Tuesday, August 8, 2017

Performing a Transparent Proxy by running PFSense Firewall + Cisco Route-Maps.

Also known as an intercepting proxy, inline proxy, or forced proxy, a transparent proxy intercepts normal communications at the network layer without requiring any special client configuration. (Wiki)

There are several options to perform a transparent proxy, being Squid one of the most popular software to implement it. The easiest way to implement a transparent proxy is setting it up as a default gateway. Nevertheless, it is not possible to implement it in a whole scenario. So, The Web Cache Communication Protocol (WCCP) was developed by Cisco Systems and adopted by other vendors. WCCP is one of the best choice to do a transparent proxy, however, it increases the complexity setup at the server side. Therefore, to simplify its implementation, I decided to do it in a different manner by running Route-Maps.

Let’s check the following configuration in order to accomplish this job.

Lab resources:
GNS3 (2.0.3) and VMWare WorkStation (12.5.7) as the framework.
Router image. (vios-adventerprisek9-m)
MLS Switch image. (vios_l2-adventerprisek9-m)
PFSense (community 2.3.4)
PC client. (Linux box 3.16.6-tinycore)

The focus of this configuration is the interactivity between PFSense and the MLS Switch.

PFSense was started from a fresh installation. Only one NIC was add, therefore the VLAN sub-interfaces had to be configured. VLAN10 has a subnet 172.16.0.0/27, while VLAN20 has a subnet 172.16.1.0/31. The default route was placed at the VLAN 20, so packets coming from VLAN10 are forwarded through VLAN20. In addition, NAT was completely disabled. Then, Squid and iftop packages were installed. Once Squid was installed and started, some basic changes were necessary to enable the transparent proxy feature.

The next step was configuring the MLS Switch (or router). IP Policy was enabled on the SVI 10, so that the packets that meet the IP policy be forwarded to PFSense, otherwise packets have to continue the established path by the routing table. Below, the router and MLS's configurations to get a better standpoint of the exposed so far

Results:
As the screenshots show, once the IP Policy is up, every packet addressed to a L4 port defined on the extended ACL, will be forwarded to the PFSense box in order to be allowed or denied, according the Squid policy already defined. For example, the first screenshot shows that the browser got access to Google. Take a look the certificate is not the Google valid certificate, instead it is a self-signed certicate, but the access was permitted. Then, the next screenshot shows that even when Facebook is carried over SSL, the access was denied.

Then, the boxes show current status of the IP Policy and each one of its elements.

Troubleshooting:
Adding a new statement that include ICMP protocol on the top of the Route-Map ACL, and launching a traceroute from the client host, can gives a step by step of how the packets are been carried. Also, iftop be launched in the PFSense box to displays the packets incoming and outgoing on each VLAN subinterface.

Conclusion:
As we can see, it is an option when a transparent-proxy needs to be run. It does not need any configuration at the client side. Also if the transparent-proxy stops, the clients still reach their destination, and the most important is, HTTP/HTTPS policies can be applied.

Monday, January 30, 2017

Implementing VLAN ACLs in a Cisco Systems Switch

How many times have we needed to protect a given host from its neighbors of the same Broadcast Domain (VLAN)? As is widely known this kind of practice is often performed by implementing local Firewalls on Workstation and Servers. For example, Linux has IPTables, Windows since the release XP includes Windows Firewall and Apple OS-X has a Firewall service as well. Also, the requirement can be performed by third party vendors such as Symantec, F-Secure and so on.

The coming example wants to demonstrate how a Cisco Systems Switch is able to perform this goal by implementing VLAN ACLs (VACL). Please keep in mind there is not any routing process involved, even more, the switch has not IP routing enabled.

The given topology has not anything specialized, only three computers connected to a switch. One of those computers runs Linux, other runs Windows and the other one runs OS-X. The Switch is a Cisco Systems Catalyst 3560, which runs IOS c3560-ipservicesk9-mz.122-55.SE11.bin. In addition, the Switch has almost its default configuration, I only added VLAN11 and some more commands, nothing special. The initial configuration is shared below.

Each computer has a static IP address assigned:
192.168.11.1/24 - - > Windows computers
192.168.11.11/24 - - > Linux box
192.168.11.111/24 - - > Apple computer

By the way, the switch has not a IP address assigned, because it is not required.

The Windows computer has Windows Firewall disabled, and the service of File and Printer Sharing (SMB) enabled.

The Linux box is publishing three folders by running python -m SimpleHTTPServer TCP-PORT. The web publishing service is assigning an unique TCP port to each folder, from 10001 to 10003.

The OS-X computer manages the switch through a serial port, sends some pings to Windows (192.168.11.1) and the Linux box (192.168.11.11), displays the content of each web folder (Text01, Text02...) and connects to the Windows computer through SMB. Everything happens as expected, everything is normal, nothing fancy.

The interesting thing starts once the VACL commands are introduced to the switch. As you can noticed in the video below, the Windows computer continues working normally. That is to say, Windows continues answering PING and sharing folders.

The lab has been focused on the Linux box. So, after introducing the VACLs commands in the switch, the Linux box stopped answering PING, also the administrator has decided to disable the TCP port 10003 to be reachable by the hosts in the same VLAN.

Let's watch a video to get a visual idea, step by step of the described topology.

The commands used by the VLAN ACL are shared below:


In addition to the first example, I would like to share one more example.

The next example consists of only two computers connected to a Cisco Systems switch. The switch has its configuration almost by default, only the switch ports have been modified as access ports. So, everything is running in VLAN1.

Each computer has two IP address statically assigned:
192.168.100.11/24 and 192.168.200.11/24 - - > Apple computer
192.168.100.12/24 and 192.168.200.12/24 - - > Linux box
As the coming video shows, each computer can PING request and reply without any problem. But, as soon the VACL commands are deployed in the Switch, the normal development changes, and the addresses corresponding to the network 192.168.200.0/24 are sudenly stopped.

The commands used by the VLAN ACL are shared below:

Conclusion:

Deploying VACLs could be considered an important security feature for our networks. Also, in this manner, an administrator can avoid applying features locally in servers and workstations, instead running this features in specialized equipment for saving performance to the host and getting a more granular control.