A botnet sinkhole is a target machine used by researchers to gather information about a particular botnet.
Sinkholing is the redirection of traffic from its original destination to one specified by the sinkhole owners. The altered destination is known as the sinkhole. (The name is a reference to a physical sinkhole, into which items apparently disappear.)
In a centralized botnet, sinkholing is straightforward. The discovery of a C&C (command and control) server makes it possible to redirect DNS requests for that server to a law enforcement computer or other analyzing machine. The specially configured DNS server can simply route the requests of the bots to a faked C&C server, where the requests provide information to researchers about the nature of the botnet. To establish this type of botnet sinkhole, researchers need the cooperation of the owner of the DNS used by the botnet, as well as knowledge of the botnet and its C&C server.
Since there is no C&C server in a decentralized or P2P botnet (peer-to-peer botnet), the researcher has to detect its method of picking up owner commands before any attempt can be made to block or analyze the botnet's communication.