Finding ID | Version | Rule ID | IA Controls | Severity |
---|---|---|---|---|
V-216655 | CISC-RT-000170 | SV-216655r531086_rule | Medium |
Description |
---|
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. |
STIG | Date |
---|---|
Cisco IOS XE Router RTR Security Technical Implementation Guide | 2021-03-26 |
Check Text ( C-17888r287922_chk ) |
---|
Review the configuration to verify the no ip unreachables command has been configured on all external interfaces as shown in the configuration example below: interface GigabitEthernet0/1 ip address x.x.x.x 255.255.255.0 no ip unreachables If ICMP unreachable notifications are sent from any external or null0 interface, this is a finding. Alternative – DODIN Backbone: Verify that the PE router is configured to rate limit ICMP unreachable messages as shown in the example below: ip icmp rate-limit unreachable 60000 ip icmp rate-limit unreachable DF 1000 Note: In the example above, packet-too-big message (ICMP Type 3 Code 4) can be sent once every second, while all other destination unreachable messages can be sent once every minute. This will avoid disrupting Path MTU Discovery for traffic traversing the backbone while mitigating the risk of an ICMP unreachable DoS attack. IF the PE router is not configured to rate limit ICMP unreachable messages, this is a finding. |
Fix Text (F-17886r287923_fix) |
---|
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 |