Measurement of DNS Traffic Caused by DDoS Attacks

We report the measurement results of Domain Name System (DNS) traffic during the periods of DDoS attacks against a Web server. The attack was caused by virus infected machines. We monitored DNS query packets at DNS cache servers of an Japanese ISP, Open Computer Networks (OCN). We especially focused on those sent by the virus to find the IP address of the target web server. By analyzing the measurement results in detail, we found that the DNS configuration change of the authoritative DNS servers of the target site caused a significant increase in the number of queries.We also show how the DNS operators mitigated those queries by changing the configuration of DNS cache servers and authoritative servers.