Dynamic ARP Inspection (DAI) is a security feature that verifies address resolution protocol (ARP) requests and responses in a network.
ARP allows hosts within a Layer 2 broadcast domain to communicate. It does this by mapping an IP address to the individual host's media access control (MAC) address. If a particular host wants to send information to another host but does not have the second host's MAC address in its ARP cache, it sends a message to all hosts throughout the domain seeking that information. The second host then responds with its MAC address.
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These transmissions can be vulnerable to man in the middle attacks. DAI stops these attacks by intercepting all ARP requests and responses and dropping packets with invalid IP-to-MAC address bindings. This approach ensures that only valid ARP requests and responses are passed through.