Chapter 12
| Security Measures
DHCP Snooping
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If the DHCP packet is from a client, such as a DISCOVER, REQUEST,
INFORM, DECLINE or RELEASE message, the packet is forwarded if MAC
address verification is disabled. However, if MAC address verification is
enabled, then the packet will only be forwarded if the client’s hardware
address stored in the DHCP packet is the same as the source MAC
address in the Ethernet header.
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If the DHCP packet is not a recognizable type, it is dropped.
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If a DHCP packet from a client passes the filtering criteria above, it will only
be forwarded to trusted ports in the same VLAN.
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If a DHCP packet is from server is received on a trusted port, it will be
forwarded to both trusted and untrusted ports in the same VLAN.
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If the DHCP snooping is globally disabled, all dynamic bindings are
removed from the binding table.
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Additional considerations when the switch itself is a DHCP client – The port(s)
through which the switch submits a client request to the DHCP server must
be configured as trusted. Note that the switch will not add a dynamic entry
for itself to the binding table when it receives an ACK message from a DHCP
server. Also, when the switch sends out DHCP client packets for itself, no
filtering takes place. However, when the switch receives any messages from
a DHCP server, any packets received from untrusted ports are dropped.
DHCP Snooping Option 82
â—† DHCP provides a relay mechanism for sending information about its DHCP
clients or the relay agent itself to the DHCP server. Also known as DHCP Option
82, it allows compatible DHCP servers to use the information when assigning IP
addresses, or to set other services or policies for clients. It is also an effective
tool in preventing malicious network attacks from attached clients on DHCP
services, such as IP Spoofing, Client Identifier Spoofing, MAC Address Spoofing,
and Address Exhaustion.
â—† DHCP Snooping must be enabled for Option 82 information to be inserted into
request packets.
â—† When the DHCP Snooping Information Option 82 is enabled, the requesting
client (or an intermediate relay agent that has used the information fields to
describe itself) can be identified in the DHCP request packets forwarded by the
switch and in reply packets sent back from the DHCP server. This information
may specify the MAC address or IP address of the requesting device (that is, the
switch in this context).
By default, the switch also fills in the Option 82 circuit-id field with information
indicating the local interface over which the switch received the DHCP client
request, including the port and VLAN ID. This allows DHCP client-server
exchange messages to be forwarded between the server and client without
having to flood them to the entire VLAN.
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