Network Security – Arp spoofing

So…. what is arp spoofing (poisoning)…. and what are it’s implications? ARP spoofing involves tricking a machine into thinking that you’re machine is, yet another. Let’s put this in IP address terms. Let’s say that is the default gatway on the network and is our target. We are given another network address – say…. Arp spoofing would tell that OUR network adapter is the place to send information destined for, (and we could also tell that WE are the rightful recipient of data sent to These is done by offering up our MAC address as the legitimate desitination to each machine through a crafted ARP response.

Now, on first reading all of this you think….. wow that sounds hard. Actually there is software readily available that does this in just a few clicks (or with a simple command line syntax.) So, it’s VERY easily done once you’re in the same subnet as the machines that are targetted. In fact, it’s possible to arp-spoof an entire subnet this way, say capturing all traffic from the subnet to the gateway. Once all that traffic is routed through the attacking machine, then…. it can be analyzed.

There is software that quickly and easily collects passwords from this stream of data, or perhaps logs web addresses, captures mail transfered, or can even alter data in the stream (maybe replacing images on a web page or directing you to a page other than you intended.)

So basically this means that ANY untrusted machine connected to your LAN (as long as it’s on your subnet or “upstream” towards the internet), could easily sniff any and all data flowing through the network. Of course, if the hostile machine is upstream, it could only sniff traffic moving outside of the LAN to the router to the internet for instance.

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