Defense Against DHCP Server DoS Attacks

Mechanism

In Figure 13-4, if a large number of attackers request IP addresses on interface1, IP addresses in the IP address pool are exhausted, and there are no IP addresses for authorized users.

A DHCP server identifies the MAC address of a client based on the client hardware address (CHADDR) field in the DHCP Request message. If an attacker continuously applies for IP addresses by changing the CHADDR field, IP addresses in the address pool on the DHCP server may be exhausted. As a result, authorized users cannot obtain IP addresses.

Figure 13-4 Defense against DHCP server DoS attacks

Solution

To prevent the DHCP server DoS attacks, set the maximum number of access DHCP clients allowed on the device or an interface after enabling DHCP snooping on the device. When the number of DHCP clients reaches the maximum value, DHCP clients cannot obtain the IP address through the device or interface.

You can enable the device to check whether the MAC address in the Ethernet frame header matches the CHADDR field in the DHCP message. If the two values match, the message is forwarded; if not, the message is discarded.