DHCP Snooping
DHCP snooping protects networks from spoofing. In the context of DHCP snooping, ports are either
trusted or not trusted.
By default, all ports are not trusted. Trusted ports are ports through which attackers cannot connect.
Manually configure ports connected to legitimate servers and relay agents as trusted.
When you enable DHCP snooping, the relay agent builds a binding table — using DHCPACK messages —
containing the client MAC address, IP addresses, IP address lease time, port, VLAN ID, and binding type.
Every time the relay agent receives a DHCPACK on a trusted port, it adds an entry to the table.
The relay agent checks all subsequent DHCP client-originated IP traffic (DHCPRELEASE, DHCPNACK, and
DHCPDECLINE) against the binding table to ensure that the MAC-IP address pair is legitimate and that the
packet arrived on the correct port. Packets that do not pass this check are forwarded to the server for
validation. This checkpoint prevents an attacker from spoofing a client and declining or releasing the real
client’s address. Server-originated packets (DHCPOFFER, DHCPACK, and DHCPNACK) that arrive on a not
trusted port are also dropped. This checkpoint prevents an attacker from acting as an imposter as a
DHCP server to facilitate a man-in-the-middle attack.
Binding table entries are deleted when a lease expires, or the relay agent encounters a DHCPRELEASE,
DHCPNACK, or DHCPDECLINE.
Dell Networking OS Behavior: Introduced in the Dell Networking OS version 7.8.1.0, DHCP snooping was
available for Layer 3 only and dependent on DHCP relay agent (ip helper-address). The Dell
Networking OS version 8.2.1.0 extends DHCP snooping to Layer 2 and you do not have to enable relay
agent to snoop on Layer 2 interfaces.
Dell Networking OS Behavior: Binding table entries are deleted when a lease expires or when the relay
agent encounters a DHCPRELEASE. The switch maintains a list of snooped VLANs. When the binding
table is exhausted, DHCP packets are dropped on snooped VLANs, while these packets are forwarded
across non-snooped VLANs. Because DHCP packets are dropped, no new IP address assignments are
made. However, DHCPRELEASE and DHCPDECLINE packets are allowed so that the DHCP snooping
table can decrease in size. After the table usage falls below the maximum limit of 4000 entries, new IP
address assignments are allowed.
NOTE: DHCP server packets are dropped on all not trusted interfaces of a system configured for
DHCP snooping. To prevent these packets from being dropped, configure ip dhcp snooping
trust on the server-connected port.

Enabling DHCP Snooping

To enable DHCP snooping, use the following commands.
1. Enable DHCP snooping globally.
CONFIGURATION mode
ip dhcp snooping
2. Specify ports connected to DHCP servers as trusted.
INTERFACE mode
Dynamic Host Configuration Protocol (DHCP) 329