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Anatomy of an ARP Poisoning Attack | WatchGuard http://www.watchguard.com/infocenter/editorial/135324.asp
Anatomy of an ARP Poisoning Attack
by, WatchGuard Network Security Analyst
Hackers lie. Skillful hackers lie well. And well-rounded hackers can lie both to people and to machines.

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Lying to people, known as social engineering, involves tactics (detailed at length by convicted hacker) such as posing as a companys employee so the companys real employees will blab secrets freely. Lying to machines involves lots of different techniques, and a commonly used one ARP Cache Poisoning is the focus of this article. ARP poisoning enables local hackers to cause general networking mayhem. Because its mostly incurable, every administrator should be aware of how this attack works.
ARP Refresher
In Foundations: What Are NIC, MAC, and ARP?, we explained that Address Resolution Protocol (ARP) is how network devices associate MAC addresses with IP Addresses so that devices on the local network can find each other. ARP is basically a form of networking roll call.
ARP, a very simple protocol, consists of merely four basic message types:
1. AnARPRequest.ComputerAasksthenetwork,WhohasthisIPaddress?
2. AnARPReply.ComputerBtellsComputerA,IhavethatIP.MyMACaddressis[whateveritis].
3. A Reverse ARP Request (RARP). Same concept as ARP Request, but Computer A asks, Who has this MAC address? 4. ARARPReply.ComputerBtellsComputerA,IhavethatMAC.MyIPaddressis[whateveritis]
All network devices have an ARP table, a short-term memory of all the IP addresses and MAC addresses the device has already matched together. The ARP table ensures that the device doesnt have to repeat ARP Requests for devices it has already communicated with.
Heres an example of a normal ARP communication. Jessica, the receptionist, tells Word to print the latest company contact list. This is her first print job today. Her computer (IP address 192.168.0.16) wants to send the print job to the offices HP LaserJet printer (IP address 192.168.0.45). So Jessicas computer broadcasts an ARP Request to the entire local network asking, Who has the IP address, 192.168.0.45? as seen in Diagram 1.
All the devices on the network ignore this ARP Request, except for the HP LaserJet printer. The printer recognizes its own IP in the request and sends an ARP Reply: Hey, my IP address is 192.168.0.45. Here is my MAC address: 00:90:7F:12:DE:7F, as in Diagram 2.
Now Jessicas computer knows the printers MAC address. It sends the print job to the correct device, and it also associates the printers MAC address of 00:90:7F:12:DE:7F with the printers IP address of 192.168.0.45 in its ARP table.
Hey ARP, Did You Know Gullible Is Not in the Dictionary?
The founders of networking probably simplified the communication process for ARP so that it would function efficiently. Unfortunately, this simplicity also leads to major insecurity. Know why my short description of ARP doesnt mention any sort of authentication method? Because in ARP, there is none.
ARP is very trusting, as in, gullible. When a networked device sends an ARP request, it simply trusts that when the ARP reply comes in, it really does come from the correct device. ARP provides no way to verify that the responding device is really who it says it is. In fact, many operating systems implement ARP so trustingly that devices that have not made an ARP request still accept ARP replies from other devices.
OK, so think like a malicious hacker. You just learned that the ARP protocol has no way of verifying ARP replies. Youve learned many devices accept ARP replies before even requesting them. Hmmm. Well, why dont I craft a perfectly valid, yet malicious, ARP reply containing any arbitrary IP and MAC address I choose? Since my victims computer will blindly accept the ARP entry into its ARP table, I can force my victims gullible computer into thinking any IP is related to any MAC address I want. Better yet, I can broadcast my faked ARP reply to my victims entire network and fool all his computers. Muahahahahaa!
Back to reality. Now you probably understand why this common technique is called ARP Cache Poisoning (or just ARP Poisoning): the attacker lies to a device on your network, corrupting or poisoning its understanding of where other devices are. This frighteningly simple procedure enables the hacker to cause a variety of networking woes, described next.
All Your ARP Are Belong To Us!
The ability to associate any IP address with any MAC address provides hackers with many attack vectors, including Denial of Service, Man in the Middle, and MAC Flooding.
