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Because the speed of DNS is so important to the performance of any connection on the ‘net, a lot of thought goes into making DNS servers fast, including optimized software that can respond to queries in milliseconds, and connecting DNS servers to the ‘net through high bandwidth links. To set the stage for massive DDoS attacks based in the DNS system, add a third point: DNS responses tend to be much larger than DNS queries. In fact, a carefully DNS response can be many times larger than the query.
To use a DNS server as an amplifier in a DDoS attack, then, the attacker sends a query to some number of publicly accessible DNS servers. The source of this query is the address of the system to be attacked. If the DNS query is carefully crafted, the attacker can send small packets that cause a number of DNS servers to send large responses to a single IP address, causing large amounts of traffic to the system under attack.
Rami Al-Dalky, Michael Rabinovich, and Mark Allman. 2018. Practical Challenge-Response for DNS. In Proceedings of the Applied Networking Research Workshop (ANRW ’18). ACM, New York, NY, USA, 74-74. DOI: https://doi.org/10.1145/3232755. Continue reading