Simple tool to set your nic into promiscuous mode and to then go in the background.
Linux, FreeBSD, Illumos/Solaris
Headers for libpcap, and libpcap itself.
./promisc [iface]
iface is optional. When iface is not specified, it will take the default interface (which has the default route).
Sometimes jail/zone-like functionality doesn't seem to route packets unless interfaces are set to IF_PROMISC. This is a better way to force your nic into promiscuous mode instead of running tcpdump or sniff in the background.