Sniffing a switched network i gave an introduction, on how a switched networked can be sniffed and what are the popular different techniques. In this article i am explaining one of the techniques i discussed in my previous article.
The ARP Process
The two main types of packet addressing are at layers 2 and 3 of the OSI model. These layer 2 addresses, or MAC addresses, are used in conjunction with whichever layer 3 addressing system you are using. In this book, in accordance with industry-standard terminology, I refer to the layer 3 addressing system as the IP addressing system. All devices on a network communicate with each other on layer 3 using IP addresses. Because switches operate on layer 2 of the OSI model, they are
cognizant of only layer 2 MAC addresses, so devices must be able to include this information in packets they construct. When a MAC address is not known, it must be obtained using the known layer 3 IP addresses to be able to forward traffic to the appropriate device. This translation process is done through the layer 2 protocol ARP. The ARP process, for computers connected to Ethernet networks, begins when one computer wishes to communicate with another. The transmitting computer first checks its ARP cache to see if it already has the MAC address associated with the IP address of the destination computer. If it does not, it sends an ARP request to the data link layer broadcast address FF:FF:FF:FF:FF:FF, as discussed in Chapter 1. As a broadcast packet, this packet is received by every computer on that particular Ethernet segment. The packet basically asks, “Which IP address owns the XX:XX:XX:XX:XX:XX MAC address?”
Devices without the destination computer’s IP address simply discard this ARP request. The destination machine replies to the packet with its MAC address via an ARP reply. At this point, the original transmitting computer now has the data link layer addressing information it needs to communicate with the remote computer, and it stores that information in its ARP cache for
How ARP Cache Poisoning Works
ARP cache poisoning, sometimes called ARP spoofing, is the process of sending ARP messages to an Ethernet switch or router with fake MAC (layer 2) addresses in order to intercept the traffic of another computer. Figure illustrates this setup.
ARP cache poisoning is an advanced form of tapping into the wire on a switched network. It is commonly used by attackers to send falsely addressed packets to client systems in order to intercept certain traffic or cause denialof- service (DoS) attacks on a target. However, it can also be a legitimate way to capture the packets of a target machine on a switched network.