In my previous CBAC post I covered how to deny all external traffic unless it is in response to a request someone on the LAN has made, e.g If you send a ping, CBAC will allow the ping reply traffic to come through the firewall.

However, this situation may not be ideal for everyone. What if you wanted to allow one or more protocols in to your network but still have the security that CBAC provides? For example, if you wanted a contractor to be able to SSH in to your network any time, day or night? I will show you how using the same topology as last time (except you will see that R3 has been renamed to SSH Source). Our goal is to allow R3 to SSH in to R2 without being blocked, while still blocking R4 (hacker) completely.


If you recall, our inbound ACL on R1 looks like this:

ip access-list extended DENY_IN
deny ip any any

So if we try to SSH in from R3 now, we should fail, which we do. See the output below:

R3#ssh -l cisco
% Destination unreachable; gateway or host down

However, if we add the following line to the ACL:

ip access-list extended DENY_IN
!!Allow SSH traffic in to the network if its source address is and it is destined for
permit tcp host host eq 22
deny ip any any

We should now be able to SSH in to R2 from R3, which we can:

R3#ssh -l cisco


But just to make sure that all of our security measures are still enforced, lets try to ping from R3 to R2:


Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to, timeout is 2 seconds:
Success rate is 0 percent (0/5)

And let’s also try to SSH in from R4 (hacker):

R4#ssh -l cisco
% Destination unreachable; gateway or host down


As you can see from the above two tests, even though R3’s SSH traffic can get through to R2, its ping traffic cannot. And you can also see that R4’s SSH traffic cannot get through at all. This is because the new ACL entry specifically says that only SSH traffic sourced from R3 and destined for R2 is allowed to enter the network.

As always, if you have any questions or have a topic that you would like me to discuss, please feel free to post a comment at the bottom of this blog entry, e-mail at, or drop me a message on Reddit (OzNetNerd).

Note: The opinions expressed in this blog are my own and not those of my employer.

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