A network covert channel violates security rules and leaks information imperceptibly. Many researchers have been studying how to construct it, and the basic methods involve exploiting packets head, packets sorting and packets timing, etc. To construct a more secret covert channel, this paper proposes a novel approach based on packet classification. With this method, covert information is encoded by modulating the varieties of packets on the Internet. The basic idea of packet classification,

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... ce of carrier and algorithm of information hiding based on packet classification are discussed. Our analysis demonstrate that the covert channels based on packet classification can not be totally eliminated with current methods.