Function of DHCP
On our networks all the different clients will usually obtain their IP address information automatically, using DHCP – Dynamic Host Configuration Protocol. This is faster and more precise than having their IP address information statically configured by manually entering the IP address, subnet mast and Default gateway into PC.
To dynamically obtain IP address information, a client makes a DHCP request. A DHCP server sends back a DHCP response in which includes information as an IP address, subnet mask, and default gateway. If an attacker puts and false DHCP server in our LAN network, the rogue DHCP server can respond to a client’s DHCP request. Even if the rogue DHCP server and the actual DHCP server both respond to the request of our PC, the PC will use the rogue DHCP server’s response if it reaches the PC faster than actual DHCP server.
More about DHCP and about DHCP attack here. Let’s se the mechanisms to mitigate the attacks or prevent them fully:
The DHCP snooping feature on Cisco and Juniper switches can be used to mitigate a DHCP server spoofing attack. With this mechanism switch ports are configured in two different state, the trusted and untrusted state. If a port is configured to be trusted, it can receive DHCP responses. In other way, if a port is untrusted, it is not allowed to receive DHCP responses, and if a false attackers DHCP response attempts to enter an untrusted port, the port will be disabled.