Snort Rule Validation Guide

G5 Cyber Security

2 weeks ago

TL;DR

This guide helps you check if your Snort rules are working as expected. We’ll cover basic syntax checks, testing with packet captures, and using the snort command to verify alerts.

1. Syntax Check

Before anything else, make sure your rules don’t have typos! Snort won’t run if they do. Use the following command:

snort -T -c /etc/snort/snort.conf

This tests the rule syntax against your configuration file. Look for errors reported in the output. Fix any issues before proceeding.

2. Rule Placement

Configuration File: Ensure rules are placed in the correct file within your Snort configuration (usually under /etc/snort/rules).
Rule Order: The order of rules matters! More specific rules should generally come before more general ones. A broad rule might catch traffic a specific rule would have handled first.
Include Statements: If using include statements in your snort.conf, verify the paths are correct and the included files exist.

3. Testing with Packet Captures

The best way to validate a rule is to see if it triggers on traffic it *should* and doesn’t trigger on traffic it *shouldn’t*. You’ll need a packet capture (.pcap file) containing the network activity you want to test.

Create a Test Capture: Use tools like tcpdump or Wireshark to create a .pcap file with traffic matching your rule’s criteria. For example, to capture HTTP GET requests on port 80:
```
tcpdump -i eth0 port 80 -w http_get.pcap
```
Run Snort: Execute Snort against the packet capture.
```
snort -r http_get.pcap -c /etc/snort/snort.conf
```
Check Alerts: Look for alerts in the alert file (usually /var/log/snort/alert). Verify that your rule generated an alert as expected.

4. Examining Rule Components

Let’s look at a typical Snort rule and how to check its parts:

alert tcp any any -> any 80 (msg:"HTTP GET Request"; content:"GET /"; http_uri; sid:1000001; rev:1;)

Header: alert tcp any any -> any 80 – Checks the protocol, source/destination IP addresses and ports. Ensure these are correct for your test traffic.
Rule Options: msg, content, http_uri, sid, rev – These define what Snort looks for in the packet content.
Content Matching: The content option is case-sensitive by default. Use nocase; to ignore case.
SID and Rev: Each rule needs a unique SID (Snort ID). rev tracks the revision number of the rule.

5. Common Issues & Troubleshooting

False Positives: If a rule triggers on traffic it shouldn’t, refine the content options or add more specific criteria.
False Negatives: If a rule doesn’t trigger when it should, double-check the content options and ensure they accurately match the packet content. Consider using different content matching keywords (e.g., pcre; for regular expressions).
Rule Disabling: Temporarily disable rules to isolate issues. Comment out the rule in your configuration file or use the -K PREPROC_RULE: option when running Snort.