What are "promiscuous mode enabled" log messages

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The following log messages are recorded when a utility has placed the network card into "promiscuous mode":

Feb 10 01:41:58    kernel: vr0: promiscuous mode disabled
Feb 10 01:41:57    kernel: vr0: promiscuous mode enabled
Feb 10 01:41:54    kernel: vr0: promiscuous mode disabled
Feb 10 01:41:54    kernel: vr0: promiscuous mode enabled
Feb 10 01:41:50    kernel: vr0: promiscuous mode disabled

Promiscuous mode is a mode where the network card will receive every packet on the interface, regardless of the target MAC address, in order to monitor traffic. This is normal if for utilities such as tcpdump.