Cisco ASA 5505 DMZ with Private VLAN Configuration

The ASA 5505 is the only model that has an 8-port switch embedded in the device. All interfaces of the ASA5505 are Layer2 switch ports and thus they support some features that you can find on Cisco switches. One of these features is called “Private Vlan”.

The concept of “Private VLAN” is very useful in DMZ environments. Here is how it can be used: Let’s say you have a firewall with an Outside interface connected to Internet, an Inside interface connected to the secure LAN, and a DMZ Interface connected to a subnet which is hosting several publicly accessible servers (e.g Web Server, Email server etc). The DMZ servers are all on the same network subnet. Thus, if one of the DMZ servers gets compromised, then the attacker can easily use this hacked server as a “stepping-stone” to access the other servers in the DMZ.

The above situation can be mitigated by using “Private VLANs”. Although the DMZ Layer2 VLAN number and Layer3 subnet will be the same for all servers, by designating each switch port of the DMZ as “Private VLAN” then the servers in the DMZ will not be allowed to communicate with each other.

Let’s see a diagram below to explain this concept.

Let’s say we have an ASA5505 with three security Zones:

• Outside Zone: Interface E0/0 in VLAN 10
• Inside Zone: Interface E0/1 in VLAN 20
• DMZ Zone: Interfaces E0/2, E0/3 in VLAN 30

Notice that in DMZ we have 2 publicly accessible servers (Web and Email Server) that they both belong in the same Layer2 vlan (VLAN30) and the same Layer3 network subnet (10.0.0.0/24).

If we don’t configure “Private Vlans”, then if the Web or Email server gets hacked, the attacker can access the other DMZ server as well. With Private VLANs, the Web and Email Servers can NOT communicate with each other although they are on the same Vlan and subnet. However, all other zones (outside and inside) are able to access the DMZ zone (and vice-versa) with no problems.

Configuration:

We are not going to see the complete config here, just the part that has to do with Private Vlan setup.

ASA5505(config)# interface ethernet 0/0
ASA5505(config-if)# switchport access vlan 10
ASA5505(config-if)# no shutdown

ASA5505(config-if)# interface ethernet 0/1
ASA5505(config-if)# switchport access vlan 20
ASA5505(config-if)# no shutdown

ASA5505(config-if)# interface ethernet 0/2
ASA5505(config-if)# switchport access vlan 30
ASA5505(config-if)# no shutdown
ASA5505(config-if)# switchport protected

ASA5505(config-if)# interface ethernet 0/3
ASA5505(config-if)# switchport access vlan 30
ASA5505(config-if)# no shutdown
ASA5505(config-if)# switchport protected

The command “switchport protected“ configures the specific physical ports as “Private VLANs”. All ports that are configured as Private Vlans can not communicate with each other.