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The Cisco BGP router must be configured to reject inbound route advertisements for any Bogon prefixes.


Overview

Finding ID Version Rule ID IA Controls Severity
V-96761 CISC-RT-000490 SV-105899r1_rule Medium
Description
Accepting route advertisements for Bogon prefixes can result in the local autonomous system (AS) becoming a transit for malicious traffic as it will in turn advertise these prefixes to neighbor autonomous systems.
STIG Date
Cisco IOS XR Router RTR Security Technical Implementation Guide 2019-07-24

Details

Check Text ( C-95597r1_chk )
Review the router configuration to verify that it will reject BGP routes for any Bogon prefixes.

Step 1: verify that an inbound route policy has been configured for each external neighbor as shown in the example below.

router bgp n
address-family ipv4 unicast
!
neighbor x.1.23.3
remote-as y
keychain YYY_KEY_CHAIN
ttl-security
address-family ipv4 unicast
route-policy BGP_FILTER in
!
!
neighbor x.1.24.4
remote-as z
keychain ZZZ_KEY_CHAIN
ttl-security
address-family ipv4 unicast
route-policy BGP_FILTER in
!
!

Step 2: Review the route policy to determine if it is filtering at a minimum BOGON prefixes as shown in the example below.

route-policy BGP_FILTER
if destination in BOGON_PREFIXES then
drop
else
pass
endif
end-policy

Step 3: Review the prefix set referenced in the route policy above has been configured containing the current Bogon prefixes as shown in the example below.

prefix-set BOGON_PREFIXES
0.0.0.0/8 le 32,
10.0.0.0/8 le 32,
100.64.0.0/10 le 32,
127.0.0.0/8 le 32,
169.254.0.0/16 le 32,
172.16.0.0/12 le 32,
192.0.2.0/24 le 32,
192.88.99.0/24 le 32,
192.168.0.0/16 le 32,
198.18.0.0/15 le 32,
198.51.100.0/24 le 32,
203.0.113.0/24 le 32,
240.0.0.0/4 le 32,
224.0.0.0/4 le 32
end-set

If the router is not configured to reject inbound route advertisements for any Bogon prefixes, this is a finding.
Fix Text (F-102439r1_fix)
Configure the router to reject inbound route advertisements for any Bogon prefixes.

Step 1: Configure a prefix set containing the current Bogon prefixes as shown below.

RP/0/0/CPU0:R2(config)#prefix-set BOGON_PREFIXES
RP/0/0/CPU0:R2(config-pfx)#0.0.0.0/8 le 32,
RP/0/0/CPU0:R2(config-pfx)#10.0.0.0/8 le 32,
RP/0/0/CPU0:R2(config-pfx)#100.64.0.0/10 le 32,
RP/0/0/CPU0:R2(config-pfx)#127.0.0.0/8 le 32,
RP/0/0/CPU0:R2(config-pfx)#169.254.0.0/16 le 32,
RP/0/0/CPU0:R2(config-pfx)#172.16.0.0/12 le 32,
RP/0/0/CPU0:R2(config-pfx)#192.0.2.0/24 le 32,
RP/0/0/CPU0:R2(config-pfx)#192.88.99.0/24 le 32,
RP/0/0/CPU0:R2(config-pfx)#192.168.0.0/16 le 32,
RP/0/0/CPU0:R2(config-pfx)#198.18.0.0/15 le 32,
RP/0/0/CPU0:R2(config-pfx)#198.51.100.0/24 le 32,
RP/0/0/CPU0:R2(config-pfx)#203.0.113.0/24 le 32,
RP/0/0/CPU0:R2(config-pfx)#240.0.0.0/4 le 32,
RP/0/0/CPU0:R2(config-pfx)#224.0.0.0/4 le 32
RP/0/0/CPU0:R2(config-pfx)#end-set

Step 2: Configure the route policy to drop routes with BOGON prefixes as shown in the example below.

RP/0/0/CPU0:R2(config)#route-policy BGP_FILTER
RP/0/0/CPU0:R2(config-rpl)#if destination in BOGON_PREFIXES then
RP/0/0/CPU0:R2(config-rpl-if)#drop
RP/0/0/CPU0:R2(config-rpl-if)#else pass endif
RRP/0/0/CPU0:R2(config-rpl)#end-policy
RP/0/0/CPU0:R2(config)#exit

Step 3: Apply the route policy to each external BGP neighbor as shown in the example.

RP/0/0/CPU0:R2(config)#router bgp xx
RP/0/0/CPU0:R2(config-bgp)#neighbor x.1.23.3
RP/0/0/CPU0:R2(config-bgp-nbr)#address-family ipv4 unicast
RP/0/0/CPU0:R2(config-bgp-nbr-af)#route-policy BGP_FILTER in
RP/0/0/CPU0:R2(config-bgp)#neighbor x.1.24.4
RP/0/0/CPU0:R2(config-bgp-nbr)#address-family ipv4 unicast
RP/0/0/CPU0:R2(config-bgp-nbr-af)#route-policy BGP_FILTER in