Title

The Cisco ACI layer 2 switch must have DHCP snooping for all user VLANs to validate DHCP messages from untrusted sources. (Cat II impact)

Discussion

In an enterprise network, devices under administrative control are trusted sources. These devices include the switches, routers, and servers in the network. Host ports and unknown DHCP servers are considered untrusted sources. An unknown DHCP server on the network on an untrusted port is called a spurious DHCP server, any device (PC, Wireless Access Point) that is loaded with DHCP server enabled. The DHCP snooping feature determines whether traffic sources are trusted or untrusted. The potential exists for a spurious DHCP server to respond to DHCPDISCOVER messages before the real server has time to respond. DHCP snooping allows switches on the network to trust the port a DHCP server is connected to and not trust the other ports. The DHCP snooping feature validates DHCP messages received from untrusted sources and filters out invalid messages as well as rate-limits DHCP traffic from trusted and untrusted sources. DHCP snooping feature builds and maintains a binding database, which contains information about untrusted hosts with leased IP addresses, and it uses the database to validate subsequent requests from untrusted hosts. Other security features, such as IP Source Guard and Dynamic Address Resolution Protocol (ARP) Inspection (DAI), also use information stored in the DHCP snooping binding database. Hence, it is imperative that the DHCP snooping feature is enabled on all VLANs.

Check Content

In the Cisco APIC GUI, navigate to Tenants and repeat the following steps for all tenants: 1. On the menu bar, click Tenants >> Tenant_name. 2. In the Navigation pane, click Policies >> Protocol >> First Hop Security. 3. Expand Feature Policy and verify DHCP Inspection is enabled and is enabled for both IPv4 and IPv6. If the switch does not have DHCP snooping enabled for all access switch ports, this is a finding.

Fix Text

In the Cisco APIC GUI, navigate to Tenants and repeat the following steps for all tenants: 1. On the menu bar, click Tenants >> Tenant_name. 2. In the Navigation pane, click Policies >> Protocol >> First Hop Security. 3. Right-click "First Hop Security" to open Create Feature Policy and fill out the form. - Check the "DHCP Inspection" option box. - Enable for both IPv4 and IPv6. 4. Click "Submit".

Additional Identifiers

Rule ID: SV-272034r1064437_rule

Vulnerability ID: V-272034

Group Title: SRG-NET-000362-L2S-000025

Expert Comments

CCIs tied to check.

Controls tied to check. These are derived from the CCIs shown above.