ARP Poisoning

ARP (address resolution protocol) operates by broadcasting a message across a network, to determine the Layer 2 address (MAC address) of a host with a predefined Layer 3 address (IP address). The host at the destination IP address sends a reply packet containing its MAC address. Once the initial ARP transaction is complete, the originating device then caches the ARP response, which is used within the Layer 2 header of packets that are sent to a specified IP address.

An ARP Spoofing attack is the egression of unsolicited ARP messages. These ARP messages contain the IP address of a network resource, such as the default gateway, or a DNS server, and replaces the MAC address for the corresponding network resource with its own MAC address. Network devices, by design, overwrite any existing ARP information in conjunction with the IP address, with the new, counterfeit ARP information. The attacker then takes the role of man in the middle; any traffic destined for the legitimate resource is sent through the attacking system. As this attack occurs on the lower levels of the OSI model, the end-user is oblivious to the attack occurrence (Figure 1).

 ARP Poisoning

Figure 1: ARP Poisoning Attack

ARP Poisoning is also capable of executing Denial of Service (DoS) attacks. The attacking system, instead of posing as a gateway and performing a man in the middle attack, can instead simply drop the packets, causing the clients to be denied service to the attacked network resource. The spoofing of ARP messages is the tributary principal of ARP Poisoning.

A Man-In-The-Middle (MITM) attack is achieved when an attacker poisons the ARP cache of two devices with the MAC address of their own Ethernet NIC. Once the ARP cache has been successfully poisoned, each of the victim devices send all their packets to the attacker when communicating to the other device. This puts the attacker in the middle of the communications path between the two victim devices; hence the name Man-In-The-Middle (MITM) attack. It allows an attacker to easily monitor all communication between victim devices. The objective of this MITM attack is to take over a session. The intent is to intercept and view the information being passed between the two victim devices.

Attack Tools:

Ettercap is a suite for man in the middle attacks on LAN. It features sniffing of live connections, content filtering on the fly and many other interesting tricks. It supports active and passive dissection of many protocols (even ciphered ones) and includes many feature for network and host analysis. Cain and Abel is another software for Microsoft Windows platforms that is able to produce an ARP spoofing attack.

Mitigation

Dynamic ARP Inspection (DAI) can be utilized to mitigate ARP poisoning attacks on local segments. An ARP poisoning attack is a method in which an attacker sends falsified ARP information to a local segment. This information is designed to corrupt the ARP cache of other devices. Often an attacker uses ARP poisoning in order to perform a man-in-the-middle attack.

DAI intercepts and validates the IP-to-MAC address relationship of all ARP packets on untrusted ports. In DHCP environments, DAI utilizes the data that is generated by the DHCP snooping feature. ARP packets that are received on trusted interfaces are not validated and invalid packets on untrusted interfaces are discarded. In non-DHCP environments, the use of ARP ACLs is required.