Cisco ASA Security Levels

The Cisco ASA Firewall uses so called “security levels” that indicate how trusted an interface is compared to another interface. The higher the security level, the more trusted the interface is. Each interface on the ASA is a security zone so by using these security levels we have different trust levels for our security zones.

An interface with a high security level can access an interface with a low security level but the other way around is not possible unless we configure an access-list that permits this traffic.

Here are a couple of examples of security levels:

• Security level 0: This is the lowest security level there is on the ASA and by default it is assigned to the “outside” interface. Since there is no lower security level this means that traffic from the outside is unable to reach any of our interfaces unless we permit it within an access-list.
• Security level 100: This is the highest security level on our ASA and by default this is assigned to the “inside” interface. Normally we use this for our “LAN”. Since this is the highest security level, by default it can reach all the other interfaces.
• Security level 1 – 99: We can create any other security levels that we want, for example we can use security level 50 for our DMZ. This means that traffic is allowed from our inside network to the DMZ (security level 100 -> 50) and also from the DMZ to the outside (security level 50 -> 0). Traffic from the DMZ however can’t go to the inside (without an access-list) because traffic from security level 50 is not allowed to reach security level 100. You can create as many security levels as you want…

Let’s take a look at a Cisco ASA firewall with three interfaces so you can see this behavior in action, here’s the topology I will use:

Above you see the Cisco ASA in the middle with three interfaces:

• Interface E0/0 as the INSIDE.
• Interface E0/1 as the OUTSIDE.
• Interface E0/2 as our DMZ.

I will use the routers so we can generate some traffic between the different security levels. Let’s configure the ASA with these interfaces:

ASA1(config)# interface E0/0
ASA1(config-if)# nameif INSIDE
INFO: Security level for "INSIDE" set to 100 by default.
ASA1(config-if)# ip address 192.168.1.254 255.255.255.0
ASA1(config-if)# no shutdown

ASA1(config)# interface E0/1
ASA1(config-if)# nameif OUTSIDE
INFO: Security level for "OUTSIDE" set to 0 by default.
ASA1(config-if)# ip address 192.168.2.254 255.255.255.0
ASA1(config-if)# no shutdown

ASA1(config)# interface E0/2
ASA1(config-if)# nameif DMZ
INFO: Security level for "DMZ" set to 0 by default.
ASA1(config-if)# security-level 50
ASA1(config-if)# ip address 192.168.3.254 255.255.255.0
ASA1(config-if)# no shutdown

The nameif command is used to specify a name for the interface, unlike the description command the name of your interface is actually used in many commands so pick something useful. As you can see the ASA recognizes INSIDE, OUTSIDE and DMZ names. It uses a default security level of 100 for INSIDE and 0 for OUTSIDE/DMZ. I manually changed the security level of the DMZ interface to 50.

Let’s see what traffic patterns are allowed now shall we? First we’ll send some pings from the ASA…

Traffic from the ASA

The ASA can reach any device on any interface:

ASA1# ping 192.168.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/10 ms

ASA1# ping 192.168.2.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.2.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/1 ms

ASA1# ping 192.168.3.3
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.3.3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/1 ms

As you can see the ASA can reach any device in each of the different security zones. This makes sense since these devices are also using the ASA as their default gateway. Next step is to test some traffic between devices in different security zones.

ASA1(config)# policy-map global_policy
ASA1(config-pmap)# class inspection_default
ASA1(config-pmap-c)# inspect icmp

Traffic from Inside

Let’s send some pings from R1 to R2 (outside) and R3 (DMZ):