Brocade BigIron RX Series Configuration Guide - Manual (Page 1162)

1084 BigIron RX Series Configuration Guide
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Dynamic ARP inspection
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ARP attacks
ARP provides IP communication within a Layer 2 broadcast domain by mapping an IP address to a
MAC address. Before a host can talk to another host, it must map the IP address to a MAC address
first. If the host does not have the mapping in its ARP table, it sends an ARP request to resolve the
mapping. All computers on the subnet will receive and process the ARP requests, and the host
whose IP address matches the IP address in the request will send an ARP reply.
An ARP poisoning attack can target hosts, switches, and routers connected to the 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. For instance, a malicious host can reply to an ARP request
with its own MAC address, thereby causing other hosts on the same subnet to store this
information in their ARP tables or replace the existing ARP entry. Furthermore, a host can send
gratuitous replies without having received any ARP requests. A malicious host can also send out
ARP packets claiming to have an IP address that actually belongs to another host (e.g. the default
router). 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 DAI works
DAI allows only valid ARP requests and responses to be forwarded.
A Brocade device on which DAI is configured does the following:
• Intercepts ARP packets received by the system CPU.
• Inspects all ARP requests and responses received on untrusted ports.
• Verifies that each of the intercepted packets has a valid IP-to-MAC address binding before
updating the local ARP table, or before forwarding the packet to the appropriate destination.
• Drops invalid ARP packets
When you enable DAI on a VLAN, by default, all member ports are untrusted. You must manually
configure trusted ports. In a typical network configuration, ports connected to host ports are
untrusted. You configure ports connected to other switches or routers as trusted.
DAI inspects ARP packets received on untrusted ports, as shown in Figure 143 . DAI carries out the
inspection based on IP-to-MAC address bindings stored in a trusted binding database. For the
BigIron RX, the binding database is the ARP table, which supports DAI, DHCP snooping, and IP
Source Guard. To inspect an ARP request packet, DAI checks the source IP and source MAC
address against the ARP table. For an ARP reply packet, DAI checks the source IP, source MAC,
destination IP, and destination MAC addresses. DAI forwards the valid packets and discards those
with invalid IP-to-MAC address bindings.
When ARP packets reach a trusted port, DAI lets them through, as shown in Figure 143 .