Amplification Hell: Revisiting Network Protocols for DDoS Abuse

Abstract—In distributed reflective denial-of-service (DRDoS) attacks, adversaries send requests to public servers (e.g., open recursive DNS resolvers) and spoof the IP address of a victim. These servers, in turn, flood the victim with valid responses and – unknowingly – exhaust its bandwidth. Recently, attackers launched DRDoS attacks with hundreds of Gb/s bandwidth of this kind. While the attack technique is well-known for a few protocols such as DNS, it is unclear if further protocols are vulnerable to similar or worse attacks. In this paper, we revisit popular UDP-based protocols of network services, online games, P2P filesharing networks and P2P botnets to assess their security against DRDoS abuse. We find that 14 protocols are susceptible to bandwidth amplification and multiply the traffic up to a factor 4670. In the worst case, attackers thus need only 0.02 % of the bandwidth that they want their victim(s) to receive, enabling far more dangerous attacks than what is known today. Worse, we identify millions of public hosts that can be abused as amplifiers. We then analyze more than 130 real-world DRDoS attacks. For this, we announce bait services to monitor their abuse and analyze darknet as well as network traffic from large ISPs. We use traffic analysis to detect both, victims and amplifiers, showing that attackers already started to abuse vulnerable protocols other than DNS. Lastly, we evaluate countermeasures against DRDoS attacks, such as preventing spoofing or hardening protocols and service configurations. We shows that carefully-crafted DRDoS attacks may evade poorly-designed rate limiting solutions. In addition, we show that some attacks evade packet-based filtering techniques, such as port-, content- or length-based filters. I.