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Intrusion Detection and Prevention Configuration

Cerberus is already an IDS Firewall. In the previous article Build Your Own IDS Firewall With pfSense the installation and configuration of Snort was covered in detail. So there is little that needs to be done further for it. We do need to add our new OPT1 WAN connection, however and rearrange our rules.

We are going to want the same overall protection on both WAN interfaces. So under Services->Snort, add both the new OPT1 interface and your LAN interface. The OPT1, Secondary ISP interface should be a clone of your Primary Interface, i.e. same pre-processor settings, same rules, as shown in Figure 7.

Squid interfaces

Figure 7: Squid interfaces

The LAN interface, on the other hand, is lightweight with just the pre-processor defaults and HTTP Inspect checked. It should handle just a few categories of rules. The idea here is to offload a few categories from your WAN interfaces to the LAN's where it would be good to know which LAN IP is being attacked and whether the attacks are coming from the inside. Examples categories would be NetBios and ICMP.

Your mileage may differ and you may want to expand the categories that generate alerts. Figure 8 shows the selected categories on Cerberus.

Alert categories

Figure 8: Alert categories

Remember, the more rules you select, the higher the probability of false positives, which can be an administration headache.

After adding the additional interfaces and configuring them, start Snort by clicking the green arrow next to the interface definition. We can test these additions to Snort by using the GRC Shields-Up Site to scan the added Secondary ISP WAN interface. Your Snort Alert log should look something like Figure 9.

Snort Alert log

Figure 9: Snort Alert log

If you have an ISP-provided router instead of just a modem, you need to either put pfSense in the DMZ or configure your router to run as a transparent bridge. Since ISP routers are a known attack vector, transparent bridging is recommended.

For example, out of the box, the Qwest branded Actiontec Q1000 has multiple ports open, including HTTPS for remote administration. For the purposes of obscuring my logged IP address in this article, Cerberus has just been put in the DMZ.

Once this is complete, you will want to reverse the changes made when testing your multi-WAN configuration and change your LAN traffic rule back to using the default gateway (our primary ISP).

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