Is there any tool on Linux to automatically modify iptables so as to block a troublesome client based on an analysis of the Apache log? I help run a site that sometimes get overwhelmed by requests from a particular user. The only solution is to add an entry in iptables to block the offending client. Its usually too late by the time I can react manually - hence, I would like some rule based mechanism to modify the iptables. I would guess that some kind of fuzzly logic or statistical analysis would be necessary.
You can use something like fail2ban, which IIRC, has an Apache log checker built in.
You might want to consider using iptables to rate-limit incoming connections. Which in its most basic setting will give you the ability to limit incoming connections to a number per minute.
For example, you might want to only allow 10 pings per minute from a single IP address. It does get a little more sophisticated than that, with the option to set burst limits on top of long term average limits.
Some good instructions on setting it up http://kevin.vanzonneveld.net/techblog/article/block_brute_force_attacks_with_iptables/
Check out OSSEC. Best log file analyzer I've used. It also supports active response based on analysis.