Massive Webserver DDoS being prepare?

I get massive access to my server's robots.txt recently (in all my other sites too). Anyone has any idea what is it?

It seems like a DDoS (Distributed Denial of Service) is being prepare or is that a new worm that hit Fedora Core 3?

Strange server logs
From the logs, all those buggy visitors (different IP addresses) were bearing Fedora Core 3 User Agent and just grab the robots.txt and run away.
tagged: http, server, ddos, hack, worm, fc3, firefox