Defending Against DDoS: Magnification
As mentioned in our last post, the predominant mechanism of network-based DDoS attacks involves flooding the pipes with standard protocols like SYN, ICMP, DNS, and NTP. But that’s not enough, so attackers now take advantage of weaknesses in the protocols to magnify the impact of their floods by an order of magnitude. This makes each compromised device far more efficient as an attack device and allows attackers to scale attacks over 400gbps (as recently reported by CloudFlare). Only a handful of organizations in the world can handle an attack of that magnitude, so DDoS + reflection + amplification is a potent combination. Fat Packets Attackers increasingly tune the size of their packets to their desired outcome. For example simple SYN packets can crush the compute capabilities of network/security devices. Combining small SYNs with larger SYN packets can also saturate the network pipe, so we see often them combined in today’s DDoS attacks. Reflection + Amplification The first technique used to magnify a DDoS attack is reflection. This entails sending requests to a large number of devices (think millions), spoofing the origination IP address of a target site. The replies to those millions of requests are reflected back to the target. The UDP-based protocols used in reflection attacks don’t require handshaking to establish new sessions, so they are spoofable. The latest wave of DDoS attacks uses reflected DNS and NTP traffic to dramatically scale the volume of traffic hitting targets. Why those two protocols? Because they provide good leverage for amplifying attacks – DNS and NTP responses are typically much bigger than requests. DNS can provide about 50x amplification because responses are that much larger than requests. And the number of open DNS resolvers which respond to any DNS request from any device make this an easy and scalable attack. Until the major ISPs get rid of these open resolvers DNS-based DDoS attacks will continue. NTP has recently become a DDoS protocol of choice because it offers almost 200x magnification. This is thanks to a protocol feature: clients can request a list of the last 600 IP addresses to access a server. To illustrate the magnitude of magnification, the CloudFlare folks reported that attack used 4,529 NTP servers, running on 1,298 different networks, each sending about 87mbps to the victim. The resulting traffic totaled about 400gbps. Even more troubling is that all those requests (to 4,500+ NTP servers) could be sent from one device on one network. Even better, other UDP-based protocols offers even greater levels of amplification. An SNMP response can be 650x the size of a request, which could theoretically be weaponized to create 1gbps+ DDoS attacks. Awesome. Stacking Attacks Of course none of these techniques existing a vacuum, so sometimes we will see them pounding a target directly, while other times attackers combine reflection and amplification to hammer a target. All the tactics in our Attacks post are in play, and taken to a new level with magnification. The underlying issue is that these attacks are enabled by sloppy network hygiene on the part of Internet service providers, who allow spoofed IP addresses for these protocols and don’t block flood attacks. These issues are largely beyond the control of a typical enterprise target, leaving victims with little option but to respond with a bigger pipe to absorb the attack. We will wrap up tomorrow, with look at the options for mitigating these attacks. Share:
