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Ethernet’s card promiscuous mode November 19, 2010

Posted by Tournas Dimitrios in Linux.

What is exactly promiscuous mode? , what would it do on my network interface card and why would i need to use it? This small article will give you a brief description of promiscuous mode as well as basic and fast instructions on how to set your ethernet adapter card to work on this mode.
First of all, promiscuous mode on an ethernet device is not the norm, for you to understand what it does please read the following definition below  :  
promiscuous mode :

  • 1) In a network, promiscuous mode allows a network device to intercept and read each network packet that arrives in its entirety. This mode of operation is sometimes given to a network snoop server that captures and saves all packets for analysis (for example, for monitoring network usage).
  • 2) In an Ethernet local area network (LAN), promiscuous mode is a mode of operation in which every data packet transmitted can be received and read by a network adapter. Promiscuous mode must be supported by each network adapter as well as by the input/output driver in the host operating system. Promiscuous mode is often used to monitor network activity.

Promiscuous mode is the opposite of non-promiscuous mode. When a data packet is transmitted in non-promiscuous mode, all the LAN devices “listen to” the data to determine if the network address included in the data packet is theirs. If it isn’t, the data packet is passed onto the next LAN device until the device with the correct network address is reached. That device then receives and reads the data.

How to change your ethernet adapter to work in promiscuous mode on a linux/unix environment?Very simple , As a root user please type the following command:  ifconfig ethx promisc .  Being ethX the number of your adapter, for example if you want to do that on ethernet adapter 0 , then type: ifconfig eth0 promisc . If you ever had used Wireshark for packet capturing and analyzing your network , behind the scenes , Wireshark forced your card in promiscuous mode .

Simple, yet useful. By the way in case you are interested in a good IDS system, please visit www.snort.org


1. Paulette - April 29, 2014

Thats amusing! But thanks again for this particular post

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