Promiscuous mode

In computer networking, promiscuous mode (often shortened to "promisc mode" or "promisc. mode") is a mode for a wired network interface controller (NIC) or wireless network interface controller (WNIC) that causes the controller to pass all traffic it receives to the central processing unit (CPU) rather than passing only the frames that the controller is intended to receive. This mode is normally used for packet sniffing that takes place on a router or on a computer connected to a hub (instead of a switch) or one being part of a WLAN. Interfaces are placed into promiscuous mode by software bridges often used with hardware virtualization.

In IEEE 802 networks such as Ethernet, token ring, and IEEE 802.11, and in FDDI, each frame includes a destination Media Access Control address (MAC address). In non-promiscuous mode, when a NIC receives a frame, it normally drops it unless the frame is addressed to that NIC's MAC address or is a broadcast or multicast frame. In promiscuous mode, however, the NIC allows all frames through, thus allowing the computer to read frames intended for other machines or network devices.

Many operating systems require superuser privileges to enable promiscuous mode. A non-routing node in promiscuous mode can generally only monitor traffic to and from other nodes within the same broadcast domain (for Ethernet and IEEE 802.11) or ring (for token ring or FDDI). Computers attached to the same network hub satisfy this requirement, which is why network switches are used to combat malicious use of promiscuous mode. A router may monitor all traffic that it routes.

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