Ettercap is a suite for man in the middle attacks on LAN. It features sniffing of live connections, content filtering on the fly and many other interesting tricks.
It supports active and passive dissection of many protocols (even ciphered ones) and includes many feature for network and host analysis.
For downloads and more information,
visit the Ettercap homepage.
I've been using this for 7+ years and it just keeps getting better and better. Today you can monitor 10G links seeing hundreds of millions of flows/day using commodity hardware. Argus comes with clients that can multiplex streams from disparate sensors and allows analytic clients to each specify their own traffic filter. Other clients support clustering and insertion into MySQL tables. Documentation can be sparse at times but there's an active and responsive mailing list.
Great tool for the classic man-in-the-middle attacks. The original developers haven't worked on since 2005, but there's a small community of developers on GitHub still fixing bugs and adding features. Check it out at https://github.com/drizztbsd/ettercap
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