ModSecurity
From FAQ:
https://github.com/SpiderLabs/ModSecurity/wiki/ModSecurity-Frequently-Asked-Questions-%28FAQ%29
Copy to clipboard
== How can I verify exactly how ModSecurity is processing rules and requests? == You need to enable the debug log with SecDebugLog and increase the log level with SecDebugLogLevel. It you set the debug log level to 9, it will tell you exactly what tasks it is completing along with what data it is acting upon. Do be aware that while the increased debug log level does help from a trouble-shooting perspective, it does negatively impact performance.
http://blog.spiderlabs.com/2011/08/modsecurity-advanced-topic-of-the-week-exception-handling.html
Todo
- Document how to have useful error requests