Defending Against DoS Attacks: Defense Part 1, the Network
In Attacks, we discussed both network-based and application-targeting Denial of Service (DoS) attacks. Given the radically different techniques between the types, it’s only logical that we use different defense strategies for each type. But be aware that aspects of both network-based and application-targeting DoS attacks are typically combined for maximum effect. So your DoS defenses need to be comprehensive, protecting against (aspects of) both types. Anti-DoS products and services you will consider defend against both. This post will focus on defending against network-based volumetric attacks. First the obvious: you cannot just throw bandwidth at the problem. Your adversaries likely have an unbounded number of bots at their disposal and are getting smarter at using shared virtual servers and cloud instances to magnify the amount of evil bandwidth at their disposal. So you can’t just hunker down and ride it out. They likely have a bigger cannon than you can handle. You need to figure out how to deal with a massive amount of traffic and separate good traffic from bad, while maintaining availability. Find a way to dump bad traffic before it hoses you somehow without throwing the baby (legitimate application traffic) out with the bathwater. We need to be clear about the volume we are talking about. Recent attacks have blasted upwards of 80-100gbps of network traffic at targets. Unless you run a peering point or some other network-based service, you probably don’t have that kind of inbound bandwidth. Keep in mind that even if you have big enough pipes, the weak link may be the network security devices connected to them. Successful DoS attacks frequently target network security devices and overwhelm their session management capabilities. Your huge expensive IPS might be able to handle 80gbps of traffic in ideal circumstances, but fall over due to session table overflow. Even if you could get a huge check to deploy another network security device in front of your ingress firewall to handle that much traffic, it’s probably not the right device for the job. Before you just call up your favorite anti-DoS service provider, ISP, or content delivery network (CDN) and ask them to scrub your traffic, that approach is no silver bullet either. It’s not like you can just flip a switch and have all your traffic instantly go through a scrubbing center. Redirecting traffic incurs latency, assuming you can even communicate with the scrubbing center (remember, your pipes are overwhelmed with attack traffic). Attackers choose a mix of network and application attacks based on what’s most effective in light of your mitigations. No, we aren’t only going to talk about more problems, but it’s important to keep everything in context. Security is not a problem you can ever solve – it’s about figuring out how much loss you can accept. If a few hours of downtime is fine, then you can do certain things to ensure you are back up within that timeframe. If no downtime is acceptable you will need a different approach. There are no right answers – just a series of trade-offs to manage to the availability requirements of your business, within the constraints of your funding and available expertise. Handling network-based attacks involves mixing and matching a number of different architectural constructs, involving both customer premise devices and network-based service offerings. Many vendors and service providers can mix and match between several offerings, so we don’t have a set of vendors to consider here. But the discussion illustrates how the different defenses play together to blunt an attack. Customer Premise-based Devices The first category of defenses is based around a device on the customer premises. These appliances are purpose-built to deal with DoS attacks. Before you turn your nose up at the idea of installing another box to solve such a specific problem, take another look at your perimeter. There is a reason you have all sorts of different devices. The existing devices already in your perimeter aren’t particularly well-suited to dealing with DoS attacks. As we mentioned, your IPS, firewall, and load balancers aren’t designed to manage an extreme number of sessions, nor are they particularly adept at dealing with obfuscated attack traffic which looks legitimate. Nor can other devices integrate with network providers (to automatically change network routes, which we will discuss later) – or include out-of-the-box DoS mitigation rules, dashboards, or forensics, built specifically to provide the information you need to ensure availability under duress. So a new category of DoS mitigation devices has emerged to deal with these attacks. They tend to include both optimized IPS-like rules to prevent floods and other network anomalies, and simple web application firewall capabilities which we will discuss in the next post. Additionally, we see a number of anti-DoS features such as session scalability, combined with embedded IP reputation capabilities, to discard traffic from known bots without full inspection. To understand the role of IP reputation, let’s recall how email connection management devices enabled anti-spam gateways to scale up to handle spam floods. It’s computationally expensive to fully inspect every inbound email, so dumping messages from known bad senders first enables inspection to focus on email that might be legitimate, and keeps mail flowing. The same methodology applies here. These devices should be as close to the perimeter as possible, to get rid of the maximum amount of traffic before the attack impacts anything else. Some devices can be deployed out-of-band as well, to monitor network traffic and pinpoint attacks. Obviously monitor-and-alert mode is less useful than blocking, which helps maintain availability in real time. And of course you will want a high-availability deployment – an outage due to a failed security device is likely to be even more embarrassing than simply succumbing to a DoS. But anti-DoS devices include their own limitations. First and foremost is the simple fact that if your pipes are overwhelmed, a device on your premises is irrelevant. Additionally, SSL attacks are increasing in frequency. It’s cheap for an army of bots to use SSL to encrypt all their
