Promiscuous mode

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In computing, promiscuous mode or promisc mode is a configuration of a network card that makes the card pass all traffic it receives to the central processing unit rather than just packets addressed to it — a feature normally used for packet sniffing.

Each packet includes the hardware (Media Access Control) address. When a network card receives a packet, it checks if the address is its own. If not, the card normally drops the packet. But in promiscuous mode, the card doesn't drop the packet, thus allowing the computer to read all packets.

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 collision domain (for Ethernet and Wireless LAN) 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.

Promiscuous mode is often used to diagnose network connectivity issues. There are programs that make use of this feature to show the user all the data being transferred over the network. Some protocols like FTP and Telnet transfer data and passwords in clear text, without encryption, and network scanners can see this data. Therefore, computer users are encouraged to stay away from insecure protocols like telnet and use more secure ones such as SSH.

[edit] Detection

As promiscuous mode can be used in a malicious way to sniff on a network, one might be interested in detecting network devices that are in promiscuous mode. There are basically two methods to do this:

If a network device is in promiscuous mode, the kernel will receive all network traffic, i. e. the CPU load will increase. Then the latency of network responses will also increase, which can be detected.
In promiscuous mode, some software might send responses to packets even though they were addressed to another machine. If you see such responses, you can be sure that the originating device is in promiscuous mode. However, experienced sniffers can prevent this (e. g. using carefully designed firewall settings). An example is sending a ping (ICMP echo request) with the wrong MAC address but the right IP address. If your firewall blocks all ICMP traffic, this will be prevented.

Promiscuous mode is heavily used by malicious application which initially do the root compromise and then start doing ARP spoofing along with IP spoofing. To do ARP spoofing , NIC must be kept in promiscuous mode. Hence detecting machines in promiscuous mode with no valid reason is an important issue in order to deal with ARP spoofing.