Transparent Firewall - Things That Go Bump on the Wire!
This blog will examine the basic setup of the transparent firewall feature available with the PIX and the ASA. This blog was based on the PIX-525 running 7.2(4) code with a Restricted license in GNS3. Here is the topology that was used:
Remember, that a transparent firewall resides WITHIN a subnet and is easy to implement in an existing network where re-addressing to introduce a firewall might be difficult. This configuration is sometimes known as a "stealth" firewall or a "bump on the wire". Thanks to the fact that the firewall lives within the subnet, instead of between it, the device has the ability to filter traffic between hosts within the subnet. Note that the traditional Layer 3 firewall can only filter traffic moving between subnets. This should remind you of the difference between a VLAN Access Control List versus a Router-based Access Control List.
Well, with the introductions out of the way, let's do what we love best, let's get to the command line.
The first thing I will do is configure and verify the transparent firewall feature and then name our PIX:
pixfirewall> en
Password:
pixfirewall# conf t
pixfirewall(config)# firewall transparent
pixfirewall(config)# show firewall
Firewall mode: Transparent
pixfirewall(config)# hostname LAYER2FIREWALL
LAYER2FIREWALL(config)#
Excellent, now that the firewall is in transparent mode, let's take care of the inside and outside interfaces. When in transparent mode, you are limited to the use of two interfaces for passing traffic. Notice how the interfaces are not configured with IP addresses.
LAYER2FIREWALL(config)# int e1
LAYER2FIREWALL(config-if)# nameif inside
INFO: Security level for "inside" set to 100 by default.
LAYER2FIREWALL(config-if)# no shut
LAYER2FIREWALL(config-if)# exit
LAYER2FIREWALL(config)# int e0
LAYER2FIREWALL(config-if)# nameif outside
INFO: Security level for "outside" set to 0 by default.
LAYER2FIREWALL(config-if)# no shut
LAYER2FIREWALL(config-if)# sh int ip brief
Interface IP-Address OK? Method Status Protocol
Ethernet0 unassigned YES unset up up
Ethernet1 unassigned YES unset up up
Ethernet2 unassigned YES unset administratively down up
Ethernet3 unassigned YES unset administratively down up
Ethernet4 unassigned YES unset administratively down up
I am sure this output looks a little strange for those of you that have not played with this feature. Just as unusual is the fact that all of the interfaces facing this device are addressed in the 10.0.0.0/24 subnet.
A requirement of the transparent firewall is that it must have an IP address assigned in global configuration mode for management access. Notice for verification we can see that our traffic forwarding interfaces are now "listening" on that IP address:
LAYER2FIREWALL(config)# ip address 10.0.0.22 255.255.255.0
LAYER2FIREWALL(config)# sh int ip brief
Interface IP-Address OK? Method Status Protocol
Ethernet0 10.0.0.22 YES unset up up
Ethernet1 10.0.0.22 YES unset up up
Ethernet2 unassigned YES unset administratively down up
Ethernet3 unassigned YES unset administratively down up
Ethernet4 unassigned YES unset administratively down up
Let us now test the "out of the box" functionality of the security device. I will initiate a Telnet session from the Inside interface to a device located on the Outside interface. This communication should be permitted due to the Adaptive Security Algorithm and the default security levels on our interfaces. Notice how we can easily verify the connection of the appliance.
R1#telnet 10.0.0.10
Trying 10.0.0.10 ... Open
User Access Verification
Password:
R0>
LAYER2FIREWALL(config)# show conn
1 in use, 1 most used
TCP outside 10.0.0.10:23 inside 10.0.0.1:25501, idle 0:00:03, bytes 102, flags UIO
Let's now permit Telnet connections from our management workstation (played by R2) and ensure we have connectivity to the PIX.
LAYER2FIREWALL(config)# telnet 10.0.0.20 255.255.255.255 Inside
R2#telnet 10.0.0.22
Trying 10.0.0.22 ... Open
User Access Verification
Password:
Type help or '?' for a list of available commands.
LAYER2FIREWALL>
Well, I am sure I will blog more on this Layer 2 firewall at a later point, but I sure do thank you for stopping by to read this initial post.
