Ethereal
The world's most popular network protocol analyzer
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http://sourceforge.net/projects/wireshark/
Wireshark is a powerful network protocol analyzer developed by an international team of networking experts. It runs on UNIX, OS X and Windows. (Looking for Ethereal? You're in the right place. We switched names in May 2006 due to trademark issues.)
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winpcap(windows packet capture)是windows平台下一个免费，公共的网络访问系统。开发winpcap这个项目的目的在于为win32应用程序提供访问网络底层的能力。