Covert channels are communication channels that allow secret transfer of information between two malicious processes by modifying the value or modulating the timing behavior of shared resources. Shared resources in covert communications vary according to the underlying covert channels. Initially, covert storage channels and covert timing channels are widely existed in information systems. More recently, the focus has shifted towards three new kinds of covert channels, namely, covert hybrid channels, covert behavior channels, and air-gap covert channels. This study surveys existing techniques for constructing covert channels that have been reported in literature, especially the covert channels that are presented in recent years. First, the definition, history, and key elements of covert channels are introduced. Covert channel analysis is also included. Second, a categorization technique is proposed for these covert channels based on the shared resources and channel characteristics. The traditional and new covert channel attack techniques are systematically analyzed based on the seven key elements of the covert channels. Third, the countermeasures for covert channels aforementioned are also demonstrated to restrict the threat brought by covert channels and to provide guidelines for future works. Finally, the challenges and problems on covert channels are provided.