The administrator must bind the ingress ACL filtering packets entering the network to the external interface on an inbound direction.

From Perimeter Router Security Technical Implementation Guide Cisco

Part of Bind ingress filters to correct interface

SV-16197r1_rule The administrator must bind the ingress ACL filtering packets entering the network to the external interface on an inbound direction.

Vulnerability discussion

Access lists are used to separate data traffic into that which it will route (permitted packets) and that which it will not route (denied packets). Secure configuration of routers makes use of access lists for restricting access to services on the router itself as well as for filtering traffic passing through the router. Inbound versus Outbound; it should be noted that some operating systems default access-lists are applied to the outbound queue. The more secure solution is to apply the access-list to the inbound queue for 3 reasons:• The router can protect itself before damage is inflicted.• The input port is still known, and can be filtered upon.• It is more efficient to filter packets before routing them.

Check content

IOS Example: interface FastEthernet 0/0 description NIPRNet link ip address 199.36.92.1 255.255.255.252 ip access-group 100 in

Fix text

Bind the ingress ACL to the external interface (inbound) and the egress ACL to the internal interface (inbound).

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