Posts Tagged ‘transparent’
As I am sure you have already seen from the blog on setting up the security device as a Layer 2 device, there are many interesting changes that occur on a PIX or ASA when configured for transparent operations. This blog highlights the major changes and guidelines that you should keep in mind when you opt for this special mode of operation.
- Number of interfaces – perhaps on of the biggest things you will want to keep in mind is the fact that you are going to be limited on the number of traffic forwarding interfaces you can use when in Layer 2 mode. When you switch to transparent mode, you are limited to the use of two traffic forwarding interfaces. On some ASA models, you may also use your dedicated management interface, but of course, the use of this port is limited for management traffic. Remember also, when in multiple context mode, you cannot share interfaces between contexts like you can when in routed mode.
- IP addressing – here is another major difference of course. In Layer 2 mode, you will assign a single IP address to the device in Global Configuration mode. This address is for remote management purposes and is required before the device will forward traffic. Once the address is assigned, all interfaces start “listening” on this address to ensure the device is responsive to its administrator. This global IP addressed assigned to the device must be in the same subnet that the forwarding interfaces are participating in. Remember, the transparent firewall is not adding a new network (subnet) to your topology.
- Default gateway – for traffic sourced from the security device itself, you can configure a default gateway on the transparent device. You can do this with the route 0 0 command.
- IPv6 support - the transparent firewall does not support IPv6.
- Non-IP traffic – you can pass non-IP traffic through the Layer 2 Mode device. Note that this is not possible on a security appliance in its default Layer 3 mode.
- More unsupported features – the Layer 2 mode device does not support – Quality of Service (QoS) or Network Address Translation (NAT).
- Multicast – the transparent mode device does not offer multicast support, but you can configure Access Control Lists (ACLs) in order to pass multicast traffic through the device.
- Inspection – with the Layer 2 mode device you can inspect traffic at Layer 2 and above. With the classic routed mode configuration, you can only inspect at Layer 3 and above.
- VPN support – the transparent mode device does support a site to site VPN configuration, but only for its management traffic.
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.