Abstract. DNS amplification attacks massively exploit open recursive DNS servers mainly for performing bandwidth consumption DDoS attacks. The amplification effect lies in the fact that DNS response messages may be substantially larger than DNS query messages. In this paper, we present and evaluate a novel and practical method that is able to distinguish between authentic and bogus DNS replies. The proposed scheme can effectively protect local DNS servers acting both proactively and reactively. Our analysis and the corresponding real-usage experimental results demonstrate that the proposed scheme offers a flexible, robust and effective solution. Keywords: DNS Security; Denial of Service; DNS Amplification Attacks; Detection and repelling mechanisms.