ettercap

Multipurpose sniffer/interceptor/logger for switched LAN
Ettercap supports active and passive dissection of many protocols
(even ciphered ones) and includes many feature for network and host
analysis.
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Data injection in an established connection and filtering (substitute
or drop a packet) on the fly is also possible, keeping the connection
synchronized.
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Many sniffing modes were implemented to give you a powerful and complete
sniffing suite. It's possible to sniff in four modes: IP Based, MAC Based,
ARP Based (full-duplex) and PublicARP Based (half-duplex).
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It has the ability to check whether you are in a switched LAN or
not, and to use OS fingerprints (active or passive) to let you know the
geometry of the LAN.