Do You See What I See: Differential Treatment of Anonymous Users

The utility of anonymous communication is undermined by a growing number of websites treating users of such services in a degraded fashion. The second-class treatment of anonymous users ranges from outright rejection to limiting their access to a subset of the service’s functionality or imposing hurdles such as CAPTCHA-solving. To date, the observation of such practices has relied upon anecdotal reports catalogued by frustrated anonymity users. We present a study to methodically enumerate and characterize, in the context of Tor, the treatment of anonymous users as second-class Web citizens.<br>We focus on first-line blocking: at the transport layer, through reset or dropped connections; and at the application layer, through explicit blocks served from website home pages. Our study draws upon several data sources: comparisons of Internet-wide port scans from Tor exit nodes versus from control hosts; scans of the home pages of top-1,000 Alexa websites through every Tor exit; and analysis of nearly a year of historic HTTP crawls from Tor network and control hosts. We develop a methodology to distinguish censorship events from incidental failures such as those caused by packet loss or network outages, and incorporate consideration of the endemic churn in web-accessible services over both time and geographic diversity. We find clear evidence of Tor blocking on the Web, including 3.67% of the top-1,000 Alexa sites. Some blocks specifically target Tor, while others result from fate-sharing when abuse-based automated blockers trigger due to misbehaving Web sessions sharing the same exit node.

匿名通信的效用正受到越来越多网站的削弱，这些网站以降级方式对待此类服务的用户。匿名用户所遭受的二等对待从直接拒绝不等，包括限制其访问服务功能子集，或设置验证码破解等障碍。迄今为止，对此类做法的观察依赖于受挫匿名用户记录的轶事报告。我们提出一项研究，旨在Tor（洋葱路由）语境下，系统地枚举并描述匿名用户作为网络二等公民的待遇。<br>我们聚焦于一线拦截：传输层通过重置或丢弃连接实现拦截；应用层则通过网站主页提供的明确拦截。本研究利用多类数据源：对比Tor出口节点与控制主机的全网端口扫描结果；通过每个Tor出口节点扫描Alexa排名前1000的网站主页；分析Tor网络与控制主机近一年的历史HTTP爬取数据。我们开发了一种方法，以区分审查事件与偶发故障（如丢包或网络中断所致故障），并纳入对网络可访问服务在时间及地理多样性维度普遍变动的考量。研究发现网络中存在明确的Tor拦截证据，涉及Alexa前1000网站中的3.67%。部分拦截专门针对Tor，另一些则源于命运共享（fate-sharing）——当共享同一出口节点的异常网络会话触发基于滥用行为的自动拦截器时，此类连带拦截便会发生。