818 CHAPTER 58: DHCP RELAY AGENT CONFIGURATION
simply conveys the message to the DHCP server, thus it does not remove the IP
address from its bindings. To solve this, the DHCP relay agent can update dynamic
bindings at a specified interval.
The DHCP relay agent uses the IP address of a client and the MAC address of the
DHCP relay interface to regularly send a DHCP-REQUEST message to the DHCP
server.
■ If the server returns a DHCP-ACK message or does not return any message
within a specified interval, which means the IP address is assignable now, the
DHCP relay agent will update its bindings by aging out the binding entry of the
IP address.
■ If the server returns a DHCP-NAK message, which means the IP address is still
in use, the relay agent will not age it out.
Follow these steps to configure dynamic binding update interval:
Enabling unauthorized DHCP servers detection
There are unauthorized DHCP servers on networks, which reply DHCP clients with
wrong IP addresses.
With this feature enabled, upon receiving a DHCP request, the DHCP relay agent
will record the IP address of the DHCP server which assigned an IP address to the
DHCP client and the receiving interface. The administrator can use this
information to check out any DHCP unauthorized servers.
Follow these steps to enable unauthorized DHCP server detection:
n
With the unauthorized DHCP server detection enabled, the device puts a record
once for each DHCP server. The administrator needs to find unauthorized DHCP
servers from the log information. After the recorded information of a DHCP server
is cleared, a new record will be put for the DHCP server.
Configuring the DHCP
Relay Agent to Support
Option 82
Prerequisites
You need to complete the following tasks before configuring the DHCP relay
agent to support Option 82.
■ Enabling DHCP
To do… Use the command… Remarks
Enter system view system-view -
Configure binding
update interval
dhcp relay security tracker
{ interval | auto }
Optional
auto by default. (auto interval is
calculated by the relay agent
according to the number of
bindings.)
To do… Use the command… Remarks
Enter system view system-view -
Enable unauthorized DHCP
server detection
dhcp relay server-detect Required
Disabled by default.
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