Denial of Service
A hacker can easily associate an operationally significant IP address to a false MAC address. For instance, a hacker can send an ARP reply associating your network routers IP address with a MAC address that doesnt exist. Your computers believe they know where your default gateway is, but in reality theyre sending any packet whose destination is not on the local segment, into the Great Bit Bucket in the Sky. In one move, the hacker has cut off your network from the Internet.
Man in the Middle
A hacker can exploit ARP Cache Poisoning to intercept network traffic between two devices in your network. For instance, lets say the hacker wants to see all the traffic between your computer, 192.168.0.12, and your Internet router, 192.168.0.1. The hacker begins by sending a malicious ARP reply (for which there was no previous request) to your router, associating his computers MAC address with 192.168.0.12 (see
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Anatomy of an ARP Poisoning Attack | WatchGuard http://www.watchguard.com/infocenter/editorial/135324.asp
Diagram 3).
Now your router thinks the hackers computer is your computer.
Next, the hacker sends a malicious ARP reply to your computer, associating his MAC Address with 192.168.0.1 (see Diagram 4).
Now your machine thinks the hackers computer is your router.
Finally, the hacker turns on an operating system feature called IP forwarding. This feature enables the hackers machine to forward any network traffic it receives from your computer to the router (shown in Diagram 5).
Now, whenever you try to go to the Internet, your computer sends the network traffic to the hackers machine, which it then forwards to the real router. Since the hacker is still forwarding your traffic to the Internet router, you remain unaware that he is intercepting all your network traffic and perhaps also sniffing your clear text passwords or hijacking your secured Internet sessions.
MAC Flooding
MAC Flooding is an ARP Cache Poisoning technique aimed at network switches. (If you need a reminder about the difference between a hub and a switch, see this sidebar.) When certain switches are overloaded they often drop into a hub mode. In hub mode, the switch is too busy to enforce its port security features and just broadcasts all network traffic to every computer in your network. By flooding a switchs ARP table with a ton of spoofed ARP replies, a hacker can overload many vendors switches and then packet sniff your network while the switch is in hub mode.
Scared? Good, Now Calm Down!
This is scary stuff. ARP Cache Poisoning is trivial to exploit yet it can result in very significant network compromise. However, before you jump to Defcon-7, notice the major mitigating factor: only local attackers can exploit ARPs insecurities. A hacker would need either physical access to your network, or control of a machine on your local network, in order to deliver an ARP Cache Poisoning attack. ARPs insecurities cant be exploited remotely.
That said, hackers have been known to gain local access to networks. Good network administrators should be aware of ARP Cache Poisoning techniques.
Since ARP Cache Poisoning results from a lack of security in a protocol that is required for TCP/IP networking to function, you cant fix it. But you can help prevent ARP attacks using the following techniques.
For Small Networks
If you manage a small network, you might try using static IP addresses and static ARP tables. Using CLI commands, such as ipconfig /all in Windows or ifconfig in NIX, you can learn the IP address and MAC address of every device in your network. Then using the arp -s command, you can add static ARP entries for all your known devices. Static means unchanging; this prevents hackers from adding spoofed ARP entries for devices in your network. You can even create a login script that would add these static entries to your PCs as they boot.
However, static ARP entries are hard to maintain; impossible in large networks. Thats because every device you add to your network has to be manually added to your ARP script or entered into each machines ARP table. But if you manage fewer than two dozen devices, this technique might work for you.
For Large Networks
If you manage a large network, research your network switchs Port Security features. One Port Security feature lets you force your switch to allow only one MAC address for each physical port on the switch. This feature prevents hackers from changing the MAC address of their machine or from trying to map more than one MAC address to their machine. It can often help prevent ARP-based Man-in-the-Middle attacks.
For All Networks
Your best defense is understanding ARP Poisoning and monitoring for it. Id highly recommend deploying an ARP monitoring tool, such as ARPwatch, to alert you when unusual ARP communication occurs. This kind of vigilance is still the greatest weapon against all kinds of attack for, as Stevenson wrote, The cruelest lies are often told in silence.
Resources:
Address Resolution Protocol Spoofing and Man-in-the-Middle Attacks
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