In this case, 3000 bytes / 50 bytes = 60x amplification We can calculate the amplification factor as follows: Bytes Out Per Request / Bytes In Per Request = Amplification Factor.The packet hits the open cache resolver and that server responds with a response packet that is much larger – for sake of ease in calculations, we will go with 3000 bytes (the actual size in 3223 bytes).The outbound packet is approximately 50 bytes (the actual size is 64 bytes).

The pack is sent to a known open cache resolver.The malware spoofs the IP address in the request to be that of the target victim.The botnet malware generates a simple DNS request for ANY record for isc.org (the first Amplification Attack domain used). A single end point – commonly a compromised computer commandeered and participating in a botnet.Outgoing payloads can vary in size from very small (around 100-200 bytes) for an A or CNAME record to up to 4000 or more bytes for a large ANY or TXT record. (I will address the security implications of DNS & TCP in a future post.)Ī typical DNS incoming payload is between 50 and 100 bytes. But the introduction of EDNS(0) extensions and subsequent adoption of these standards means the DNS payload packet size can reach up to a maximum 4096 bytes before fragmentation and subsequent retry via TCP. DNS request packet payload size used to be limited to 512 bytes in size, according to the RFC. There is no connection handshake, making it extremely easy to spoof the source IP address in the incoming packets. UDP connections are stateless and essentially a best-effort protocol. Remember, DSN is an application-layer protocol that makes use of UDP connections primarily. Either way, you don’t want to be in that position. Technically reflective distributed denial of service attacks (rDDoS), these attacks can manifest themselves in two ways: either you are the target or the middle man participating in an attack on another network. The most prevalent type of attack we see is DNS Amplification. I presented a talk at BSides Las Vegas on the topic and wanted to take some time to delve into more technical details regarding some of the attacks we have seen, as well as review some mitigation strategies. In a prior post, I gave a broad overview of some of the challenges we face in securing unrestricted DNS resolvers.

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