Enabling Unauthorized Dhcp Server Detection; Enabling Dhcp Starvation Attack Protection - HP A7500 Series Configuration Manual

If the server returns a DHCP-ACK message or does not return any message within a specified
•
interval, the DHCP relay agent ages out the entry.
• If the server returns a DHCP-NAK message, the relay agent keeps the entry.
Follow these steps to configure periodic refresh of dynamic client entries:
To do...
Enter system view
Enable periodic refresh of
dynamic client entries
Configure the refresh interval

Enabling unauthorized DHCP server detection

Unauthorized DHCP servers may assign wrong IP addresses to DHCP clients.
With unauthorized DHCP servers detection enabled, the DHCP relay agent checks whether a request
contains Option 54 (Server Identifier Option). If yes, the DHCP relay agent records the IP address of
each detected DHCP server that assigned an IP address to a requesting DHCP client in the option, and
records the receiving interface. The administrator can use this information to check for unauthorized
DHCP servers.
Follow these steps to enable unauthorized DHCP server detection:
To do...
Enter system view
Enable unauthorized DHCP server
detection
NOTE:
The relay agent logs a DHCP server only once.

Enabling DHCP starvation attack protection

A DHCP starvation attack occurs when an attacker constantly sends forged DHCP requests using
different MAC addresses in the chaddr field to a DHCP server. This exhausts the IP address resources of
the DHCP server so legitimate DHCP clients cannot obtain IP addresses. The DHCP server may also fail
to work because of exhaustion of system resources.
To relieve a DHCP starvation attack that uses DHCP packets encapsulated with different source
•
MAC addresses, you can limit the number of ARP entries that a Layer 3 interface can learn or MAC
addresses that a Layer 2 port can learn. You can also configure an interface that has learned the
maximum MAC addresses to discard packets whose source MAC addresses are not in the MAC
address table.
• To prevent a DHCP starvation attack that uses DHCP requests encapsulated with the same source
MAC address, enable MAC address check on the DHCP relay agent. With this function enabled,
the DHCP relay agent compares the chaddr field of a received DHCP request with the source MAC
Use the command...
system-view
dhcp relay security refresh
enable
dhcp relay security tracker
{ interval | auto }
Use the command...
system-view
dhcp relay server-detect
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Remarks
—
Optional
Enabled by default.
Optional
auto by default. (auto interval is calculated
by the relay agent according to the number
of client entries.)
Remarks
—
Required
Disabled by default.