Configure Custom Local Snort Rules in Snort2 on FTD

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Updated:April 11, 2024

Document ID:221875

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Introduction

This document describes the procedure to configure Custom Local Snort Rules in Snort2 on Firewall Threat Defense (FTD).

Prerequisites

Requirements

Cisco recommends that you have knowledge of these topics:

Cisco Firepower Management Center (FMC)
Firewall Threat Defense (FTD)

Components Used

The information in this document is based on these software and hardware versions:

Cisco Firepower Management Center for VMWare 7.4.1
Cisco Firepower 2120 7.4.1

The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, ensure that you understand the potential impact of any command.

Background Information

Custom Local Snort Rule refers to a user-defined rule that you can create and implement within the Snort intrusion detection and prevention system that is integrated into the FTD. When you create a custom local Snort rule in Cisco FTD, you are essentially defining a new pattern or set of conditions that the Snort engine can watch for. If network traffic matches the conditions specified in your custom rule, Snort can take the action defined in the rule, such as generating an alert or dropping the packet. Administrators use custom local Snort rules to address specific threats that are not covered by the general rule sets.

In this document, you are introduced how to configure and verify a Custom Local Snort Rule designed to detect and drop HTTP response packets containing a specific string (username).

Caution: Creating Custom Local Snort Rules and providing support for them falls outside of TAC support coverage. Therefore, this document can be used as a reference only, and ask that you create and manage these custom rules at your own discretion and responsibility.

Configure

Network Diagram

This document introduces the configuration and verification for Custom Local Snort Rule in Snort2 on this diagram.

Network Diagram

Configuration

This is the configuration of Custom Local Snort Rule to detect and drop HTTP response packets containing a specific string (username).

Step 1. Confirm Snort Version

Navigate to Devices > Device Management on FMC, click Device tab. Confirming the snort version is Snort2.

Snort Version

Step 2. Create a Custom Local Snort Rule in Snort 2

Navigate to Objects > Intrusion Rules > Snort 2 All Rules on FMC, click Create Rule button.

Create Custom Rule

Input necessary info for Custom Local Snort Rule.

Intrusion : custom_http_sig
Action : alert
Protocol : tcp
flow : Established, To Client

content : username (Raw Data)

Input Necessary Info for Rule

Step 3. Confirm Custom Local Snort Rule

Navigate to Policies > Intrusion Policies on FMC, click Snort 2 Version button.

Confirm Custom Rule

Navigate to Rules > Category > local on FMC, confirm the detail of Custom Local Snort Rule.

Detail of Custom Rule

Step 4. Change Rule Action

Click State button, set the State to Drop and Generate Events and click OK button.

Change the Rule Action

Click Policy Information button, click Commit Changes button to save changes.

Commit Changes

Step 5. Associate Intrusion Policy with Access Control Policy (ACP) Rule

Navigate to Policies > Access Control on FMC, associate Intrusion Policy with ACP.

Associate with ACP Rule

Step 6. Deploy Changes

Deploy the changes to FTD.

Deploy Changes

Verify

Custom Local Snort Rule is Not Triggered

Step 1. Set Contents of File in HTTP Server

Set the contents of the test.txt file on HTTP server side to user.

Step 2. Initial HTTP Request

Access the HTTP Server (192.168.20.1/test.txt) from the browser of the client (192.168.10.1) and confirm that the HTTP communication is permitted.

Initial HTTP Request 1 Initial HTTP Request

Custom Local Snort Rule is Triggered

Step 1. Set Contents of File in HTTP Server

Set the contents of the test.txt file on HTTP server side to username.

Step 2. Initial HTTP Request

Access the HTTP Server (192.168.20.1/test.txt) from the browser of the client (192.168.10.1) and confirm that the HTTP communication is blocked.

Initial HTTP Request 2 Initial HTTP Request

Step 3. Confirm Intrusion Event

Navigate to Analysis > Intrusions > Events on FMC, confirm the Intrusion Event is generated by the Custom Local Snort Rule.

Intrusion Event

Click Packets tab, confirm the detail of Intrusion Event.

Detail of Intrusion Event

Troubleshoot

Run system support trace command to confirm the behavior on FTD. In this example, the HTTP traffic is blocked by the IPS rule (gid 1, sid 1000001).

> system support trace

Enable firewall-engine-debug too? [n]: y
Please specify an IP protocol: tcp
Please specify a client IP address: 192.168.10.1
Please specify a client port: 
Please specify a server IP address: 192.168.20.1
Please specify a server port:

192.168.20.1-80 - 192.168.10.1-50075 6 AS 1-1 CID 0 Firewall: allow rule, 'ftd_acp', allow
192.168.20.1-80 - 192.168.10.1-50075 6 AS 1-1 CID 0 IPS Event: gid 1, sid 1000001, drop

192.168.20.1-80 - 192.168.10.1-50075 6 AS 1-1 CID 0 Snort id 3, NAP id 2, IPS id 1, Verdict BLOCKFLOW
192.168.20.1-80 - 192.168.10.1-50075 6 AS 1-1 CID 0 ===> Blocked by IPS