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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.
Read 7 reviews.
Latest release: version 0.8.0-Lacassagne on Sept. 21, 2013 (11 months, 2 weeks ago).