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

Overview

Finding ID	Version	Rule ID	IA Controls	Severity
V-96699	CISC-RT-000170	SV-105837r1_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 XR Router RTR Security Technical Implementation Guide	2019-12-20

Details

Check Text ( C-95535r1_chk )
Review the configuration to verify the ipv4 unreachables disable command has been configured on all external interfaces as shown in the configuration example below. interface GigabitEthernet0/0/0/1 ipv4 address x.11.1.2 255.255.255.252 ipv4 unreachables disable Note: ICMP unreachables must also be configured. On the Null0 interface if it is used to black hole traffic. 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 ipv4 rate-limit unreachable 60000 ip icmp ipv4 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 denial of service attack. IF the PE router is not configured to rate limit ICMP unreachable messages, this is a finding.

Check Text ( C-95535r1_chk )

Review the configuration to verify the ipv4 unreachables disable command has been configured on all external interfaces as shown in the configuration example below.

interface GigabitEthernet0/0/0/1
ipv4 address x.11.1.2 255.255.255.252
ipv4 unreachables disable

Note: ICMP unreachables must also be configured. On the Null0 interface if it is used to black hole traffic.

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 ipv4 rate-limit unreachable 60000
ip icmp ipv4 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 denial of service attack.

IF the PE router is not configured to rate limit ICMP unreachable messages, this is a finding.

Fix Text (F-102375r1_fix)
Disable ip unreachables on all external interfaces as shown below. RP/0/0/CPU0:R3(config)#int g0/0/0/1 RP/0/0/CPU0:R3(config-if)#ipv4 unreachables disable Alternative – DODIN Backbone Configure the PE router to rate limit ICMP unreachable messages as shown in the example below. RP/0/0/CPU0:R3(config)#icmp ipv4 rate-limit unreachable df 1000 RP/0/0/CPU0:R3(config)#icmp ipv4 rate-limit unreachable 60000 RP/0/0/CPU0:R3(config)#end

Fix Text (F-102375r1_fix)

Disable ip unreachables on all external interfaces as shown below.

RP/0/0/CPU0:R3(config)#int g0/0/0/1
RP/0/0/CPU0:R3(config-if)#ipv4 unreachables disable

Alternative – DODIN Backbone

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

RP/0/0/CPU0:R3(config)#icmp ipv4 rate-limit unreachable df 1000
RP/0/0/CPU0:R3(config)#icmp ipv4 rate-limit unreachable 60000
RP/0/0/CPU0:R3(config)#end