline. And attackers without the resources or desire to assemble and manage botnets can just rent them. Yes, a number of folks offer DDoS as a service (DDoSaaS, for the acronym hounds), so it couldn’t be easier for attackers to harness the resources to knock down a victim. And it’s not expensive according to McAfee, which recorded DDoS costs from $2/hour for short attacks, up to $1,000 to take a site down for a month. It is a bit scary to think you could knock down someone’s site for 4 hours for less than a cup of coffee. But when you take a step back and consider the easy availability of compromised devices, servers, and cloud servers, DDoS is a very easy service to add to an attacker’s arsenal. Our next post will discuss tactics for magnifying the impact of a DDoS attack – including encryption and reflection – to make attacks an order of magnitude more effective. Share:

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