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Attackers, in particular botnet controllers, use stealthy messaging systems to set up large-scale command and control. To systematically understand the potential capability of attackers, we investigate the feasibility of using domain name service (DNS) as a stealthy botnet command-and-control channel. We describe and quantitatively analyze several techniques that can be used to effectively hide malicious DNS activities at the network level. Our experimental evaluation makes use of a two-month-long 4.6-GB campus network data set and 1 million domain names obtained from alexa.com. We conclude that the DNS-based stealthy command-and-control channel (in particular, the codeword mode) can be very powerful for attackers, showing the need for further research by defenders in this direction. The statistical analysis of DNS payload as a countermeasure has practical limitations inhibiting its large-scale deployment.