Over last few weeks, I have been fiddling around with this tool to test one of the applications. I found the tool has some real good capabilities. Sniffing over a switched network is not easy. However, using Ettercap, I managed it quite nicely.
In an Ethernet network computers communicate with each other via Ethernet MAC addresses. So, there is a mechanism needed for matching of IP addresses with the addresses in an ethernet network. The mechanism is called ARP (Address Resolution Protocol).
What ARP does is exactly what most people do, when they have to find Mr. X in a crowd of people - they shout loud enough, so that everyone can hear them and expect Mr. X to answer, if he is there. When he answers, we will know who is he. When ARP wants to know whats the Ethernet address matching a given IP address it uses an Ethernet technic, called BROADCASTING, with which the datagram is addressed to all the workstations in the network. The broadcast-datagram sent by ARP contains a request for the IP address. Every computer, received that request compares the requested address with its own IP address and if they match, it sends an ARP reply back to the asking computer. After rreceiving the reply, the asking computer can get the Ethernet address of the computer it is looking for, from his reply. After the computer finds an Ethernet address, he stores it in its ARP cache (ARP table), so he won't need to look for it the next time he wants to send a datagram to the same address. However, it is not good this information to be stored forever (the Ethernet adapter of the other host may be replaced for some reasonm and the entry for the computer's IP in the ARP cache will become invalid). So the entries in the ARP cache expire after a period of time. Most operating systems will replace an entry in their ARP cache even if they haven't sent and ARP request before. That allows a MITM (Man-In-The-Middle) attack to be performed.
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