DHCP spoof attack
A DHCP Spoof attack is a illegal DHCP server setup in a LAN environment which configures the clients with wrong IP information. The illegal DHCP server can configure the clients with it self as the default gateway, in which case all non-local traffic will be sent to the illegal DHCP server where the intruder can inspect or alter the packets before they are sent on to the real gateway. this is known as a man in the middle attack.
Protecting against DHCP spoof attack
On the access switches you can protect against spoofing
ip dhcp spoofing interface GigabitEthernet0/1 description Trunk to Distribution layer and DHCP server ip dhcp snooping trust
Switch# <input>show ip dhcp snooping</input> Switch DHCP snooping is enabled DHCP Snooping is configured on the following VLANs: 10 30-40 100 200-220 Insertion of option 82 information is enabled Interface Trusted Rate limit (pps) --------- ------- ---------------- FastEthernet0/1 no 20 FastEthernet0/2 no 20 GigabitEthernet0/1 yes none