This is in response to all the iptables + strings mails, talking about blocking such things as nimda and codered, etc.
The following link has two things. A tutorial on getting strings matching to work (it is NOT enabled by default), and within that article there is a link to my rejectstrings shell script which does exactly what you people want. http://articles.linuxguru.net/view/125 Enjoy, and pass the article around.
