  Smart DNS (SDNS) services advertise access to "geofenced" content (typically,
video streaming sites such as Netflix or Hulu) that is normally inaccessible
unless the client is within a prescribed geographic region. SDNS is simple to
use and involves no software installation. Instead, it requires only that users
modify their DNS settings to point to an SDNS resolver. The SDNS resolver
"smartly" identifies geofenced domains and, in lieu of their proper DNS
resolutions, returns IP addresses of proxy servers located within the geofence.
These servers then transparently proxy traffic between the users and their
intended destinations, allowing for the bypass of these geographic
restrictions.
  This paper presents the first academic study of SDNS services. We identify a
number of serious and pervasive privacy vulnerabilities that expose information
about the users of these systems. These include architectural weaknesses that
enable content providers to identify which requesting clients use SDNS. Worse,
we identify flaws in the design of some SDNS services that allow {\em any}
arbitrary third party to enumerate these services' users (by IP address), even
if said users are currently offline. We present mitigation strategies to these
attacks that have been adopted by at least one SDNS provider in response to our
findings.
