This is a simple nagios plugin for monitoring domain expiration, that I've maintained and extended.
If you just want to copy the plugin onto your nagios machine, replace PLUGINS_DIR in the command: wget --no-check-certificate https://raw.githubusercontent.com/jasonmce/check_dom ain/master/check_domain -O PLUGINS_DIR/check_domain : and make sure it's executable by nagios.
Enjoy!