In a previous blog, I’ve written about the differences between Firepower Threat Defense and ASA software. And although the basic OS appears to be ASA with Snort set in between ASA ingress and egress, some basic concepts of the ASA (or actually the PIX) have disappeared in FTD. And it will have an impact once you migrate from ASA (with or without firepower) to FTD devices..
The Cisco ASA firewall has implemented the concept of security levels on interfaces. Each interface must have an interface name and interface level (IP-address is only necessary if you use routed or IRB interfaces). The concept for this is quite easy, let’s take the basic ASA edge deployment into mind.

With interface security levels the ASA keeps two principles into mind when a packet comes into the firewall for a new connection:

• Traffic is always allowed from a higher security level to a lower security level, unless an access-list prohibts this
• Traffic is never allowed from a lower security level to a higher security level, unless there is an access-list explicitly permitting the traffic and a static translation is created

So, in this example, the client with internal IP 10.1.1.10 is allowed to go outbound to the webserver on the Internet, but unless you configure a specific access-list, traffic is not allowed from the Internet to the inside mail server.
If you then take into account that IOS/PIX/ASA access-lists always have an implicit deny ip any any statement in the access-list, so even if you configure an access-list (except of course permit ip any any), traffic will be blocked by that implicit match. So it’s relatively easy to tie down the traffic flows, right?

In FTD things are working quite differently.. In FTD there is not really the concept of an access list with an implicitly deny any any. Within FTD, an access policy is used to define the policies that need to be applied to one or more FTD devices. One or more rules, which are run sequentially, combined define the access policy. Each rule in itself contains several elements that can be combined to define the policy rule. Each rule consists following elements:

FTD Policy actions

There are different actions that FTD can take once a packet has matched the access rule, being

For grouping purposes it is possible to group access rules together. Using this approach, it is a very flexible and powerfull mechanism to define the policy. It is a single policy for both ingress and egress traffic, so double access list policies are not required anymore, which is a great benefit. Also, rules are run through sequentially, so after a packet matches the rule, the specific actions are executed.

But what happens if no rule inside the policy is matched? Well, then the default Action comes into play. As soon as you create a new access policy, FMC asks what the default action will be of the new policy

Within a Firepower on ASA rule, I usually choose the “Intrusion Prevention” or “network discovery” rule, as there is still a firewall outside the FirePower module that has access-lists as well.

But in FTD that is not true!! If you choose “network discovery” or “intrusion prevention“, you will basically allow alltraffic to traverse through the firewall as the interface security levels are in essence equal. So it is then basically a router with some security power! I ran into this issue when I converted a Firepower on ASA device to FTD recently. This is something you have to be aware off.

If you want to change this behaviour in an access policy at a later stage, just go into the access policy and click the pull down menu at the bottom of the policy and select “Access control: Block All Traffic”

So in summary, if you start working with FTD and come from an ASA background, make sure your that you define the access rules both ways and have explicit block rules in your access policy. Either via the default action or per traffic-flow in a category.