Technically, it is a third party nginx module, available as a package for many UNIX-like platforms. This module, by default, reads a small subset of simple (and readable) rules containing 99% of known patterns involved in website vulnerabilities. For example,
dropare not supposed to be part of a URI.
Being very simple, those patterns may match legitimate queries, it is the Naxsi's administrator duty to add specific rules that will whitelist legitimate behaviours. The administrator can either add whitelists manually by analyzing nginx's error log, or (recommended) start the project with an intensive auto-learning phase that will automatically generate whitelisting rules regarding a website's behaviour.
In short, Naxsi behaves like a DROP-by-default firewall, the only task is to add required ACCEPT rules for the target website to work properly.
Why is it different?
Contrary to most Web Application Firewalls, Naxsi doesn't rely on a signature base like an antivirus, and thus cannot be circumvented by an "unknown" attack pattern. Naxsi is Free software (as in freedom) and free (as in free beer) to use.
What does it run on?
Naxsi should be compatible with any nginx version.
It depends on
libpcrefor its regexp support, and is reported to work great on NetBSD, FreeBSD, OpenBSD, Debian, Ubuntu and CentOS.