I recently had a problem on CentOS 5 where the best solution I could think of required running a command when a network interface is brought up. I’m used to having this functionality as a part of the networking scripts on Debian-derived distributions, but I discovered that Red Hat-derived distributions lack it. I’ve written a small daemon called nethook to provide it. You can find the code and documentation for nethook on github.
Update: Moved project from google code to github.