launchctl: Run script when network interface is added/comes up?

I'm looking to run a script when 'bridge100' interface is added with an IP. How is this accomplished? I see this post to add the following, however I'm not sure how one can understand that 'bridge100' was the interface added and also has an IP just by looking at these files change timestamps: Label ifup.ddns LowPriorityIO ProgramArguments /Users/Shared/bin/ddns-update.sh WatchPaths /etc/resolv.conf /Library/Preferences/SystemConfiguration/NetworkInterfaces.plist /Library/Preferences/SystemConfiguration/com.apple.airport.preferences.plist RunAtLoad or a watch on: /Library/Preferences/SystemConfiguration/com.apple.airport.preferences.plist