Manual

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Chapter 27 DHCP Snooping Configuration
27.1 Introduction to DHCP Snooping
DHCP Snooping means that the switch monitors the IP-getting process of DHCP CLIENT via
DHCP protocol. It prevents DHCP attacks and illegal DHCP SERVER by setting trust ports and
untrust ports. And the DHCP messages from trust ports can be forwarded without being
verified. In typical settings, trust ports are used to connect DHCP SERVER or DHCP RELAY
Proxy, and untrust ports are used to connect DHCP CLINET. The switch will forward the DCHP
request messages from untrust ports, but not DHCP reply ones. If any DHCP reply messages
is received from a untrust port, besides giving an alarm, the switch will also implement
designated actions on the port according to settings, such as “shutdown”, or distributing a
“blackhole”. If DHCP Snooping binding is enabled, the switch will save binding information
(including its MAC address, IP address, IP lease, VLAN number and port number) of each
DHCP CLINET on untrust ports in DHCP snooping binding table With such information, DHCP
Snooping can combine modules like dot1x and ARP, or implement user-access-control
independently.
Defense against Fake DHCP Server: once the switch intercepts the DHCP Server reply
packets(including DHCPOFFER, DHCPACK, and DHCPNAK) , it will alarm and respond
according to the situation(shutdown the port or send Black hole) .
Defense against DHCP over load attacks: To avoid too many DHCP messages attacking
CPU, users should limit the DHCP speed of receiving packets on trusted and non-trusted ports.
Record the binding data of DHCP: DHCP SNOOPING will record the binding data allocated
by DHCP SERVER while forwarding DHCP messages, it can also upload the binding data to
the specified server to backup it. The binding data is mainly used to configure the dynamic
users of dot1x user based ports. Please refer to the chapter called“dot1x configuration” to find
more about the usage of dot1x use-based mode.
Add binding ARP: DHCP SNOOPING can add static binding ARP according to the binding
data after capturing binding data, thus to avoid ARP cheating.
Add trusted users: DHCP SNOOPING can add trusted user list entries according to the
parameters in binding data after capturing binding data; thus these users can access all
resources without DOT1X authentication.
Automatic Recovery: A while after the switch shut down the port or send blockhole, it should
automatically recover the communication of the port or source MAC and send information to
Log Server via syslog.
LOG Function: When the switch discovers abnormal received packets or automatically
recovers, it should send syslog information to Log Server.