ARP spoofing

ARP spoofing^[1] is a technique whereby an attacker sends fake ("spoofed") Address Resolution Protocol (ARP) messages onto a Local Area Network. Generally, the aim is to associate the attacker's MAC address with the IP address of another host (such as the default gateway), causing any traffic meant for that IP address to be sent to the attacker instead.

ARP spoofing may allow an attacker to intercept data frames on a LAN, modify the traffic, or stop the traffic altogether. Often the attack is used as an opening for other attacks, such as denial of service, man in the middle, or session hijacking attacks.^[2]

The attack can only be used on networks that make use of the Address Resolution Protocol (ARP), and is limited to local network segments.^[3]

The Address Resolution Protocol (ARP) is a widely used protocol for resolving network layer addresses into link layer addresses.^[4]

When an Internet Protocol (IP) datagram is sent from one host to another on a local area network, the destination IP address must be converted into a MAC address for transmission via the data link layer.^[3] When another host's IP address is known, and its MAC address is needed, a broadcast packet is sent out on the local network. This packet is known as an ARP request. The destination machine with the IP in the ARP request then responds with an ARP reply, which contains the MAC address for that IP.^[3]

ARP is a stateless protocol. Network hosts will automatically cache any ARP replies they receive, regardless of whether or not they requested them. Even ARP entries which have not yet expired will be overwritten when a new ARP reply packet is received. There is no method in the ARP protocol by which a host can authenticate the peer from which the packet originated. This behavior is the vulnerability which allows ARP spoofing to occur.^[2][3]

The basic principle behind ARP spoofing is to exploit the above mentioned vulnerabilities in the ARP protocol by sending spoofed ARP messages onto the LAN. ARP spoofing attacks can be run from a compromised host on the LAN, or from an attacker's machine that is connected directly to the target LAN.

Generally, the goal of the attack is to associate the attacker's MAC address with the IP address of a target host, so that any traffic meant for the target host will be sent to the attacker's MAC instead. The attacker could then choose to:

1. Inspect the packets, and forward the traffic to the actual default gateway (interception)
2. Modify the data before forwarding it (man-in-the-middle attack).
3. Launch a denial-of-service attack by causing some or all of the packets on the network to be dropped

IP-to-MAC mappings in the local ARP cache can be statically defined, and then hosts can be directed to ignore all ARP reply packets.^[5] While static entries provide perfect security against spoofing if the operating systems handles them correctly, they result in quadratic maintenance efforts as IP-MAC mappings of all machines in the network have to be distributed to all other machines.

Software that detects ARP spoofing generally relies on some form of certification or cross-checking of ARP responses. Uncertified ARP responses are then blocked. These techniques may be integrated with the DHCP server so that both dynamic and static IP addresses are certified. This capability may be implemented in individual hosts or may be integrated into Ethernet switches or other network equipment. The existence of multiple IP addresses associated with a single MAC address may indicate an ARP spoof attack, although there are legitimate uses of such a configuration. In a more passive approach a device listens for ARP replies on a network, and sends a notification via email when an ARP entry changes.^{[citation needed]}

Operating systems react differently, e.g. Linux ignores unsolicited replies, but on the other hand uses seen requests from other machines to update its cache. Solaris accepts updates on entries only after a timeout. In Microsoft Windows, the behavior of the ARP cache can be configured through several registry entries under HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters, ArpCacheLife, ArpCacheMinReferenceLife, ArpUseEtherSNAP, ArpTRSingleRoute, ArpAlwaysSourceRoute, ArpRetryCount.^[6]

AntiARP^[7] also provides Windows-based spoofing prevention at the kernel level. ArpStar is a Linux module for kernel 2.6 and Linksys routers that drops invalid packets that violate mapping, and contains an option to repoison/heal.

The simplest form of certification is the use of static, read-only entries for critical services in the ARP cache of a host. This prevents only simple attacks and does not scale on a large network, since the mapping has to be set for each pair of machines resulting in approximately n² ARP entries that have to be configured when n machines are present: On every machine there must be an ARP entry for every other machine on the network, which are n ARP entries on every of the n machines.

The techniques that are used in ARP spoofing can also be used to implement redundancy of network services. For example, some software allows a backup server to issue a gratuitous ARP request in order to take over for a defective server and transparently offer redundancy.^{[8] [9]}

ARP spoofing is often used by developers to debug IP traffic between two hosts when a switch is in use.

This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "ARP spoofing" – news · newspapers · books · scholar · JSTOR (January 2012) (Learn how and when to remove this message)

• DNS spoofing
• IP address spoofing
• MAC spoofing

• Steve Gibson (2005-12-11). "ARP Cache Poisoning". GRC.