Dell PowerSwitch S4820T 9.50.1 Configuration Guide for the S4820T System - Page 327
DHCP Snooping, Introduced in Dell Networking OS version 7.8.1.0
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• associate client MAC addresses with a relay agent to prevent offering an IP address to a client spoofing the same MAC address on a different relay agent. • assign IP addresses according to the relay agent. This prevents generating DHCP offers in response to requests from an unauthorized relay agent. The server echoes the option back to the relay agent in its response, and the relay agent can use the information in the option to forward a reply out the interface on which the request was received rather than flooding it on the entire VLAN. The relay agent strips Option 82 from DHCP responses before forwarding them to the client. To insert Option 82 into DHCP packets, follow this step. • Insert Option 82 into DHCP packets. CONFIGURATION mode ip dhcp relay information-option [trust-downstream] For routers between the relay agent and the DHCP server, enter the trust-downstream option. • Manually reset the remote ID for Option 82. CONFIGURATION mode ip dhcp relay information-option remote-id DHCP Snooping DHCP snooping protects networks from spoofing. In the context of DHCP snooping, ports are either trusted or not trusted. By default, all ports are not trusted. Trusted ports are ports through which attackers cannot connect. Manually configure ports connected to legitimate servers and relay agents as trusted. When you enable DHCP snooping, the relay agent builds a binding table - using DHCPACK messages - containing the client MAC address, IP addresses, IP address lease time, port, VLAN ID, and binding type. Every time the relay agent receives a DHCPACK on a trusted port, it adds an entry to the table. The relay agent checks all subsequent DHCP client-originated IP traffic (DHCPRELEASE, DHCPNACK, and DHCPDECLINE) against the binding table to ensure that the MAC-IP address pair is legitimate and that the packet arrived on the correct port. Packets that do not pass this check are forwarded to the server for validation. This checkpoint prevents an attacker from spoofing a client and declining or releasing the real client's address. Server-originated packets (DHCPOFFER, DHCPACK, and DHCPNACK) that arrive on a not trusted port are also dropped. This checkpoint prevents an attacker from acting as an imposter as a DHCP server to facilitate a man-in-the-middle attack. Binding table entries are deleted when a lease expires, or the relay agent encounters a DHCPRELEASE, DHCPNACK, or DHCPDECLINE. Dell Networking OS Behavior: Introduced in Dell Networking OS version 7.8.1.0, DHCP snooping was available for Layer 3 only and dependent on DHCP relay agent (ip helper-address). Dell Networking OS version 8.2.1.0 extends DHCP snooping to Layer 2 and you do not have to enable relay agent to snoop on Layer 2 interfaces. Dell Networking OS Behavior: Binding table entries are deleted when a lease expires or when the relay agent encounters a DHCPRELEASE. Line cards maintain a list of snooped VLANs. When the binding table Dynamic Host Configuration Protocol (DHCP) 327
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