There have been several recent reports that botnet communication between bot-infected computers and Command and Control servers (C&C servers) using the Domain Name System (DNS) protocol has been used by many cyber attackers. In particular, botnet communication based on the DNS TXT record type has been observed in several kinds of botnet attack. Unfortunately, the DNS TXT record type has many forms of legitimate usage, such as hostname description. In this paper, in order to detect and block out botnet communication based on the DNS TXT record type, we first differentiate between legitimate and suspicious usages of the DNS TXT record type and then analyze real DNS TXT query data obtained from our campus network. We divide DNS queries sent out from an organization into three types-via-resolver, and indirect and direct outbound queriesand analyze the DNS TXT query data separately. We use a 99-day dataset for via-resolver DNS TXT queries and an 87-day dataset for indirect and direct outbound DNS TXT queries. The results of our analysis show that about 30%, 8% and 19% of DNS TXT queries in via-resolver, indirect and direct outbound queries, respectively, could be identified as suspicious DNS traffic. Based on our analysis, we also consider a comprehensive botnet detection system and have designed a prototype system.
DNS (Domain Name System) based name resolution is one of the most fundamental Internet services for both of the Internet users and Internet service providers. In normal DNS based name resolution process, the corresponding NS (Name Server) records are required prior to sending a DNS query to the authoritative DNS servers. However, in recent years, DNS based botnet communication has been observed in which botnet related network traffic is transferred via DNS queries and responses. In particular, it has been observed that, in some types of malware, DNS queries will be sent to the C&C servers using an IP address directly without obtaining the corresponding NS records in advance. In this paper, we propose a novel mechanism to detect and block abnormal DNS traffic by analyzing the achieved NS record history in intranet. In the proposed mechanism, all DNS traffic of an intranet will be captured and analyzed in order to extract the legitimate NS records and the corresponding glue A records (the IP address(es) of a name server) which will be stored in a white list database. Then all the outgoing DNS queries will be checked and those destined to the IP addresses that are not included in the white list will be blocked as abnormal DNS traffic. We have implemented a prototype system and evaluated the functionality in an SDN-based experimental network. The results showed that the prototype system worked well as we expected and accordingly we consider that the proposed mechanism is capable of detecting and blocking some specific types of abnormal DNS-based botnet communication.
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