The Cisco perimeter router must be configured to enforce approved authorizations for controlling the flow of information between interconnected networks in accordance with applicable policy.

Information flow control regulates authorized information to travel within a network and between interconnected networks. Controlling the flow of network traffic is critical so it does not introduce any unacceptable risk to the network infrastructure or data. An example of a flow control restriction is blocking outside traffic claiming to be from within the organization. For most routers, internal information flow control is a product of system design.

Review the router configuration to verify that ACLs are configured to allow or deny traffic for specific source and destination addresses as well as ports and protocols. In the example below, the router is peering BGP with DISN. ICMP echo and echo-reply packets are allowed for troubleshooting connectivity. WWW traffic is permitted inbound to the NIPRNet host-facing web server (x.12.1.22).

ipv4 access-list EXTERNAL_ACL_INBOUND 10 permit tcp host x.11.1.1 eq bgp host x.11.1.2 20 permit tcp host x.11.1.1 host x.11.1.2 eq bgp 30 permit icmp host x.11.1.1 host x.11.1.2 echo 40 permit icmp host x.11.1.1 host x.11.1.2 echo-reply 50 deny ipv4 any host x.11.1.1 log 60 permit tcp any host x.12.1.22 eq www 70 permit tcp any any established 80 deny ipv4 any any log-input … … … interface GigabitEthernet0/0/0/1 ipv4 address x.11.1.2 255.255.255.252 ipv4 access-group EXTERNAL_ACL_INBOUND ingress

If the router is not configured to enforce approved authorizations for controlling the flow of information between interconnected networks, this is a finding.

This requirement is not applicable for the DODIN Backbone.

Step 1: Configure an ACL to allow or deny traffic as shown in the example below.

RP/0/0/CPU0:R3(config)#ipv4 access-list EXTERNAL_ACL_INBOUND RP/0/0/CPU0:R3(config-ipv4-acl)#permit tcp host x.11.1.1 eq bgp host x.11.1.2 RP/0/0/CPU0:R3(config-ipv4-acl)#permit tcp host x.11.1.1 host x.11.1.2 eq bgp RP/0/0/CPU0:R3(config-ipv4-acl)#permit icmp host x.11.1.1 host x.11.1.2 echo RP/0/0/CPU0:R3(config-ipv4-acl)#permit icmp host x.11.1.1 host x.11.1.2 echo-reply RP/0/0/CPU0:R3(config-ipv4-acl)#deny ip any host x.11.1.1 log-input RP/0/0/CPU0:R3(config-ipv4-acl)#permit tcp any host x.12.1.22 eq www RP/0/0/CPU0:R3(config-ipv4-acl)#permit tcp any any established RP/0/0/CPU0:R3(config-ipv4-acl)#deny ip any any log-input RP/0/0/CPU0:R3(config-ipv4-acl)#exit

Step 2: Apply the ACL inbound on all external interfaces.

RP/0/0/CPU0:R3(config)#int g0/0/0/1 RP/0/0/CPU0:R3(config-if)#ipv4 access-group EXTERNAL_ACL_INBOUND in RP/0/0/CPU0:R3(config-if)#end