In order to protect user privacy or guarantee free access to the Internet, the network covert channel has become a hot research topic. It refers to an information channel in which the messages are covertly transmitted under the network environment. In recent years, many new construction schemes of network covert channels are proposed. But at the same time, network covert channel has also received the attention of censors, leading to many attacks. The network covert channel refers to an information channel in which the messages are covertly transmitted under the network environment. Many users exploit the network covert channel to protect privacy or guarantee free access to the Internet. Previous construction schemes of the network covert channel are based on information steganography, which can be divided into CTCs and CSCs. In recent years, there are some covert channels constructed by changing the transmission network architecture. On the other side, some research work promises that the characteristics of emerging network may better fit the construction of the network covert channel. In addition, the covert channel can also be constructed by changing the transmission network architecture. The proxy and anonymity communication technology implement this construction scheme. In this paper, we divide the key technologies for constructing network covert channels into two aspects: communication content level (based on information steganography) and transmission network level (based on proxy and anonymity communication technology). We give an comprehensively summary about covert channels at each level. We also introduce work for the three new types of network covert channels (covert channels based on streaming media, covert channels based on blockchain, and covert channels based on IPv6). In addition, we present the attacks against the network covert channel, including elimination, limitation, and detection. Finally, the challenge and future research trend in this field are discussed.

中文翻译：

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网络隐蔽通道建设关键技术综述

为了保护用户隐私或保证免费访问互联网，网络秘密渠道已成为研究的热点。它是指在网络环境下秘密传输消息的信息通道。近年来，提出了许多新的网络隐蔽通道建设方案。但是同时，网络秘密通道也受到了审查员的注意，导致了许多攻击。网络秘密信道是指在网络环境下秘密地发送消息的信息信道。许多用户利用网络秘密渠道来保护隐私或保证免费访问Internet。网络隐蔽通道的先前构建方案是基于信息隐写技术的，可以分为CTC和CSC。最近几年，通过更改传输网络架构可以构建一些隐蔽通道。另一方面，一些研究工作承诺，新兴网络的特征可能更适合网络隐蔽通道的建设。另外，还可以通过改变传输网络架构来构造隐蔽信道。代理和匿名通信技术实现此构造方案。在本文中，我们将构建网络隐蔽通道的关键技术分为两个方面：通信内容级别（基于信息隐写）和传输网络级别（基于代理和匿名通信技术）。我们对每个级别的秘密渠道进行了全面的总结。我们还介绍了三种新型网络隐秘通道（基于流媒体的隐秘通道，基于区块链的隐秘通道和基于IPv6的隐秘通道）的工作。此外，我们还介绍了针对网络秘密通道的攻击，包括消除，限制和检测。最后，讨论了该领域的挑战和未来的研究趋势。