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By: Athul K

How to Write Custom Network Intrusion Detection Rules


In the world of network security, the ability to detect and respond to potential threats is crucial. Network Intrusion Detection Systems (NIDS) act as vigilant guardians, constantly analyzing network traffic to identify signs of malicious activity. Snort, an open-source NIDS tool, empowers security professionals to create custom rules for spotting specific patterns within this vast sea of data. In this blog, we'll take a comprehensive journey through the process of creating custom NIDS rules using Snort.

Before diving into creating custom rules, let's understand the key components that make up a Snort rule:

1. Action: Specifies the action to be taken when a rule matches, such as generating alerts or dropping packets.

2. Protocol: Defines the network protocol the rule applies to, such as TCP, UDP, or ICMP

3. Source/Destination IP: Specifies the source and destination IP addresses of the traffic.

4. Source/Destination Port: Sets the source and destination port numbers.

5. Rule Options: This section contains attributes that refine the rule's behavior. Here, you can define patterns to detect within packet payloads or headers.

Steps for writing Intrusion Rules

1. Define the Rule Header

The rule starts with a header that outlines its basic characteristics:

alert tcp any any -> $HOME_NET 22 (msg:"SSH Brute Force Attempt"; sid:100001;)

* alert: Signals the rule to generate an alert upon detection.

* tcp: Specifies the rule's applicability to TCP traffic.

* any any: Matches any source IP and any source port.

* $HOME_NET: Refers to the network defined as the home network in Snort's configuration.

* 22: The port number associated with SSH.

* msg: Provides a descriptive message for the generated alert.

* sid: Uniquely identifies the rule.

2. Add Rule Options

The rule options section is where the rule's behavior is defined. For our purpose, we'll focus on the content within packet payloads to identify the pattern of a failed SSH login:

alert tcp any any -> $HOME_NET 22 (msg:"SSH Brute Force Attempt"; content:"Failed password"; sid:100001;)

* content:"Failed password": Matches packets containing the "Failed password" string.

3. Refine Rule Precision

Enhancing the rule's precision is essential. We can further refine the rule to target repeated failed login attempts from a single source IP within a specific time frame.

alert tcp $EXTERNAL_NET any -> $HOME_NET 22 (msg:"SSH Brute Force Attempt"; content:"Failed password"; threshold: type threshold, track by_src, count 5, seconds 60; sid:100001;)

* $EXTERNAL_NET: Represents the network external to your own.

* threshold: Sets a triggering threshold for the rule.

* type threshold, track by_src, count 5, seconds 60: Specifies that the rule activates when five failed login attempts from the same source IP occur within 60 seconds.

4. Implement Action

In the realm of network security, rule detection often requires action. This can involve logging the event, sending alert notifications, or even blocking the source IP. Here, we add the action component.

alert tcp $EXTERNAL_NET any -> $HOME_NET 22 (msg:"SSH Brute Force Attempt"; content:"Failed password"; threshold: type threshold, track by_src, count 5, seconds 60; sid:100001; rev:1;)

* rev: Indicates the revision of the rule.

Developing custom Snort rules is a critical component of a robust Network Intrusion Detection System (NIDS) strategy. A deep understanding of rule composition and their interactions empowers security professionals to finely tune their NIDS for effective threat detection. In this blog, we will walk you through the process of creating a personalized Snort rule to identify brute-force SSH login attempts.

As you navigate the landscape of network security, it is vital to emphasize the importance of thorough testing in a controlled environment before deploying these rules on your production network. This practice ensures the reliability of your NIDS, minimizing false alarms and ensuring comprehensive threat coverage.

To read more about network-based intrusion detection & prevention systems, refer to our blog An Overview of Network-based Intrusion Detection & Prevention Systems

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