GFW主动探测系统研究报告
英文原文来自 https://blog.torproject.org/blog/learning-more-about-gfws-active-probing...
Roya, David, Nick, nweaver, Vern, 和我刚刚完成了关于GFW主动探测系统的研究。这个系统在几年前就被用来探测翻墙工具,比如Tor。我们在之前的博文中介绍过GFW主动探测系统是如何工作的。但有几个问题我们没有回答。比如这个系统的物理结构是怎样的。那些用来主动探测的IP是归GFW所有的么? 有猜测GFW短时间内劫持了部分IP来用来主动探测,但没有证据。这次研究回答了这些问题。
因为这个实验是网络检测,我们从收集数据开始。我们创建了3个数据库,包括了GFW主动探测的小时/月份/年份的数据。这3个不同时段的数据库让我们从不同角度了解GFW主动探测的工作原理。我们公开了其中的两个数据库,你可以重复我们的实验,或者自己分析。
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通常来说,如果Tor的某个网桥代理被GFW检测并封锁,它会一直被封锁。但是这意味着网桥代理完全无法访问吗? 我们让中国的VPS一直连接我们控制的网桥代理。我们发现,每25小时,中国的VPS可以短暂的连接到我们的代理网桥。下图显示了这个现象。每个数据点表示中国的VPS试图与网桥代理建立连接。中国联通和中国教育网都有这个周期性现象。有时候,网络安全设备在更新规则时会默认允许所有流量,但我们不知道GFW周期性现象是不是因为这个原因导致的。
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我们找到了规律,GFW主动探测的TCP头暗示那几千个IP都来自与同一个地方。下图显示了数据包的初始序号和时间。每个数据点都是一个主动探测连接。如果每个主动探测都是从不同地方发出的,我们应该看到随机的数据点,因为数据包的初始序号是随机选择的。但是下图显示主动探测连接虽然来自不同IP,但是非常有规律。我们认为主动探测的初始序号是按照时间产生的。
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我们发现GFW主动探测不仅仅针对了Tor。GFW还对 SoftEther 和GoAgent进行了主动探测。这说明主动探测系统是模块化的。GFW工程师能比较简单的对新翻墙软件改进主动探测功能。
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GFW能(部分的)模拟 vanilla Tor protocol, obfs2, and obfs3 来主动探测网桥代理。有趣的是, node-Tor 因为是使用JavaScript 编写而导致不同的代码实现,从而对主动探测免疫。人为修改了Tor回复能躲避GFW的主动探测,但这应该不是长久之计。
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在2012年,主动探测系统每15分钟扫描一次。但现在,主动探测系统能实时扫描。平均来说,中国用户连接Tor网桥代理后半秒内就有主动探测连接。
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我们使用 traceroute 发现,GFW的主动探测系统是有状态的,但没有办法重组TCP流。
幸运的是,我们有 pluggable transports 来防止主动探测。ScrambleSuit 和 obfs4 能使用预先分享的密钥来防止主动探测。 Meek 使用CDN来代理流量,虽然这不能阻止主动探测,但封锁造成的额外伤害会很大。我们在开发翻墙工具的同时需要注重可用性。大力开发的翻墙工具若很难使用,是没有价值的。这是用户界面的重要性所在。
最后,这是我们的论文,数据库和代码,和研究页面。若你有任何问题和反馈,欢迎联系我们。
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And I have just completed a GFW Active Detection System. The system in a few years ago was used to detect circumvention tools, such as Tor. We are introduced in a previous blog post how GFW active detection system works. But there are several questions we did not answer. For example, the physical structure of the system is like. Those for active detection of IP is owned by GFW all of it? There is speculation GFW short time hijacked a part of the IP to be used to take the initiative to detect, but there is no evidence. This study answers these questions.
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