  Mobile devices that connect to the Internet via cellular networks are rapidly
becoming the primary medium for accessing Web content. Cellular service
providers (CSPs) commonly deploy Web proxies and other middleboxes for
security, performance optimization and traffic engineering reasons. However,
the prevalence and policies of these Web proxies are generally opaque to users
and difficult to measure without privileged access to devices and servers. In
this paper, we present a methodology to detect the presence of Web proxies
without requiring access to low-level packet traces on a device, nor access to
servers being contacted. We demonstrate the viability of this technique using
controlled experiments, and present the results of running our approach on
several production networks and popular Web sites. Next, we characterize the
behaviors of these Web proxies, including caching, redirecting, and content
rewriting. Our analysis can identify how Web proxies impact network
performance, and inform policies for future deployments. Last, we release an
Android app called Proxy Detector on the Google Play Store, allowing average
users with unprivileged (non-rooted) devices to understand Web proxy
deployments and contribute to our IRB-approved study. We report on results of
using this app on 11 popular carriers from the US, Canada, Austria, and China.
