Applying CBAC Inspection to Local Traffic
By stretch | Thursday, April 21, 2011 at 5:17 a.m. UTC
Cisco's context-based access control allows for the filtering of traffic based on application-layer protocols. CBAC makes use of two components to filter traffic traversing an untrusted interface: a set of inspection rules for traffic egressing the interface and an access list to permit or deny ingress traffic. Traffic originating on the internal network is inspected as it leaves the router and an entry is appended to the incoming access list to allow return traffic from the same session.
One peculiar aspect of CBAC is that the outgoing inspection rules match only traffic passing through the router; traffic generated by the router itself is not inspected. At first this may seem an insignificant detail, but because the router's traffic isn't being inspected, an exception for it in the inbound access list isn't created and the router won't receive any return traffic.
For illustration, let's create a simple inspection rule to match ICMP and apply it to the router's external interface in the outbound direction:
Router(config)# ip inspect name Outbound icmp Router(config)# interface f0/0 Router(config-if)# ip inspect Outbound out
Next, we create a "deny any" access list and apply it inbound on the same interface:
Router(config)# ip access-list extended Inbound Router(config-ext-nacl)# deny ip any any Router(config-ext-nacl)# interface f0/0 Router(config-if)# ip access-group Inbound in
Our CBAC configuration should allow only ICMP traffic originating from within the trusted network to return. We can verify that pinging the external router R1 from R3 succeeds:
R3# ping 10.0.12.1 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 10.0.12.1, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 4/8/16 ms
However, R2 itself cannot reach R1:
R2# ping 10.0.12.1 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 10.0.12.1, timeout is 2 seconds: ..... Success rate is 0 percent (0/5)
This is because, as mentioned, traffic originating from the router itself is not by default included in the inspection rule applied to the external interface. To fix this, we need to append the router-traffic argument to our inspection rule:
R2(config)# ip inspect name Outbound icmp ? alert Turn on/off alert audit-trail Turn on/off audit trail router-traffic Enable inspection of sessions to/from the router timeout Specify the inactivity timeout time R2(config)# ip inspect name Outbound icmp router-traffic
Now R2 can successfully ping R1:
R2# ping 10.0.12.1 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 10.0.12.1, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 4/10/28 ms
router-traffic argument must be appended independently to each inspection rule in a named set.
I had intended to include an IPv6 example of CBAC as well, but it unfortunately appears broken on the versions of IOS I have available.
About the Author
Jeremy Stretch is a network engineer living in the Raleigh-Durham, North Carolina area. He is known for his blog and cheat sheets here at Packet Life. You can reach him by email or follow him on Twitter.
Posted in Security
April 21, 2011 at 5:48 a.m. UTC
Nice tip! I teach the CCNA-Security and am spending less and less time on CBAC and focusing more on ZBF. A lot more complicated for the students, but a great way to really exercise some network engineering skills...;)
April 22, 2011 at 11:49 a.m. UTC
I am studying for the SECURE exam and I did not know this argument. Thanks a lot
April 24, 2011 at 3:26 a.m. UTC
Nice write up. I discoevered this a while ago while troubleshooting CBAC not inspecting transit traffic, and remembered the router was also performing NAT on transit traffic, which causes said traffic to be originated from the router.
July 25, 2011 at 4:16 p.m. UTC
Thank you Jeremy. Your explanation about 'to the router and through the router' with regard to the actions of the IP inspection rule was wonderful. I have marked your site as a favourite.