What does Guardian do?
Intercepts any or all web server errors. Detailed documentation describes how to configure the web server for error handling.
Emails an administrator when an error occurs, including: which document the visitor linked from; which document they tried to visit; the visitor's IP, hostname, and browser.
Logs the error to an industry-standard tab-delimited file for later review or import into a spreadsheet application.
Versatile pattern-matching filter rules allow the administrator to disable email or logging for certain error types (prevents flooding).
Secure web admin interface allows administrator to review and edit filter rules, review and edit logs, and create and customize special error messages.
How Guardian stands out
Guardian belongs to a general class of error-handling Perl CGI scripts. Most of them are Apache-specific scripts that log and/or send an email. The Guardian differs in that:
It can handle errors - by replacing content or redirecting users - in addition to simply reporting errors.
It comes with a secure admin interface. Even though it is one of the most configurable and customizable scripts available, absolutely no customization of the Perl source code is required. (If you like customizing the source you are still welcome to do so.)
Behavior can be controlled through pattern matching. This lets you return a certain custom message for some file types, a different message for other file types, and yet another custom message for people linking in from a specific site.
It is designed to run in a more platform independent fashion, working for Apache, Microsoft IIS and Zeus web servers.
Guardian can send mail via a command-line sendmail program, like most, or via an SMTP server. SMTP support allows the Guardian to work on Windows and other sendmail-deprived systems.
The 401 Authorization Required handler will notify you when visitors try to access password-protected folders. It will tell you what usernames they are testing, which can help you assist a lost user or recognize a probe early on.
Why would I want to use the Guardian?
Though there are excellent tools for tracking the details of legitimate visitors, these often require that the visitor view a real document. What about sites that link to files that have moved? Are you losing hundreds of hits every day because you renamed a file? Guardian alerts you of the "backdoor" visitors who would otherwise go unnoticed.