Dhcp Filtering; Overview; Limitations; Cli Examples - Dell PowerConnect M6220 Configuration Manual

DHCP Filtering

This section describes the Dynamic Host Configuration Protocol (DHCP) Filtering feature.

Overview

DHCP filtering provides security by filtering untrusted DHCP messages. An untrusted message is a
message that is received from outside the network or firewall, and that can cause traffic attacks within
network.
You can use DHCP Filtering as a security measure against unauthorized DHCP servers. A known attack
can occur when an unauthorized DHCP server responds to a client that is requesting an IP address. The
unauthorized server can configure the gateway for the client to be equal to the IP address of the server. At
that point, the client sends all of its IP traffic destined to other networks to the unauthorized machine,
giving the attacker the possibility of filtering traffic for passwords or employing a 'man-in-the-middle'
attack.
DHCP filtering works by allowing the administrator to configure each port as a trusted or untrusted port.
The port that has the authorized DHCP server should be configured as a trusted port. Any DHCP
responses received on a trusted port will be forwarded. All other ports should be configured as untrusted.
Any DHCP (or BootP) responses received on the ingress side will be discarded.

Limitations

• Port Channels (LAGs): If an interface becomes a member of a LAG, DHCP filtering is no longer
operationally enabled on the interface. Instead, the interface follows the configuration of the LAG
port. End user configuration for the interface remains unchanged. When an interface is no longer a
member of a LAG, the current end user configuration for that interface automatically becomes
effective.
• Mirroring: If an interface becomes a probe port, DHCP filtering can no longer become operationally
enabled on the interface. End user configuration for the interface remains unchanged. When an
interface no longer acts as a probe port, the current end user configuration for that interface
automatically becomes effective.
• DHCP Relay: When DHCP Filtering is administratively enabled, the IP Helper function must check
whether a port is trusted before a DHCP (or BootP) response is forwarded on the port. If the port is
untrusted, the response is dropped. The forwarding of DHCP or BootP request is unaffected.
• If DHCP Filtering is administratively disabled, the operation of the DHCP relay function is
unaffected.

CLI Examples

The commands shown below show examples of configuring DHCP Filtering for the switch and for
individual interfaces.
Example #1: Enable DHCP Filtering for the Switch
console#configure
Switching Configuration
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