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 is capable of intercepting traffic on a network
segment, capturing passwords, and conducting man-in-the-middle attacks
against a number of common protocols.

It supports active and passive dissection of many protocols (even
ciphered ones) and includes many feature for network and host analysis.

It is free software, licensed under the terms of the
GNU General Public License.

Ettercap-NG requires libnet. A SlackBuild for this is available at
SlackBuilds.org.
