Network steganography is an emerging research field, which exploits packet protocol headers, protocol mechanism, packet payload, etc. to establish secret communication between two parties over a computer network. The detection of such hidden communication has not been part of intrusion detection systems that are primarily used to detect malicious activities such as viruses and malwares. In this paper, we propose two packet-length based steganographic techniques to implement a covert channel. We examine and analyze the packet lengths of normal traffic to show that our techniques can cope with traffic anomaly detection methods and does not introduce noticeable traffic overhead.