Dell PowerSwitch S4820T Configuration Guide for the S4820T System 9.14.1.0 - Page 309

Dell(conf-if-vl-4)# ip helper-address vrf vrf1 100.0.0.1 Dell(conf-if-vl-4)# ipv6 helper-address vrf vrf1 100::1 Dell(conf-if-vl-4)# ip dhcp relay source-interface loopback 3 Dell(conf-if-vl-4)# ipv6 dhcp relay source-interface loopback 3 3 In the below configuration, the DHCP relay source interface is not configured in the VLAN interface. So, the DHCP relay uses the configured global DHCP relay source interface to forward the packets from the DHCP client to server. Dell(conf)# interface Vlan 5 Dell(conf-if-vl-4)# ip vrf forwarding vrf1 Dell(conf-if-vl-4)# ip address 4.0.0.1/24 Dell(conf-if-vl-4)# ipv6 address 4::1/64 Dell(conf-if-vl-4)# tagged TenGigE 1/4 Dell(conf-if-vl-4)# ip helper-address vrf vrf1 100.0.0.1 Dell(conf-if-vl-4)# ipv6 helper-address vrf vrf1 100::1 Configure Secure DHCP DHCP as defined by RFC 2131 provides no authentication or security mechanisms. Secure DHCP is a suite of features that protects networks that use dynamic address allocation from spoofing and attacks. • Option 82 • DHCP Snooping • Dynamic ARP Inspection • Source Address Validation Option 82 RFC 3046 (the relay agent information option, or Option 82) is used for class-based IP address assignment. The code for the relay agent information option is 82, and is comprised of two sub-options, circuit ID and remote ID. Circuit ID Remote ID This is the interface on which the client-originated message is received. This identifies the host from which the message is received. The value of this sub-option is the MAC address of the relay agent that adds Option 82. The DHCP relay agent inserts Option 82 before forwarding DHCP packets to the server. The server can use this information to: • track the number of address requests per relay agent. Restricting the number of addresses available per relay agent can harden a server against address exhaustion attacks. • 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. Dynamic Host Configuration Protocol (DHCP) 309