ARP Inspection Configuration

Definition

Dynamic ARP Inspection (DAI) is a Layer 2 security feature that validates ARP (Address Resolution Protocol) packets against a trusted binding database to prevent ARP spoofing and poisoning attacks. ARP inspection configuration involves enabling DAI on network switches, defining trusted and untrusted ports, building the DHCP snooping binding database, and setting rate limits to protect against ARP-based denial-of-service attacks.

How It Works

DAI intercepts all ARP packets on untrusted ports and validates them against the DHCP snooping binding database, which maps IP addresses to MAC addresses on specific switch ports. ARP packets that do not match a valid binding are dropped. Trusted ports, typically uplinks to other switches or DHCP servers, bypass DAI inspection. The DHCP snooping binding database is automatically populated as clients obtain addresses from DHCP servers. For statically addressed devices, ARP access control lists (ARP ACLs) define explicit IP-to-MAC bindings. Rate limiting on untrusted ports prevents ARP flooding attacks that could overwhelm the switch CPU. Additional validation checks can verify source MAC consistency between the Ethernet header and ARP body, and destination MAC validation in ARP responses. DAI logging captures all dropped ARP packets with source information for security investigation.

Why It Matters

ARP spoofing is one of the most effective Layer 2 attacks, enabling man-in-the-middle interception, session hijacking, and credential theft on local networks. By sending forged ARP responses, an attacker can redirect traffic intended for the default gateway through their own system. Without DAI, any device on the local network can impersonate any other device's MAC address. ARP spoofing tools are freely available and require minimal technical skill to operate, making this a common attack vector in both external and insider threat scenarios.