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Fat AP and Cloud AP V200R008C00 CLI-based Configuration Guide

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Application Scenarios for DHCP Snooping

Application Scenarios for DHCP Snooping

Defense Against Bogus DHCP Server Attacks


Because DHCP servers and DHCP clients lack authentication mechanisms between them, each DHCP server newly configured on a network assigns IP addresses and other network parameters to DHCP clients. If the assigned IP addresses and other network parameters are incorrect, errors may occur on the network.

In Figure 26-12, authorized and unauthorized DHCP servers can receive DHCP Discover messages broadcast by DHCP clients.

Figure 26-12  DHCP client sending DHCP Discover messages

If a bogus DHCP server sends a bogus DHCP Reply message with an incorrect gateway address, Domain Name System (DNS) server address, or IP address to a DHCP client, as shown in Figure 26-13, the DHCP client cannot obtain the correct IP address and required information. The authorized user then fails to access the network and user information security is compromised.

Figure 26-13  Bogus DHCP server attack


To prevent attacks from a bogus DHCP server, configure the trusted interface and untrusted interfaces on the device.

You can configure the interface directly or indirectly connected to the authorized DHCP server as the trusted interface and other interfaces as untrusted interfaces. The device then discards DHCP Reply messages received on untrusted interfaces, preventing bogus DHCP server attacks, as shown in Figure 26-14.

Figure 26-14  Trusted interface and untrusted interfaces

Defense Against DHCP Flood Attacks


On a DHCP network, if a large number of DHCP messages are sent to the device within a short period of time, device performance may deteriorate, preventing the device from working properly. This kind of attack is known as a DHCP flood attack.


To prevent DHCP flood attacks, enable DHCP snooping and enable the device to check the rate at which DHCP messages are sent to the processing unit. The device then limits the rate at which it sends DHCP messages to the processing unit and discards those that exceed the rate.

Defense Against Bogus DHCP Message Attacks


An authorized DHCP client that has obtained an IP address sends a DHCP Request message or Release message to extend the lease time or to release the IP address. If an attacker continuously sends DHCP Request messages to the DHCP server to extend the lease time, the IP address cannot be reclaimed or obtained by authorized users. If an attacker sends forged DHCP Release messages to the DHCP server, the authorized user may be disconnected.


To prevent bogus DHCP message attacks, use the DHCP snooping binding table. The device checks DHCP Request messages and Release messages against binding entries to determine whether the messages are valid. Specifically, the device checks whether the VLAN IDs, IP addresses, MAC addresses, and interface IDs in DHCP messages match binding entries. If a message matches a binding entry, the device forwards the message. Otherwise, the device discards the message.

Defense Against DHCP Server DoS Attacks


In Figure 26-15, if a large number of attackers request IP addresses on interface1, IP addresses in the IP address pool are exhausted. This results in no IP addresses being available 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 value of 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 26-15  Defense against DHCP server DoS attacks


To prevent DHCP server DoS attacks, enable DHCP snooping on the device and then set the maximum number of access DHCP clients allowed on the device or an interface. Only the allowed number of DHCP clients can obtain an 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 value of the CHADDR field in the DHCP message. If the two values match, the message is forwarded. Otherwise, the message is discarded.

Typical Application of the Option 82 Field

The DHCP Relay Agent Information Option (Option 82) field records the location of a DHCP client. A DHCP snooping-enabled device or a DHCP relay agent inserts the Option 82 field into a DHCP Request message to notify the DHCP server of the DHCP client location. Based on the Option 82 field, the DHCP server can properly assign an IP address and other configurations to the DHCP client, facilitating DHCP client security.

Figure 26-16  Application of the Option 82 field

In Figure 26-16, clients use DHCP to obtain IP addresses. To improve network security, the administrator configures the device to control network access of clients connected to Interface1.

The DHCP server cannot detect the DHCP client location based only on the DHCP Request message. As a result, users in the same VLAN have the same rights to access network resources.

To address this issue, the administrator can enable the Option 82 insertion function after DHCP snooping is enabled on AP1. Upon receiving a DHCP Request message, AP1 inserts the Option 82 field in the message to notify the DHCP server of the DHCP client location. The Option 82 field contains the MAC address, VLAN ID, and interface number of the client. The DHCP server can properly assign an IP address and other configurations to the client based on the IP address assignment or security policies on the server.

The Option 82 field records only the location of a DHCP client and is encapsulated in a DHCP Request message sent to the DHCP server. To deploy different IP address assignment or security policies for different clients, the DHCP server must support the Option 82 field and be configured with IP address assignment or security policies.

Updated: 2019-01-11

Document ID: EDOC1000176006

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