I've recently been hit pretty hard with a nasty DDoS attack on a site of mine.

With http://wiki.nginx.org/HttpLimitReqModule and http://wiki.nginx.org/HttpLimitZoneModule I was able to mitigate the attack reasonably well, but neither of these modules do what I'd really like to have done - temporarily serve only a plain 4xx or 5xx error message to any IP that is exhibiting clearly abusive behavior, like requesting the exact same page over and over again, or attempting to make a large amount of parallel connections (100+) at once. This should be in effect for a configurable amount of time, but just 10 minutes or so would have worked well against my recent attack, as each attacking bot only tends to attack for a few minutes before its replaced by another bot.

So my suggestion is either adding functionality to the Limit modules, or adding a new DDoS module which would use a set amount of memory to keep tab of all IPs that have requested anything in the past 10-15 seconds, and can mitigate DDoS attacks by analyzing recent request and connect patterns by IP.