Arp Cache Poisoning; How Arp Prevents Cache Poisoning - Cisco SF220-24 Administration Manual

Configuring Security
Configuring Dynamic ARP Inspection
Cisco 220 Series Smart Switches Administration Guide Release 1.1.0.x

ARP Cache Poisoning

A malicious user can attack hosts, switches, and routers connected to a Layer 2
network by poisoning the ARP caches of systems connected to the subnet and by
intercepting traffic intended for other hosts on the subnet. This situation can
happen because ARP allows a gratuitous reply from a host even if an ARP request
was not received. After the attack, all traffic from the device under attack flows
through the attacker's computer and then to the router, switch, or host.

How ARP Prevents Cache Poisoning

The ARP inspection feature relates to interfaces as either trusted or untrusted (see
Security > ARP Inspection > Interface Settings page).
Interfaces are classified by the user as follows:
• Trusted—Packets are not inspected.
• Untrusted—Packets are inspected as described above.
ARP inspection is performed only on untrusted interfaces. ARP packets that are
received on the trusted interface are simply forwarded.
Upon packet arrival on untrusted interfaces the following logic is implemented:
• Search the ARP access control rules for the packet's IP/MAC addresses. If
the IP address is found and the MAC address in the list matches the
packet's MAC address, then the packet is valid.
• If the packet's IP address was not found, and DHCP Snooping is enabled for
the packet's VLAN, search the DHCP Snooping Binding database for the
packet's <VLAN - IP address> pair. If the <VLAN - IP address> pair was
found, and the MAC address and the interface in the database match the
packet's MAC address and ingress interface, the packet is valid.
• If the packet's IP address was not found in the ARP access control rules or in
the DHCP Snooping Binding database the packet is invalid and is dropped.
A SYSLOG message is generated.
• If a packet is valid, it is forwarded and the ARP cache is updated.
If the ARP Packet Validation option is selected (on the Properties page), the
following additional validation checks are performed:
• Source MAC Address—Compares the packet's source MAC address in
the Ethernet header against the sender's MAC address in the ARP request.
This check is performed on both ARP requests and responses.
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