A post for students in network security class:
ARP spoofing is a powerful technique used by hackers to intercept and view unencrypted web traffic on a local area network (LAN). This attack exploits a fundamental vulnerability in the Address Resolution Protocol (ARP), which the internet uses to map a device’s IP address (where it is on the network) to its MAC address (who it is). Because ARP was not designed with security in mind, it allows an attacker to “pretend” to be someone else on the network.
Every router maintains an ARP table to manage these mappings and reduce the need for constant queries. In the first phase of an ARP spoofing attack, the hacker sends a fake ARP response to a victim, stating that the hacker’s MAC address is the correct mapping for the router’s IP address. Once the victim’s machine accepts this fake packet, its ARP table updates, causing all subsequent internet traffic to be sent to the attacker’s machine instead of the legitimate router. This creates a man-in-the-middle scenario where the hacker sits invisibly between the victim and the internet.
To perform this attack effectively, the sources recommend a specific technical workflow using the dsniff tool suite. First, an attacker must identify targets on the network using netdiscover, a tool that scans for active IP and MAC addresses by issuing ARP queries.
Once the victim’s IP and the router’s IP are identified, the attacker must enable IP forwarding on their own machine. This step is critical; without it, the victim would lose internet access and quickly realize something is wrong. By enabling forwarding, the attacker ensures the victim’s packets still reach the router after being inspected.
The actual spoofing is executed using the arpspoof tool. The attacker must run two instances of the tool: one to trick the victim into believing the attacker is the router, and another to trick the router into believing the attacker is the victim. Once the traffic is flowing through the attacker’s network interface card (NIC), tools like urlsnarf can be used to extract a list of URLs the victim visits in real-time. For more detailed theft, tools like Wireshark allow the hacker to analyze raw packets and reconstruct unencrypted TCP streams to steal private data.
Defending against ARP spoofing is challenging, but the most effective protection is encrypting your traffic. Since spoofing only allows attackers to read unencrypted data, using HTTPS ensures that even if traffic is intercepted, the contents remain secure. Additionally, security professionals can use the Scapy library in Python to write scripts that heuristically detect attacks by monitoring ARP tables for sudden, suspicious changes in IP-to-MAC mappings.
An ARP spoofing attack is essentially like a dishonest postal worker who tells you they are the mailman and tells the post office they are you; they get to read all your letters before passing them along, and neither you nor the post office ever realizes a third party was involved.