The Domain Name System (DNS) was originally designed with the assumption that the DNS will return the same answer to any given query regardless of who may have issued the query, and that all data in the DNS is thus visible. Such an assumption can no longer be justified for private Internet hosts, particularly mobile laptops and PDAs. IP addresses in the DNS reveal a host's geographic location and corporate affiliation to anyone that is interested without the host's knowledge or consent. This paper identifies an attack that allows anyone on the Internet to covertly monitor mobile devices to construct detailed profiles including user identity, daily commute patterns, and travel itineraries. We identify a growing number of users vulnerable to this attack (two million and climbing), and covertly monitor over one hundred thousand of them. We demonstrate the feasibility and severity of such an attack in today's Internet. We further propose shortterm and long-term defenses for the attack.