Dell S3048-ON, Configuration Manual

Page: 259 / 1036

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Total number of Entries in the table : 4
Dynamic ARP Inspection
Dynamic address resolution protocol (ARP) inspection prevents ARP spoofing by forwarding only ARP frames that have been validated
against the DHCP binding table.
ARP is a stateless protocol that provides no authentication mechanism. Network devices accept ARP requests and replies from any device.
ARP replies are accepted even when no request was sent. If a client receives an ARP message for which a relevant entry already exists in
its ARP cache, it overwrites the existing entry with the new information.
The lack of authentication in ARP makes it vulnerable to spoofing. ARP spoofing is a technique attackers use to inject false IP-to-MAC
mappings into the ARP cache of a network device. It is used to launch man-in-the-middle (MITM), and denial-of-service (DoS) attacks,
among others.
A spoofed ARP message is one in which the MAC address in the sender hardware address field and the IP address in the sender protocol
field are strategically chosen by the attacker. For example, in an MITM attack, the attacker sends a client an ARP message containing the
attacker’s MAC address and the gateway’s IP address. The client then thinks that the attacker is the gateway, and sends all internet-bound
packets to it. Likewise, the attacker sends the gateway an ARP message containing the attacker’s MAC address and the client’s IP address.
The gateway then thinks that the attacker is the client and forwards all packets addressed to the client to it. As a result, the attacker is able
to sniff all packets to and from the client.
Other attacks using ARP spoofing include:
Broadcast An attacker can broadcast an ARP reply that specifies FF:FF:FF:FF:FF:FF as the gateway’s MAC address, resulting
in all clients broadcasting all internet-bound packets.
MAC flooding An attacker can send fraudulent ARP messages to the gateway until the ARP cache is exhausted, after which,
traffic from the gateway is broadcast.
Denial of service An attacker can send a fraudulent ARP messages to a client to associate a false MAC address with the gateway
address, which would blackhole all internet-bound packets from the client.
NOTE: Dynamic ARP inspection (DAI) uses entries in the L2SysFlow CAM region, a sub-region of SystemFlow. One CAM entry is
required for every DAI-enabled VLAN. You can enable DAI on up to 16 VLANs on a system. However, the ExaScale default CAM
profile allocates only nine entries to the L2SysFlow region for DAI. You can configure 10 to 16 DAI-enabled VLANs by allocating
more CAM space to the L2SysFlow region before enabling DAI.
SystemFlow has 102 entries by default. This region is comprised of two sub-regions: L2Protocol and L2SystemFlow. L2Protocol has 87
entries; L2SystemFlow has 15 entries. Six L2SystemFlow entries are used by Layer 2 protocols, leaving nine for DAI. L2Protocol can
have a maximum of 100 entries; you must expand this region to capacity before you can increase the size of L2SystemFlow. This is
relevant when you are enabling DAI on VLANs. If, for example, you want to enable DAI on 16 VLANs, you need seven more entries; in
this case, reconfigure the SystemFlow region for 122 entries using the
layer-2 eg-acl value fib value frrp value
ing-acl value learn value l2pt value qos value system-flow 122
command.
The logic is as follows:
L2Protocol has 87 entries by default and must be expanded to its maximum capacity, 100 entries, before L2SystemFlow can be
increased; therefore, 13 more L2Protocol entries are required. L2SystemFlow has 15 entries by default, but only nine are for DAI; to
enable DAI on 16 VLANs, seven more entries are required. 87 L2Pr 13 additional L2Pr 15 L2Syst 7 additional
L2SystemFlow equals 122.
Dynamic Host Configuration Protocol (DHCP) 259