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How to detect DDoS attacks
DDoS attacks are on the rise -- again. While they usually strike without warning, there are some red flags to be aware of. Rapid detection is key to surviving such an attack.
DDoS attacks often arrive suddenly and unexpectedly. One minute, a website is actively processing client requests, and the next, it's not, with users getting 503 Service Unavailable error response messages or the dreaded loading spinner.
Without proper monitoring in place, site admins might only become aware there is a problem when users start to complain. And, unless a malicious actor has sent extortion demands threatening to launch an attack, it might not even be clear at first why a site is unreachable. Consider, for example, a mention in a news article or a product featured in a video that has gone viral or been commented on by an influencer.
It's, therefore, important to establish whether an outage is due to heavier-than-normal legitimate traffic or a flood of malicious traffic in a DDoS attack.
Indicators of DDoS attacks
The following could indicate a DDoS attack is occurring:
- A single IP or range of IP addresses making excessive and consecutive requests.
- Heavy traffic from a single geographical location or device.
- Unusual traffic patterns.
- Service persistently responding with 500 Internal Server Error or 503 Server Unavailable error messages indicating it is unavailable or unable to handle requests.
- Alerts about bandwidth, memory or CPU issues.
- Packet TTLs (time-to-live) expiring due to an attack consuming excessive bandwidth.
DDoS attacks can target different Open Systems Interconnection (OSI) layers, but Layers 3, 4 and 7 are the most popular because they are relatively easy to launch and can potentially have an enormous impact.
Multivector DDoS attacks target multiple layers of the OSI model at the same time. For example, a multivector DDoS attack might include a DNS amplification attack that targets Layers 3 and 4, as well as an HTTP flood that targets Layer 7.
The five most common DDoS attack vectors for Q2 2024, according to Cloudflare research, were DNS, SYN, RST, User Datagram Protocol and Generic Routing Encapsulation.
How to detect DDoS attacks
Detecting the aforementioned signs quickly and accurately is key to mitigating DDoS attacks. It is vital to build automated DDoS detection methods into cloud and on-premises infrastructure so preventative measures can be taken immediately and before excessive damage is done.
Two methods to detect DDoS attacks are inline packet inspection and out-of-band detection via traffic flow analysis. Both can be deployed on-premises or via the cloud.
Inline packet examination tools sit in front of an IT infrastructure and monitor all traffic. Devices such as load balancers, firewalls and intrusion prevention systems can provide inline detection and mitigation. These tools, however, are easily overwhelmed by today's hypervolumetric attacks. It is better to deploy dedicated inline packet examination DDoS mitigation appliances that use machine learning to spot abnormal traffic and activity. As soon as a DDoS attack is detected, dedicated DDoS mitigation tools adjust volumetric and protocol protection configurations to filter out malicious traffic. Note, however, this runs the danger of false positives and blocking genuine requests. Inspecting every data packet also causes increased latency.
Out-of-band tools overcome the difficulties of deep packet inspection at scale and unwanted false alarms. These tools passively analyze flow data from NetFlow, J-Flow, sFlow and IP Flow Information Export-enabled routers and switches to detect attacks. Although they can't automatically adjust protection configurations, they can send alerts or automatically trigger steps to mitigate the attack via routing traffic to a centralized data cleansing station that filters legitimate traffic.
Michael Cobb, CISSP-ISSAP, is a renowned security author with more than 20 years of experience in the IT industry.
