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Fat AP and Cloud AP V200R008C00 CLI-based Configuration Guide

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Huawei uses machine translation combined with human proofreading to translate this document to different languages in order to help you better understand the content of this document. Note: Even the most advanced machine translation cannot match the quality of professional translators. Huawei shall not bear any responsibility for translation accuracy and it is recommended that you refer to the English document (a link for which has been provided).
Understanding DHCP Snooping

Understanding DHCP Snooping

DHCP Snooping Fundamentals

DHCP snooping provides the trusted interface and listening functions.

Trusted Interface

DHCP snooping involves two interface roles: trusted interface and untrusted interfaces, through which DHCP snooping ensures that DHCP clients obtain IP addresses from a valid DHCP server.

If a bogus DHCP server exists on a network, DHCP clients may obtain incorrect IP addresses and network configuration parameters from it, leading to communication failures. The trusted interface controls the source of DHCP Reply messages to prevent bogus DHCP servers from assigning IP addresses and other configurations to other DHCP clients.

Trusted interface and untrusted interfaces process DHCP messages as follows:
  • The device receives DHCP ACK messages, NAK messages, and Offer messages through the trusted interface.
  • The device discards DHCP ACK messages, NAK messages, and Offer messages on untrusted interfaces.
NOTE:

The administrator configures the interface directly or indirectly connected to an authorized DHCP server as the trusted interface, and other interfaces as untrusted interfaces. This ensures that DHCP clients obtain IP addresses from authorized DHCP servers.

Listening

After DHCP snooping is enabled, the device generates a DHCP snooping binding table by listening to DHCP Request messages and Reply messages. A binding entry contains the MAC address, IP address, interface number, and Virtual Local Area Network (VLAN) ID of the DHCP client.

The DHCP snooping binding entries are aged out when the DHCP release expires, or the entries are deleted when users send DHCP Release packets to release IP addresses.

The administrator needs to record IP addresses of DHCP clients and identify the mappings between the IP addresses and MAC addresses of the DHCP clients. The DHCP snooping binding table helps the administrator conveniently record the mappings.

The DHCP snooping binding table records the mapping between IP addresses and MAC addresses of DHCP clients. The device can check DHCP messages against the DHCP snooping binding table to prevent attacks initiated by unauthorized users.

To ensure that the device obtains parameters such as MAC addresses for generating a DHCP snooping binding table, configure DHCP snooping on the Layer 2 access devices or the first DHCP relay agent from the device to the DHCP server.

Option 82 Supported by DHCP Snooping

Overview

In the traditional method of dynamically allocating IP addresses, a DHCP server cannot determine the DHCP client location from the received DHCP Request message. As a result, DHCP clients in the same VLAN have the same rights to access network resources. This brings challenges in security control because the network administrator cannot control network access of clients in the same VLAN.

RFC 3046 defines the DHCP Relay Agent Information Option (Option 82), which 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. In this case, the DHCP server assigns an IP address and other configurations to the DHCP client, implementing security control over the DHCP client.

The Option 82 field contains two commonly used suboptions: circuit ID and remote ID. The circuit ID distinguishes the VLAN ID and interface number of a client, and the remote ID distinguishes the MAC address of the device to which the client connects.

When functioning as a DHCP relay agent, a device supports the Option 82 field regardless of whether DHCP snooping is enabled. However, when functioning as an access device on a Layer 2 network, the device supports the Option 82 field only after DHCP snooping is enabled.

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.

The Option 82 field is different from parameters recorded in a DHCP snooping binding table. The device adds the Option 82 field to the DHCP Request message when a DHCP client requests an IP address (at this time, the client does not have an IP address). However, a DHCP snooping binding entry is generated based on the DHCP ACK messages received from the DHCP server (at this time, an IP address has been assigned to the client).

Implementation
A device uses the following modes to insert the Option 82 field into a DHCP Request message:
  • Insert mode: Upon receiving a DHCP Request message without the Option 82 field, the device inserts the Option 82 field. If the DHCP Request message contains the Option 82 field, the device checks whether the Option 82 field contains a remote ID. If a remote ID is present, the device retains the Option 82 field. Otherwise, the device inserts a remote ID.

  • Rebuild mode: Upon receiving a DHCP Request message without the Option 82 field, the device inserts the Option 82 field. If the DHCP Request message contains the Option 82 field, the device deletes the original Option 82 field and inserts the Option 82 field set by the administrator.

The device handles reply messages from the DHCP server in the same way regardless of whether the Insert or Rebuild mode is used.

  • If the DHCP Request messages received by the device do not contain the Option 82 field, the device deletes the Option 82 field from the DHCP Reply messages and then forwards them to the DHCP client.
  • If the DHCP Request messages contain the Option 82 field, the device changes the Option 82 format in the DHCP Reply messages to that in the DHCP Request messages and then forwards the reply messages to the DHCP client.
  • If the DHCP Reply messages do not contain the Option 82 field, the device directly forwards the messages.
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Updated: 2019-01-11

Document ID: EDOC1000176006

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