Chapter 35 Inspecting And Tracking Dhcp Packets; Dynamic Arp Inspection; Arp Attacks - Dell PowerConnect B-RX Configuration Manual

Bigiron rx series configuration guide v02.8.00

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Inspecting and Tracking DHCP Packets

For enhanced network security, you can configure the Brocade device to inspect and keep track of

Dynamic Host Configuration Protocol (DHCP) assignments. To do so, use the following features.

Dynamic ARP Inspection – Intercepts and examines all ARP request and response

packets in a subnet, and blocks all packets that have invalid IP to MAC address bindings

DHCP Snooping – Filters replay DHCP packets from untrusted ports (those connected to

host ports), and allows DHCP packets from trusted ports (those connected to DHCP

IP Source Guard – Permits traffic with valid source IP addresses only, which is learned

from Dynamic ARP Inspection or DHCP snooping

This feature is only supported on Layer 3 code.

Dynamic ARP Inspection (DAI) enables the Brocade device to intercept and examine all ARP request

and response packets in a subnet and discard those packets with invalid IP to MAC address

bindings. DAI can prevent common man-in-the-middle (MiM) attacks such as ARP cache poisoning,

and disallow mis-configuration of client IP addresses.

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.

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