Tcpreplay is aimed at testing the performance of a Network Intrusion Detection
System by replaying real background network traffic in which to hide attacks.

Tcpreplay allows you to control the speed at which the traffic is replayed,
and can replay arbitrary libpcap traces.

Unlike programmatically-generated artificial traffic which doesn't exercise
the application/protocol inspection that a NIDS performs, and doesn't
reproduce the real-world anomalies that appear on production networks
(asymmetric routes, traffic bursts/lulls, fragmentation, retransmissions,
etc.), tcpreplay allows for exact replication of real traffic seen on real
networks. 

WWW: http://sourceforge.net/projects/tcpreplay/