We've got four servers in use for a mission-critical application that all need constant connectivity to each other--six always-up connections in total. I need a way to monitor these connections and fire off, at the very least, an email when any one of them goes down. I can find centralised solutions, but nothing that really fits this bill. Any suggestions?
EDIT: Went ahead and rolled my own in Ruby. Nagios looks like a decent bit of kit, though--would've gone with it otherwise.