Counteract DNS Attacks on SIP Proxies Using Bloom Filters

SIP proxies play an important part in VoIP services. A Denial of Service (DoS) attack on them may cause the failure of the whole network. We investigate such a DoS attack by exploiting DNS queries. A SIP proxy needs to resolve domain names for processing a message. However, a DNS resolution may take a while. To avoid being blocked, a proxy suspends the processing task of the current message during its name resolution, so that it can continue to deal with other messages. Later when the answer is received, the suspended task will be resumed. It is an asynchronous implementation of DNS queries. Unfortunately, this implementation consumes memory storage and also brings troubles like a race condition. An attacker can collect a list of domain names which take seconds to resolve. Then, the attacker sends to a victim SIP proxy messages which contain these domain names. As a result, the victim proxy has to suspend a number of messages in a short while. Our experiments show that a SIP proxy can be easily crashed by such an attack and thus be not available anymore. To solve the problem, we analyzed the reasons that make a DNS query time-consuming, and then proposed a prevention scheme using bloom filters to blacklist suspicious DNS authoritative servers. Results of our experiments show it efficiently mitigates the attack with a reasonable false positive rate.