CISC-RT-000240 - The Cisco perimeter router must be configured to deny network traffic by default and allow network traffic by exception.

Information

A deny-all, permit-by-exception network communications traffic policy ensures that only connections that are essential and approved are allowed.

This requirement applies to both inbound and outbound network communications traffic. All inbound and outbound traffic must be denied by default. Firewalls and perimeter routers should only allow traffic through that is explicitly permitted. The initial defense for the internal network is to block any traffic at the perimeter that is attempting to make a connection to a host residing on the internal network. In addition, allowing unknown or undesirable outbound traffic by the firewall or router will establish a state that will permit the return of this undesirable traffic inbound.

Solution

This requirement is not applicable for the DODIN Backbone.

Step 1: Configure an inbound ACL to deny all other traffic by default 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 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 ingress filter to 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

See Also

https://dl.dod.cyber.mil/wp-content/uploads/stigs/zip/U_Cisco_IOS-XR_Router_Y24M07_STIG.zip

Item Details

Category: SYSTEM AND COMMUNICATIONS PROTECTION

References: 800-53|SC-7(5), CAT|I, CCI|CCI-001109, Rule-ID|SV-216752r531087_rule, STIG-ID|CISC-RT-000240, STIG-Legacy|SV-105849, STIG-Legacy|V-96711, Vuln-ID|V-216752

Plugin: Cisco

Control ID: a2c7ed9d52841cd742deff66cfa8c84dd74c3556ca28b9366e18a75290cdd9fc