The Cisco router must be configured to have Internet Control Message Protocol (ICMP) unreachable messages disabled on all external interfaces.

An XCCDF Rule

Description

<VulnDiscussion>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.</VulnDiscussion><FalsePositives></FalsePositives><FalseNegatives></FalseNegatives><Documentable>false</Documentable><Mitigations></Mitigations><SeverityOverrideGuidance></SeverityOverrideGuidance><PotentialImpacts></PotentialImpacts><ThirdPartyTools></ThirdPartyTools><MitigationControl></MitigationControl><Responsibility></Responsibility><IAControls></IAControls>

ID: SV-216565r856184_rule
Severity: Medium
References: CCI-002385
Updated: 9 months ago

Step 1: Disable ip unreachables on all external interfaces.

R4(config)#int g0/1
R4(config-if)#no ip unreachables

Step 2: Disable ip unreachables on the Null0 interface if it is used to backhole packets.
R4(config-if)#int null 0
R4(config-if)#no ip unreachables

Alternative – DODIN Backbone:

Configure the PE router to rate limit ICMP unreachable messages as shown in the example below:

R4(config)#ip icmp rate-limit unreachable df 100
R4(config)#ip icmp rate-limit unreachable 100000
R4(config)#end

Alternative – Non DODIN Backbone. 

An alternative for non-backbone networks (i.e. enclave, base, camp, etc.) is to filter messages generated by the router and silently drop ICMP Administratively Prohibited and Host Unreachable messages using the following configuration steps:  

Step 1: Configure ACL to include ICMP Type 3 Code 1 (Host Unreachable) and Code 13 (Administratively Prohibited) as shown in the example below:

R2(config)#ip access-list ext ICMP_T3C1C13
R2(config-ext-nacl)#permit icmp any any host-unreachable
R2(config-ext-nacl)#permit icmp any any administratively-prohibited
R2(config-ext-nacl)#exit

Step 2: Create a route map to forward these ICMP messages to the Null0 interface.
 
R2(config)#route-map LOCAL_POLICY
R2(config-route-map)#match ip address ICMP_T3C1C13
R2(config-route-map)#set interface Null0
R2(config-route-map)#exit

Step 3: Configure no ip unreachables on the Null0 interface.

R2(config)#int null 0
R2(config-if)#no ip unreachables
R2(config-if)#exit

Step 4: Apply the policy to filter messages generated by the router.

R2(config)#ip local policy route-map LOCAL_POLICY
R2(config)#end

The Cisco router must be configured to have Internet Control Message Protocol (ICMP) unreachable messages disabled on all external interfaces.

Description

Remediation - Manual Procedure