The NSX Tier-0 Gateway router must be configured to have Internet Control Message Protocol (ICMP) unreachable notifications disabled on all external interfaces.

The ICMP supports IP traffic by relaying information about paths, routes, and network conditions. Routers automatically send ICMP messages under a wide variety of conditions. Host unreachable ICMP messages are commonly used by attackers for network mapping and diagnosis.

If the Tier-0 Gateway is deployed in an Active/Active HA mode, this is Not Applicable. From the NSX Manager web interface, go to Security >> Policy Management >> Gateway Firewall >> Gateway Specific Rules, and choose each Tier-0 Gateway in the drop-down. Review each Tier-0 Gateway Firewall rule to verify one exists to drop ICMP unreachable messages. If a rule does not exist to drop ICMP unreachable messages, this is a finding.

To configure a shared rule to drop ICMP unreachable messages, do the following: From the NSX Manager web interface, go to Security >> Policy Management >> Gateway Firewall >> All Shared Rules. Click "Add Rule" (add a policy first, if needed) and under "Services", select "ICMP Destination Unreachable" and "Apply". Enable logging and under the "Applied To" field select the target Tier-0 gateways and click "Publish" to enforce the new rule. Note: A rule can also be created under Gateway Specific Rules to meet this requirement.