Configuring DHCP Filtering

C H A P T E R 28

This chapter describes the Dynamic Host Configuration Protocol (DHCP) Filtering feature and how to configure DHCP filtering.

This chapter contains the following topics:

Section , Understanding Dynamic Host Configuration Protocol (DHCP) Filtering

Section , Configuring DHCP Filtering

Understanding Dynamic Host Configuration Protocol (DHCP) Filtering

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.

The following limitations exist:

Port Channels (LAGs)--If an interface becomes a member of a LAG, DHCP filtering is no longer become 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. Instead, the interface follows the configuration of the LAG port. 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.

Operation without DHCP Relay--On platforms in which the DHCP relay feature is not included, hardware support must be available for the DHCP Filtering feature to operate.

DHCP Relay--When DHCP Filtering is administratively enabled, the DHCP relay 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.

If Hardware support is available for DHCP Filtering, DHCP Filtering may be enabled both routing and non-routing interfaces.

If Hardware support is unavailable, DHCP Filtering may be enabled only on routed interfaces and only on interfaces enabled for DHCP relay.

Configuring DHCP Filtering

The following CLI commands show examples of configuring DHCP Filtering for the switch and for individual interfaces.

Example 1: Enable DHCP Filtering for the Switch

config
     ip dhcp filtering
     exit
exit

Example 2: Enable DHCP Filtering for an Interface

config
     interface 0/11
           ip dhcp filtering trust
           exit
     exit

Example 3: Show DHCP Filtering Configuration

show ip dhcp filtering
 
Switch DHCP Filtering is Enabled
 
Interface    Trusted
-----------  ----------
1/0/1        No
1/0/2        No
1/0/3        No
1/0/4        No
1/0/5        No
1/0/6        No
1/0/7        No
1/0/8        No
1/0/9        No
1/0/10       No
1/0/11       Yes
1/0/12       No
1/0/13       No
1/0/14       No
1/0/15       No