Enabling Dhcp Starvation Attack Protection; Configuring The Dhcp Relay Agent To Release An Ip Address - HPE FlexNetwork 10500 Series Configuration Manual

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 might also fail to work because of exhaustion of system resources. The following methods are
available to relieve or prevent such attacks.
•
To relieve a DHCP starvation attack that uses DHCP packets encapsulated with different
source MAC addresses, you can use one of the following methods:
Limit the number of ARP entries that a Layer 3 interface can learn.

Set the MAC learning limit on a Layer 2 port, and disable unknown frame forwarding when

the MAC learning limit is reached.
•
To prevent a DHCP starvation attack that uses DHCP requests encapsulated with the same
source MAC address, you can enable MAC address check on the DHCP relay agent. The
DHCP relay agent compares the chaddr field of a received DHCP request with the source MAC
address in the frame header. If they are the same, the DHCP relay agent forwards the request
to the DHCP server. If not, the relay agent discards the request.
Enable MAC address check only on the DHCP relay agent directly connected to the DHCP clients. A
DHCP relay agent changes the source MAC address of DHCP packets before sending them.
A MAC address check entry has an aging time. When the aging time expires, both of the following
occur:
•
The entry ages out.
•
The DHCP relay agent rechecks the validity of DHCP requests sent from the MAC address in
the entry.
To enable MAC address check:
Step
1. Enter system view.
2. Set the aging time for MAC
address check entries.
3. Enter the interface view.
4. Enable MAC address check.
Configuring the DHCP relay agent to release an
IP address
Configure the relay agent to release the IP address for a relay entry. The relay agent sends a
DHCP-RELEASE message to the server and meanwhile deletes the relay entry. Upon receiving the
DHCP-RELEASE message, the DHCP server releases the IP address.
To configure the DHCP relay agent to release an IP address:
Command
system-view
dhcp relay check mac-address
aging-time time
interface interface-type
interface-number
dhcp relay check mac-address
72
Remarks
N/A
The default aging time is 30
seconds.
This command takes effect
only after you execute the
dhcp relay check
mac-address command.
N/A
By default, MAC address
check is disabled.