ARP Poisoning

ARP Poisoning

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Introduction This article is meant to teach how ARP works and how one can go about poisoning the ARP cache and enable them to completely sniff traffic over a switched network. This article assumes that you already have access to a switched network. ARP Poisoning is a way of tricking computers over a switched network to send traffic through you before going to other computers or out to the internet.

ARP Address Resolution Protocol (ARP) is a dynamic protocol to map a 32bit IP Address to a 48bit physical hardware address (MAC Address). If one system over a network wants to communicate with another system over a network, it will first check if it already knows that systems MAC Address and if not it will send out an ARP broadcast which will look for the hardware address of the destination system. There are four types of ARP messages but the main two are ARP Request and ARP Reply. When a system starts broadcasting an ARP Message it sends out an ARP Request. An ARP Request is a message sent to the broadcast address, the message contains the sender’s IP Address and MAC Address and requests the MAC Address of the given IP, and then it waits for an ARP Reply. An ARP Reply replies to the ARP Request and tells the computer sending the ARP Request what its MAC Address is. The ARP Cache is a temporary storage place that holds a table with MAC Address’s and IP Address’s. If a computer wants to talk to another computer and it doesn’t already have its MAC address stored it will send an ARP Request. If the Computer that is sending the ARP Reply does not have the requesting computers MAC Address it as well will save it to cache. So now both computers have the MAC Address. A system cannot communicate with another until it has its MAC Address. ARP is a stateless protocol with no authentication built in so any ARP Reply, whether there was a request or not will update the ARP Cache on a computer. All systems will accept an ARP Reply regardless if there was an ARP Request sent.

The Switch Media Access Control (MAC) is a standard addressing system for all Ethernet devices. Most networks use switching devices and in a switched network packets are only sent to the port they are destined to according to their destination MAC Address. Switches maintain a table that associates MAC Address’s with certain ports. A switch constructs a route table by extracting the source MAC Address from the Ethernet frame of each packet processed. If any entry in the route table does not exist the switch will forward the packet out all of its ports.

Within a switched network packets are only sent to the destination device making it, so other devices cannot see the traffic.

Poisoning There are a few tricks to manipulating a network to send traffic through you before sending it to the packets to the destination device. One of these methods is referred to as ARP Poisoning and it is when you send a customized ARP Reply to different computers across the network tricking their computers into updating their ARP cache with new MAC Address’s (Your MAC Address). So now each time computer1 wants to send a message to computer2 it gets the MAC address of computer2’s IP and sends the message to that MAC address. But if that MAC address is changed to your MAC address, by poisoning the ARP Cache the message will be sent to you instead. After packets are sent to you, you must forward the packets to the computer it was meant to go in the first place or DoS will be caused and the hosts will not be able to communicate anymore. Another factor that you must weigh in are timeouts, if there is no traffic over the network, after a timeout period the ARP cache of the computers across a network will be flushed out and you will need to send another constructed ARP reply to the hosts so that traffic is once again forwarded to you. One way to fix this is to automatically send ARP Replies every 10 seconds or so to the hosts that you want to poison.

Sniffing Sniffing is the act of capturing packets that aren’t necessarily meant for public viewings. When you sniff packets across a network you can come across many interesting things such as emails, instant messages, and even passwords to email accounts and ftp accounts and many other types of passwords which in my experience are more often than not, left unencrypted. There are many tools out there that will automatically scan packets for username and password info. You can also see what websites the person is going to.

Wireless If an access point is connected directly to a hub or a switch than it leaves the entire wireless network open to ARP Poisoning. Wireless internet is becoming more and more used and it is hard to be anywhere that does not have a wireless access point, especially in well populated areas. This leaves a huge security risk to most networks because in theory someone with a laptop could go into the lobby of a business and get on their network by cracking their WEP key or just simply connecting if they don’t even have WEP. The attacker would then just need to poison the ARP Cache of the different computers across the network and then forward all traffic through you. You would get their passwords and usernames, the websites they go to and anything else that you feel would be fun to look at.